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itemprop="item" href="/cpp/category/exceptions-and-raii/"><span itemprop="name">Exceptions and RAII</span></a><meta itemprop="position" content="1"></li><li itemscope="" itemprop="itemListElement" itemtype="https://schema.org/ListItem" class="breadcrumbs__item breadcrumbs__item--active"><span class="breadcrumbs__link" itemprop="name">Placeholders</span><meta itemprop="position" content="2"></li></ul></nav><div class="tocCollapsible_ETCw theme-doc-toc-mobile tocMobile_ITEo"><button type="button" class="clean-btn tocCollapsibleButton_TO0P">On this page</button></div><div class="theme-doc-markdown markdown"><header><h1>Placeholders</h1></header><p>Here we will try to implement some placeholders that you can find in other
languages, but I miss them in the C++. I&#x27;m taking the inspiration from languages
like Rust (all that we will implement) or Kotlin (<code>TODO</code>) that have them
implemented.</p>
<p>You may ask what placeholders do we need in the code, in our case we will be
talking about TODOs and unexpected situations, such as not implemented branches.</p>
<p>Namely we will implement</p>
<ul>
<li><code>todo</code>,</li>
<li><code>unimplemented</code>, and</li>
<li><code>unreachable</code>.</li>
</ul>
<h2 class="anchor anchorWithStickyNavbar_LWe7" id="design">Design<a href="#design" class="hash-link" aria-label="Direct link to Design" title="Direct link to Design">​</a></h2>
<p>If we take the two languages mentioned above as examples, there are at least two
ways how to implement them:</p>
<ol>
<li><em>panic</em> when they are reached (as they do in Rust), or</li>
<li><em>raise</em> an exception when they are reached (as they do in Kotlin).</li>
</ol>
<p>I will choose raising an exception, since the closest equivalent of <em>panic</em> in
C++ would be <code>assert</code>s that are (by default) disabled in the <em>release builds</em>.</p>
<p>However I am too lazy to do:</p>
<div class="language-cpp codeBlockContainer_Ckt0 theme-code-block" style="--prism-background-color:hsl(230, 1%, 98%);--prism-color:hsl(230, 8%, 24%)"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-cpp codeBlock_bY9V thin-scrollbar" style="background-color:hsl(230, 1%, 98%);color:hsl(230, 8%, 24%)"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token keyword" style="color:hsl(301, 63%, 40%)">throw</span><span class="token plain"> </span><span class="token function" style="color:hsl(221, 87%, 60%)">todo</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token comment" style="color:hsl(230, 4%, 64%)">// or</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token keyword" style="color:hsl(301, 63%, 40%)">throw</span><span class="token plain"> </span><span class="token function" style="color:hsl(221, 87%, 60%)">todo</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token string" style="color:hsl(119, 34%, 47%)">&quot;optional note&quot;</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><br></span></code></pre><div class="buttonGroup__atx"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><span class="copyButtonIcons_eSgA" aria-hidden="true"><svg viewBox="0 0 24 24" class="copyButtonIcon_y97N"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_LjdS"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></span></button></div></div></div>
<p>Therefore we will implement exceptions and also wrap them in functions, so that
we can do:</p>
<div class="language-cpp codeBlockContainer_Ckt0 theme-code-block" style="--prism-background-color:hsl(230, 1%, 98%);--prism-color:hsl(230, 8%, 24%)"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-cpp codeBlock_bY9V thin-scrollbar" style="background-color:hsl(230, 1%, 98%);color:hsl(230, 8%, 24%)"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token function" style="color:hsl(221, 87%, 60%)">todo</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token comment" style="color:hsl(230, 4%, 64%)">// or</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token function" style="color:hsl(221, 87%, 60%)">todo</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token string" style="color:hsl(119, 34%, 47%)">&quot;optional note&quot;</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><br></span></code></pre><div class="buttonGroup__atx"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><span class="copyButtonIcons_eSgA" aria-hidden="true"><svg viewBox="0 0 24 24" class="copyButtonIcon_y97N"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_LjdS"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></span></button></div></div></div>
<div class="theme-admonition theme-admonition-tip admonition_xJq3 alert alert--success"><div class="admonitionHeading_Gvgb"><span class="admonitionIcon_Rf37"><svg viewBox="0 0 12 16"><path fill-rule="evenodd" d="M6.5 0C3.48 0 1 2.19 1 5c0 .92.55 2.25 1 3 1.34 2.25 1.78 2.78 2 4v1h5v-1c.22-1.22.66-1.75 2-4 .45-.75 1-2.08 1-3 0-2.81-2.48-5-5.5-5zm3.64 7.48c-.25.44-.47.8-.67 1.11-.86 1.41-1.25 2.06-1.45 3.23-.02.05-.02.11-.02.17H5c0-.06 0-.13-.02-.17-.2-1.17-.59-1.83-1.45-3.23-.2-.31-.42-.67-.67-1.11C2.44 6.78 2 5.65 2 5c0-2.2 2.02-4 4.5-4 1.22 0 2.36.42 3.22 1.19C10.55 2.94 11 3.94 11 5c0 .66-.44 1.78-.86 2.48zM4 14h5c-.23 1.14-1.3 2-2.5 2s-2.27-.86-2.5-2z"></path></svg></span>tip</div><div class="admonitionContent_BuS1"><p>Wrapping them in a function (or macro) will allow us to do a little magic trick.</p></div></div>
<h2 class="anchor anchorWithStickyNavbar_LWe7" id="implementation">Implementation<a href="#implementation" class="hash-link" aria-label="Direct link to Implementation" title="Direct link to Implementation">​</a></h2>
<p>We&#x27;re going to utilize the exceptions, so we&#x27;ll need to include the <code>exception</code>
header and we will start with a simple <code>_todo</code> exception class.</p>
<div class="language-cpp codeBlockContainer_Ckt0 theme-code-block" style="--prism-background-color:hsl(230, 1%, 98%);--prism-color:hsl(230, 8%, 24%)"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-cpp codeBlock_bY9V thin-scrollbar" style="background-color:hsl(230, 1%, 98%);color:hsl(230, 8%, 24%)"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token macro property directive-hash" style="color:hsl(5, 74%, 59%)">#</span><span class="token macro property directive keyword" style="color:hsl(301, 63%, 40%)">include</span><span class="token macro property" style="color:hsl(5, 74%, 59%)"> </span><span class="token macro property string" style="color:hsl(119, 34%, 47%)">&lt;exception&gt;</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token macro property directive-hash" style="color:hsl(5, 74%, 59%)">#</span><span class="token macro property directive keyword" style="color:hsl(301, 63%, 40%)">include</span><span class="token macro property" style="color:hsl(5, 74%, 59%)"> </span><span class="token macro property string" style="color:hsl(119, 34%, 47%)">&lt;string&gt;</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain" style="display:inline-block"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token keyword" style="color:hsl(301, 63%, 40%)">class</span><span class="token plain"> </span><span class="token class-name" style="color:hsl(35, 99%, 36%)">_todo</span><span class="token plain"> </span><span class="token operator" style="color:hsl(221, 87%, 60%)">:</span><span class="token plain"> </span><span class="token base-clause keyword" style="color:hsl(301, 63%, 40%)">public</span><span class="token base-clause"> std</span><span class="token base-clause double-colon punctuation" style="color:hsl(119, 34%, 47%)">::</span><span class="token base-clause class-name" style="color:hsl(35, 99%, 36%)">exception</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">{</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"> std</span><span class="token double-colon punctuation" style="color:hsl(119, 34%, 47%)">::</span><span class="token plain">string cause</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain" style="display:inline-block"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">public</span><span class="token operator" style="color:hsl(221, 87%, 60%)">:</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"> </span><span class="token function" style="color:hsl(221, 87%, 60%)">_todo</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"> </span><span class="token operator" style="color:hsl(221, 87%, 60%)">:</span><span class="token plain"> </span><span class="token function" style="color:hsl(221, 87%, 60%)">cause</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token string" style="color:hsl(119, 34%, 47%)">&quot;not yet implemented&quot;</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">{</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">}</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"> </span><span class="token function" style="color:hsl(221, 87%, 60%)">_todo</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token plain">std</span><span class="token double-colon punctuation" style="color:hsl(119, 34%, 47%)">::</span><span class="token plain">string</span><span class="token operator" style="color:hsl(221, 87%, 60%)">&amp;&amp;</span><span class="token plain"> excuse</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"> </span><span class="token operator" style="color:hsl(221, 87%, 60%)">:</span><span class="token plain"> </span><span class="token function" style="color:hsl(221, 87%, 60%)">cause</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token string" style="color:hsl(119, 34%, 47%)">&quot;not yet implemented: &quot;</span><span class="token plain"> </span><span class="token operator" style="color:hsl(221, 87%, 60%)">+</span><span class="token plain"> excuse</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">{</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">}</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">virtual</span><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">const</span><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">char</span><span class="token operator" style="color:hsl(221, 87%, 60%)">*</span><span class="token plain"> </span><span class="token function" style="color:hsl(221, 87%, 60%)">what</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">const</span><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">throw</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">{</span><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">return</span><span class="token plain"> cause</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token function" style="color:hsl(221, 87%, 60%)">c_str</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">}</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">}</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><br></span></code></pre><div class="buttonGroup__atx"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><span class="copyButtonIcons_eSgA" aria-hidden="true"><svg viewBox="0 0 24 24" class="copyButtonIcon_y97N"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_LjdS"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></span></button></div></div></div>
<p>In this case we have 2 constructors:</p>
<ol>
<li>default constructor without any parameters that will return just
<code>not yet implemented</code></li>
<li>and one parametrized with an “excuse” that will return string like:
<code>not yet implemented: ‹excuse›</code></li>
</ol>
<p>If we were to use it now, we would need to do something like:</p>
<div class="language-cpp codeBlockContainer_Ckt0 theme-code-block" style="--prism-background-color:hsl(230, 1%, 98%);--prism-color:hsl(230, 8%, 24%)"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-cpp codeBlock_bY9V thin-scrollbar" style="background-color:hsl(230, 1%, 98%);color:hsl(230, 8%, 24%)"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token macro property directive-hash" style="color:hsl(5, 74%, 59%)">#</span><span class="token macro property directive keyword" style="color:hsl(301, 63%, 40%)">include</span><span class="token macro property" style="color:hsl(5, 74%, 59%)"> </span><span class="token macro property string" style="color:hsl(119, 34%, 47%)">&quot;placeholders.hpp&quot;</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain" style="display:inline-block"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token keyword" style="color:hsl(301, 63%, 40%)">int</span><span class="token plain"> </span><span class="token function" style="color:hsl(221, 87%, 60%)">main</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">{</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">throw</span><span class="token plain"> </span><span class="token function" style="color:hsl(221, 87%, 60%)">_todo</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">return</span><span class="token plain"> </span><span class="token number" style="color:hsl(35, 99%, 36%)">0</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">}</span><br></span></code></pre><div class="buttonGroup__atx"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><span class="copyButtonIcons_eSgA" aria-hidden="true"><svg viewBox="0 0 24 24" class="copyButtonIcon_y97N"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_LjdS"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></span></button></div></div></div>
<h2 class="anchor anchorWithStickyNavbar_LWe7" id="wrapping-in-a-function">Wrapping in a function<a href="#wrapping-in-a-function" class="hash-link" aria-label="Direct link to Wrapping in a function" title="Direct link to Wrapping in a function">​</a></h2>
<p>I am a lazy person, so we will wrap the exception in a function that will throw
it:</p>
<div class="language-cpp codeBlockContainer_Ckt0 theme-code-block" style="--prism-background-color:hsl(230, 1%, 98%);--prism-color:hsl(230, 8%, 24%)"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-cpp codeBlock_bY9V thin-scrollbar" style="background-color:hsl(230, 1%, 98%);color:hsl(230, 8%, 24%)"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token keyword" style="color:hsl(301, 63%, 40%)">void</span><span class="token plain"> </span><span class="token function" style="color:hsl(221, 87%, 60%)">todo</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">{</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">throw</span><span class="token plain"> </span><span class="token function" style="color:hsl(221, 87%, 60%)">_todo</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">}</span><br></span></code></pre><div class="buttonGroup__atx"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><span class="copyButtonIcons_eSgA" aria-hidden="true"><svg viewBox="0 0 24 24" class="copyButtonIcon_y97N"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_LjdS"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></span></button></div></div></div>
<p>This can be used like:</p>
<div class="language-cpp codeBlockContainer_Ckt0 theme-code-block" style="--prism-background-color:hsl(230, 1%, 98%);--prism-color:hsl(230, 8%, 24%)"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-cpp codeBlock_bY9V thin-scrollbar" style="background-color:hsl(230, 1%, 98%);color:hsl(230, 8%, 24%)"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token macro property directive-hash" style="color:hsl(5, 74%, 59%)">#</span><span class="token macro property directive keyword" style="color:hsl(301, 63%, 40%)">include</span><span class="token macro property" style="color:hsl(5, 74%, 59%)"> </span><span class="token macro property string" style="color:hsl(119, 34%, 47%)">&quot;placeholders.hpp&quot;</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain" style="display:inline-block"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token keyword" style="color:hsl(301, 63%, 40%)">int</span><span class="token plain"> </span><span class="token function" style="color:hsl(221, 87%, 60%)">main</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">{</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"> </span><span class="token function" style="color:hsl(221, 87%, 60%)">todo</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">return</span><span class="token plain"> </span><span class="token number" style="color:hsl(35, 99%, 36%)">0</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">}</span><br></span></code></pre><div class="buttonGroup__atx"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><span class="copyButtonIcons_eSgA" aria-hidden="true"><svg viewBox="0 0 24 24" class="copyButtonIcon_y97N"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_LjdS"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></span></button></div></div></div>
<h2 class="anchor anchorWithStickyNavbar_LWe7" id="magic-trick">Magic trick<a href="#magic-trick" class="hash-link" aria-label="Direct link to Magic trick" title="Direct link to Magic trick">​</a></h2>
<p>At the beginning I&#x27;ve mentioned that by wrapping the exceptions in a helper
functions that will throw them, we can do a nice magic trick <!-- -->😄<!-- --> This trick
will consist of formatted string and for that we will use
<a href="https://en.cppreference.com/w/cpp/utility/format/format" target="_blank" rel="noopener noreferrer"><code>std::format</code></a> that is
available since C++20.</p>
<p>We just need to add one more overload for our <code>todo()</code>:</p>
<div class="language-cpp codeBlockContainer_Ckt0 theme-code-block" style="--prism-background-color:hsl(230, 1%, 98%);--prism-color:hsl(230, 8%, 24%)"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-cpp codeBlock_bY9V thin-scrollbar" style="background-color:hsl(230, 1%, 98%);color:hsl(230, 8%, 24%)"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token macro property directive-hash" style="color:hsl(5, 74%, 59%)">#</span><span class="token macro property directive keyword" style="color:hsl(301, 63%, 40%)">include</span><span class="token macro property" style="color:hsl(5, 74%, 59%)"> </span><span class="token macro property string" style="color:hsl(119, 34%, 47%)">&lt;format&gt;</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain" style="display:inline-block"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token keyword" style="color:hsl(301, 63%, 40%)">template</span><span class="token operator" style="color:hsl(221, 87%, 60%)">&lt;</span><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">class</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token plain"> Args </span><span class="token operator" style="color:hsl(221, 87%, 60%)">&gt;</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token keyword" style="color:hsl(301, 63%, 40%)">void</span><span class="token plain"> </span><span class="token function" style="color:hsl(221, 87%, 60%)">todo</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token plain">std</span><span class="token double-colon punctuation" style="color:hsl(119, 34%, 47%)">::</span><span class="token plain">format_string</span><span class="token operator" style="color:hsl(221, 87%, 60%)">&lt;</span><span class="token plain">Args</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token operator" style="color:hsl(221, 87%, 60%)">&gt;</span><span class="token plain"> fmt</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">,</span><span class="token plain"> Args</span><span class="token operator" style="color:hsl(221, 87%, 60%)">&amp;&amp;</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token plain"> args</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">{</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">throw</span><span class="token plain"> </span><span class="token function" style="color:hsl(221, 87%, 60%)">_todo</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token plain">std</span><span class="token double-colon punctuation" style="color:hsl(119, 34%, 47%)">::</span><span class="token function" style="color:hsl(221, 87%, 60%)">format</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token plain">fmt</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">,</span><span class="token plain"> args</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">}</span><br></span></code></pre><div class="buttonGroup__atx"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><span class="copyButtonIcons_eSgA" aria-hidden="true"><svg viewBox="0 0 24 24" class="copyButtonIcon_y97N"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_LjdS"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></span></button></div></div></div>
<h2 class="anchor anchorWithStickyNavbar_LWe7" id="finishing-off-with-2-more-exceptions">Finishing off with 2 more exceptions<a href="#finishing-off-with-2-more-exceptions" class="hash-link" aria-label="Direct link to Finishing off with 2 more exceptions" title="Direct link to Finishing off with 2 more exceptions">​</a></h2>
<p>Now we can repeat the same process for the other two exceptions I&#x27;ve mentioned</p>
<ul>
<li><code>unimplemented</code>, and</li>
<li><code>unreachable</code>.</li>
</ul>
<p>In the end we should end up with something like this:</p>
<div class="language-cpp codeBlockContainer_Ckt0 theme-code-block" style="--prism-background-color:hsl(230, 1%, 98%);--prism-color:hsl(230, 8%, 24%)"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-cpp codeBlock_bY9V thin-scrollbar" style="background-color:hsl(230, 1%, 98%);color:hsl(230, 8%, 24%)"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token macro property directive-hash" style="color:hsl(5, 74%, 59%)">#</span><span class="token macro property directive keyword" style="color:hsl(301, 63%, 40%)">include</span><span class="token macro property" style="color:hsl(5, 74%, 59%)"> </span><span class="token macro property string" style="color:hsl(119, 34%, 47%)">&lt;exception&gt;</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token macro property directive-hash" style="color:hsl(5, 74%, 59%)">#</span><span class="token macro property directive keyword" style="color:hsl(301, 63%, 40%)">include</span><span class="token macro property" style="color:hsl(5, 74%, 59%)"> </span><span class="token macro property string" style="color:hsl(119, 34%, 47%)">&lt;format&gt;</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token macro property directive-hash" style="color:hsl(5, 74%, 59%)">#</span><span class="token macro property directive keyword" style="color:hsl(301, 63%, 40%)">include</span><span class="token macro property" style="color:hsl(5, 74%, 59%)"> </span><span class="token macro property string" style="color:hsl(119, 34%, 47%)">&lt;string&gt;</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain" style="display:inline-block"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token keyword" style="color:hsl(301, 63%, 40%)">class</span><span class="token plain"> </span><span class="token class-name" style="color:hsl(35, 99%, 36%)">_todo</span><span class="token plain"> </span><span class="token operator" style="color:hsl(221, 87%, 60%)">:</span><span class="token plain"> </span><span class="token base-clause keyword" style="color:hsl(301, 63%, 40%)">public</span><span class="token base-clause"> std</span><span class="token base-clause double-colon punctuation" style="color:hsl(119, 34%, 47%)">::</span><span class="token base-clause class-name" style="color:hsl(35, 99%, 36%)">exception</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">{</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"> std</span><span class="token double-colon punctuation" style="color:hsl(119, 34%, 47%)">::</span><span class="token plain">string cause</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain" style="display:inline-block"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">public</span><span class="token operator" style="color:hsl(221, 87%, 60%)">:</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"> </span><span class="token function" style="color:hsl(221, 87%, 60%)">_todo</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"> </span><span class="token operator" style="color:hsl(221, 87%, 60%)">:</span><span class="token plain"> </span><span class="token function" style="color:hsl(221, 87%, 60%)">cause</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token string" style="color:hsl(119, 34%, 47%)">&quot;not yet implemented&quot;</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">{</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">}</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"> </span><span class="token function" style="color:hsl(221, 87%, 60%)">_todo</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token plain">std</span><span class="token double-colon punctuation" style="color:hsl(119, 34%, 47%)">::</span><span class="token plain">string</span><span class="token operator" style="color:hsl(221, 87%, 60%)">&amp;&amp;</span><span class="token plain"> excuse</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"> </span><span class="token operator" style="color:hsl(221, 87%, 60%)">:</span><span class="token plain"> </span><span class="token function" style="color:hsl(221, 87%, 60%)">cause</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token string" style="color:hsl(119, 34%, 47%)">&quot;not yet implemented: &quot;</span><span class="token plain"> </span><span class="token operator" style="color:hsl(221, 87%, 60%)">+</span><span class="token plain"> excuse</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">{</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">}</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">virtual</span><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">const</span><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">char</span><span class="token operator" style="color:hsl(221, 87%, 60%)">*</span><span class="token plain"> </span><span class="token function" style="color:hsl(221, 87%, 60%)">what</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">const</span><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">throw</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">{</span><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">return</span><span class="token plain"> cause</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token function" style="color:hsl(221, 87%, 60%)">c_str</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">}</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">}</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain" style="display:inline-block"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token keyword" style="color:hsl(301, 63%, 40%)">void</span><span class="token plain"> </span><span class="token function" style="color:hsl(221, 87%, 60%)">todo</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">{</span><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">throw</span><span class="token plain"> </span><span class="token function" style="color:hsl(221, 87%, 60%)">_todo</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">}</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain" style="display:inline-block"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token keyword" style="color:hsl(301, 63%, 40%)">template</span><span class="token plain"> </span><span class="token operator" style="color:hsl(221, 87%, 60%)">&lt;</span><span class="token keyword" style="color:hsl(301, 63%, 40%)">class</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token plain"> Args</span><span class="token operator" style="color:hsl(221, 87%, 60%)">&gt;</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token keyword" style="color:hsl(301, 63%, 40%)">void</span><span class="token plain"> </span><span class="token function" style="color:hsl(221, 87%, 60%)">todo</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token plain">std</span><span class="token double-colon punctuation" style="color:hsl(119, 34%, 47%)">::</span><span class="token plain">format_string</span><span class="token operator" style="color:hsl(221, 87%, 60%)">&lt;</span><span class="token plain">Args</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token operator" style="color:hsl(221, 87%, 60%)">&gt;</span><span class="token plain"> fmt</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">,</span><span class="token plain"> Args</span><span class="token operator" style="color:hsl(221, 87%, 60%)">&amp;&amp;</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token plain"> args</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">{</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">throw</span><span class="token plain"> </span><span class="token function" style="color:hsl(221, 87%, 60%)">_todo</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token plain">std</span><span class="token double-colon punctuation" style="color:hsl(119, 34%, 47%)">::</span><span class="token function" style="color:hsl(221, 87%, 60%)">format</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token plain">fmt</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">,</span><span class="token plain"> args</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">}</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain" style="display:inline-block"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token keyword" style="color:hsl(301, 63%, 40%)">class</span><span class="token plain"> </span><span class="token class-name" style="color:hsl(35, 99%, 36%)">_unimplemented</span><span class="token plain"> </span><span class="token operator" style="color:hsl(221, 87%, 60%)">:</span><span class="token plain"> </span><span class="token base-clause keyword" style="color:hsl(301, 63%, 40%)">public</span><span class="token base-clause"> std</span><span class="token base-clause double-colon punctuation" style="color:hsl(119, 34%, 47%)">::</span><span class="token base-clause class-name" style="color:hsl(35, 99%, 36%)">exception</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">{</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"> std</span><span class="token double-colon punctuation" style="color:hsl(119, 34%, 47%)">::</span><span class="token plain">string cause</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain" style="display:inline-block"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">public</span><span class="token operator" style="color:hsl(221, 87%, 60%)">:</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"> </span><span class="token function" style="color:hsl(221, 87%, 60%)">_unimplemented</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"> </span><span class="token operator" style="color:hsl(221, 87%, 60%)">:</span><span class="token plain"> </span><span class="token function" style="color:hsl(221, 87%, 60%)">cause</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token string" style="color:hsl(119, 34%, 47%)">&quot;not implemented&quot;</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">{</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">}</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"> </span><span class="token function" style="color:hsl(221, 87%, 60%)">_unimplemented</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token plain">std</span><span class="token double-colon punctuation" style="color:hsl(119, 34%, 47%)">::</span><span class="token plain">string</span><span class="token operator" style="color:hsl(221, 87%, 60%)">&amp;&amp;</span><span class="token plain"> excuse</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"> </span><span class="token operator" style="color:hsl(221, 87%, 60%)">:</span><span class="token plain"> </span><span class="token function" style="color:hsl(221, 87%, 60%)">cause</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token string" style="color:hsl(119, 34%, 47%)">&quot;not implemented: &quot;</span><span class="token plain"> </span><span class="token operator" style="color:hsl(221, 87%, 60%)">+</span><span class="token plain"> excuse</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">{</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">}</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">virtual</span><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">const</span><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">char</span><span class="token operator" style="color:hsl(221, 87%, 60%)">*</span><span class="token plain"> </span><span class="token function" style="color:hsl(221, 87%, 60%)">what</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">const</span><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">throw</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">{</span><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">return</span><span class="token plain"> cause</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token function" style="color:hsl(221, 87%, 60%)">c_str</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">}</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">}</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain" style="display:inline-block"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token keyword" style="color:hsl(301, 63%, 40%)">void</span><span class="token plain"> </span><span class="token function" style="color:hsl(221, 87%, 60%)">unimplemented</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">{</span><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">throw</span><span class="token plain"> </span><span class="token function" style="color:hsl(221, 87%, 60%)">_unimplemented</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">}</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain" style="display:inline-block"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token keyword" style="color:hsl(301, 63%, 40%)">template</span><span class="token plain"> </span><span class="token operator" style="color:hsl(221, 87%, 60%)">&lt;</span><span class="token keyword" style="color:hsl(301, 63%, 40%)">class</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token plain"> Args</span><span class="token operator" style="color:hsl(221, 87%, 60%)">&gt;</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token keyword" style="color:hsl(301, 63%, 40%)">void</span><span class="token plain"> </span><span class="token function" style="color:hsl(221, 87%, 60%)">unimplemented</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token plain">std</span><span class="token double-colon punctuation" style="color:hsl(119, 34%, 47%)">::</span><span class="token plain">format_string</span><span class="token operator" style="color:hsl(221, 87%, 60%)">&lt;</span><span class="token plain">Args</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token operator" style="color:hsl(221, 87%, 60%)">&gt;</span><span class="token plain"> fmt</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">,</span><span class="token plain"> Args</span><span class="token operator" style="color:hsl(221, 87%, 60%)">&amp;&amp;</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token plain"> args</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">{</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">throw</span><span class="token plain"> </span><span class="token function" style="color:hsl(221, 87%, 60%)">_unimplemented</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token plain">std</span><span class="token double-colon punctuation" style="color:hsl(119, 34%, 47%)">::</span><span class="token function" style="color:hsl(221, 87%, 60%)">format</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token plain">fmt</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">,</span><span class="token plain"> args</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">}</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain" style="display:inline-block"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token keyword" style="color:hsl(301, 63%, 40%)">class</span><span class="token plain"> </span><span class="token class-name" style="color:hsl(35, 99%, 36%)">_unreachable</span><span class="token plain"> </span><span class="token operator" style="color:hsl(221, 87%, 60%)">:</span><span class="token plain"> </span><span class="token base-clause keyword" style="color:hsl(301, 63%, 40%)">public</span><span class="token base-clause"> std</span><span class="token base-clause double-colon punctuation" style="color:hsl(119, 34%, 47%)">::</span><span class="token base-clause class-name" style="color:hsl(35, 99%, 36%)">exception</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">{</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"> std</span><span class="token double-colon punctuation" style="color:hsl(119, 34%, 47%)">::</span><span class="token plain">string cause</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain" style="display:inline-block"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">public</span><span class="token operator" style="color:hsl(221, 87%, 60%)">:</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"> </span><span class="token function" style="color:hsl(221, 87%, 60%)">_unreachable</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"> </span><span class="token operator" style="color:hsl(221, 87%, 60%)">:</span><span class="token plain"> </span><span class="token function" style="color:hsl(221, 87%, 60%)">cause</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token string" style="color:hsl(119, 34%, 47%)">&quot;entered unreachable code&quot;</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">{</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">}</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"> </span><span class="token function" style="color:hsl(221, 87%, 60%)">_unreachable</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token plain">std</span><span class="token double-colon punctuation" style="color:hsl(119, 34%, 47%)">::</span><span class="token plain">string</span><span class="token operator" style="color:hsl(221, 87%, 60%)">&amp;&amp;</span><span class="token plain"> excuse</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"> </span><span class="token operator" style="color:hsl(221, 87%, 60%)">:</span><span class="token plain"> </span><span class="token function" style="color:hsl(221, 87%, 60%)">cause</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token string" style="color:hsl(119, 34%, 47%)">&quot;entered unreachable code: &quot;</span><span class="token plain"> </span><span class="token operator" style="color:hsl(221, 87%, 60%)">+</span><span class="token plain"> excuse</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">{</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">}</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">virtual</span><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">const</span><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">char</span><span class="token operator" style="color:hsl(221, 87%, 60%)">*</span><span class="token plain"> </span><span class="token function" style="color:hsl(221, 87%, 60%)">what</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">const</span><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">throw</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">{</span><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">return</span><span class="token plain"> cause</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token function" style="color:hsl(221, 87%, 60%)">c_str</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">}</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">}</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain" style="display:inline-block"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token keyword" style="color:hsl(301, 63%, 40%)">void</span><span class="token plain"> </span><span class="token function" style="color:hsl(221, 87%, 60%)">unreachable</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">{</span><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">throw</span><span class="token plain"> </span><span class="token function" style="color:hsl(221, 87%, 60%)">_unreachable</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">}</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain" style="display:inline-block"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token keyword" style="color:hsl(301, 63%, 40%)">template</span><span class="token plain"> </span><span class="token operator" style="color:hsl(221, 87%, 60%)">&lt;</span><span class="token keyword" style="color:hsl(301, 63%, 40%)">class</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token plain"> Args</span><span class="token operator" style="color:hsl(221, 87%, 60%)">&gt;</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token keyword" style="color:hsl(301, 63%, 40%)">void</span><span class="token plain"> </span><span class="token function" style="color:hsl(221, 87%, 60%)">unreachable</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token plain">std</span><span class="token double-colon punctuation" style="color:hsl(119, 34%, 47%)">::</span><span class="token plain">format_string</span><span class="token operator" style="color:hsl(221, 87%, 60%)">&lt;</span><span class="token plain">Args</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token operator" style="color:hsl(221, 87%, 60%)">&gt;</span><span class="token plain"> fmt</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">,</span><span class="token plain"> Args</span><span class="token operator" style="color:hsl(221, 87%, 60%)">&amp;&amp;</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token plain"> args</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">{</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">throw</span><span class="token plain"> </span><span class="token function" style="color:hsl(221, 87%, 60%)">_unreachable</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token plain">std</span><span class="token double-colon punctuation" style="color:hsl(119, 34%, 47%)">::</span><span class="token function" style="color:hsl(221, 87%, 60%)">format</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token plain">fmt</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">,</span><span class="token plain"> args</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">}</span><br></span></code></pre><div class="buttonGroup__atx"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><span class="copyButtonIcons_eSgA" aria-hidden="true"><svg viewBox="0 0 24 24" class="copyButtonIcon_y97N"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_LjdS"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></span></button></div></div></div>
<div class="theme-admonition theme-admonition-info admonition_xJq3 alert alert--info"><div class="admonitionHeading_Gvgb"><span class="admonitionIcon_Rf37"><svg viewBox="0 0 14 16"><path fill-rule="evenodd" d="M7 2.3c3.14 0 5.7 2.56 5.7 5.7s-2.56 5.7-5.7 5.7A5.71 5.71 0 0 1 1.3 8c0-3.14 2.56-5.7 5.7-5.7zM7 1C3.14 1 0 4.14 0 8s3.14 7 7 7 7-3.14 7-7-3.14-7-7-7zm1 3H6v5h2V4zm0 6H6v2h2v-2z"></path></svg></span>info</div><div class="admonitionContent_BuS1"><p>Final source code: <a href="/files/cpp/exceptions-and-raii/placeholders/placeholders.hpp" target="_blank" rel="noopener noreferrer"><code>placeholders.hpp</code></a></p></div></div>
<h2 class="anchor anchorWithStickyNavbar_LWe7" id="post-mortem">Post-mortem<a href="#post-mortem" class="hash-link" aria-label="Direct link to Post-mortem" title="Direct link to Post-mortem">​</a></h2>
<p>One of the things, I&#x27;ve forgotten about, is the fact that static analysis of
your code has no way to know those helper functions we&#x27;ve created as shortcuts
don&#x27;t return and just throw the exception right away. Therefore we need to mark
them with <code>[[noreturn]]</code> to let the static analysis know that we <strong>never</strong>
return from such functions. For example:</p>
<div class="language-cpp codeBlockContainer_Ckt0 theme-code-block" style="--prism-background-color:hsl(230, 1%, 98%);--prism-color:hsl(230, 8%, 24%)"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-cpp codeBlock_bY9V thin-scrollbar" style="background-color:hsl(230, 1%, 98%);color:hsl(230, 8%, 24%)"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token punctuation" style="color:hsl(119, 34%, 47%)">[</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">[</span><span class="token plain">noreturn</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">]</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">]</span><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">void</span><span class="token plain"> </span><span class="token function" style="color:hsl(221, 87%, 60%)">unreachable</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">{</span><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">throw</span><span class="token plain"> </span><span class="token function" style="color:hsl(221, 87%, 60%)">_unreachable</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">}</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain" style="display:inline-block"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token keyword" style="color:hsl(301, 63%, 40%)">template</span><span class="token plain"> </span><span class="token operator" style="color:hsl(221, 87%, 60%)">&lt;</span><span class="token keyword" style="color:hsl(301, 63%, 40%)">class</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token plain"> Args</span><span class="token operator" style="color:hsl(221, 87%, 60%)">&gt;</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">[</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">[</span><span class="token plain">noreturn</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">]</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">]</span><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">void</span><span class="token plain"> </span><span class="token function" style="color:hsl(221, 87%, 60%)">unreachable</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token plain">std</span><span class="token double-colon punctuation" style="color:hsl(119, 34%, 47%)">::</span><span class="token plain">format_string</span><span class="token operator" style="color:hsl(221, 87%, 60%)">&lt;</span><span class="token plain">Args</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token operator" style="color:hsl(221, 87%, 60%)">&gt;</span><span class="token plain"> fmt</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">,</span><span class="token plain"> Args</span><span class="token operator" style="color:hsl(221, 87%, 60%)">&amp;&amp;</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token plain"> args</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">{</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">throw</span><span class="token plain"> </span><span class="token function" style="color:hsl(221, 87%, 60%)">_unreachable</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token plain">std</span><span class="token double-colon punctuation" style="color:hsl(119, 34%, 47%)">::</span><span class="token function" style="color:hsl(221, 87%, 60%)">format</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token plain">fmt</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">,</span><span class="token plain"> args</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">}</span><br></span></code></pre><div class="buttonGroup__atx"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><span class="copyButtonIcons_eSgA" aria-hidden="true"><svg viewBox="0 0 24 24" class="copyButtonIcon_y97N"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_LjdS"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></span></button></div></div></div></div><footer class="theme-doc-footer docusaurus-mt-lg"><div class="theme-doc-footer-edit-meta-row row"><div class="col"><a href="https://github.com/mfocko/blog/tree/main/cpp/07-exceptions-and-raii/2023-11-24-placeholders.md" target="_blank" rel="noopener noreferrer" 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