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62 lines
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<div class="titlepage"><div><div><h1 style="clear: both">construct<T></h1></div></div></div>
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<p>First, we need a base definition for <code>make<T></code>:</p>
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<div class="code-snippet"><div class="highlight"><pre class="chroma"><code class="language-c++" data-lang="c++"><span class="k">template</span> <span class="o"><</span><span class="k">class</span><span class="err"> </span><span class="nc">T</span><span class="o">></span> <span class="k">struct</span> <span class="n">make</span>
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<span class="p">{</span>
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<span class="n">outcome</span><span class="o">::</span><span class="n">result</span><span class="o"><</span><span class="n">T</span><span class="o">></span> <span class="k">operator</span><span class="p">()()</span> <span class="k">const</span> <span class="k">noexcept</span>
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<span class="p">{</span> <span class="c1">//
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</span><span class="c1"></span> <span class="k">static_assert</span><span class="p">(</span><span class="o">!</span><span class="n">std</span><span class="o">::</span><span class="n">is_same</span><span class="o"><</span><span class="n">T</span><span class="p">,</span> <span class="n">T</span><span class="o">>::</span><span class="n">value</span><span class="p">,</span> <span class="c1">//
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</span><span class="c1"></span> <span class="s">"make<T>() was not specialised for the type T supplied"</span><span class="p">);</span>
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<span class="p">}</span>
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<span class="p">};</span>
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</code></pre></div><a href="https://github.com/boostorg/outcome/tree/master/doc/src/snippets/constructors.cpp#L152" class="code-snippet-url" target="_blank">View this code on Github</a></div>
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<p>This fails a static assert if the type is ever instantiated unspecialised.</p>
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<p>We then specialise for <code>make<file_handle></code>:</p>
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<div class="code-snippet"><div class="highlight"><pre class="chroma"><code class="language-c++" data-lang="c++"><span class="k">template</span> <span class="o"><></span> <span class="k">struct</span> <span class="n">make</span><span class="o"><</span><span class="n">file_handle</span><span class="o">></span>
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<span class="p">{</span>
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<span class="n">file_handle</span><span class="o">::</span><span class="n">path_type</span> <span class="n">_path</span><span class="p">;</span>
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<span class="n">file_handle</span><span class="o">::</span><span class="n">mode</span> <span class="n">_mode</span><span class="p">{</span><span class="n">file_handle</span><span class="o">::</span><span class="n">mode</span><span class="o">::</span><span class="n">read</span><span class="p">};</span>
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<span class="c1">// Any other args, default initialised if necessary, follow here ...
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</span><span class="c1"></span>
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<span class="n">outcome</span><span class="o">::</span><span class="n">result</span><span class="o"><</span><span class="n">file_handle</span><span class="o">></span> <span class="k">operator</span><span class="p">()()</span> <span class="k">const</span> <span class="k">noexcept</span> <span class="c1">//
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</span><span class="c1"></span> <span class="p">{</span>
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<span class="k">return</span> <span class="n">file_handle</span><span class="o">::</span><span class="n">file</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">move</span><span class="p">(</span><span class="n">_path</span><span class="p">));</span>
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<span class="p">}</span>
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<span class="p">};</span>
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</code></pre></div><a href="https://github.com/boostorg/outcome/tree/master/doc/src/snippets/constructors.cpp#L163" class="code-snippet-url" target="_blank">View this code on Github</a></div>
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<p>Because this is a struct, we can list initialise <code>make</code>, and use
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default member initialisers to implement default arguments. This can get
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you surprisingly far before you need to start writing custom constructors.</p>
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<p>But in more complex code, you will usually provide all the initialisation overloads that
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you would for the constructors of your main type. You then implement a single phase 2 constructing
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function which accepts <code>make<YOURTYPE></code> as input, and construct solely from
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that source.</p>
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</div><p><small>Last revised: February 08, 2019 at 22:18:08 UTC</small></p>
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