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57 lines
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<title>Alternatives - Boost.Outcome documentation</title>
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<div class="titlepage"><div><div><h1 style="clear: both">Alternatives</h1></div></div></div>
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<p>No doubt many will dislike the two-stage invocation pattern i.e.</p>
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<div class="highlight"><pre class="chroma"><code class="language-c++" data-lang="c++"><span class="n">make</span><span class="o"><</span><span class="n">file_handle</span><span class="o">></span><span class="p">{</span><span class="s">"hello"</span><span class="p">}();</span>
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</code></pre></div>
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<p>So let us examine the most obvious alternative: a templated free function <code>make<T></code>.</p>
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<p>Due to the inability to partially specialise templated functions in C++, you
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need to use tagged overloading e.g.</p>
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<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">Args</span><span class="o">></span>
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<span class="kr">inline</span> <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="n">make</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">in_place_type_t</span><span class="o"><</span><span class="n">file_handle</span><span class="o">></span><span class="p">,</span> <span class="n">Args</span><span class="o">&&</span> <span class="p">...</span> <span class="n">args</span><span class="p">)</span>
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<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">forward</span><span class="o"><</span><span class="n">Args</span><span class="o">></span><span class="p">(</span><span class="n">args</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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<span class="c1">// Now you must always write this:
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</span><span class="c1"></span><span class="n">make</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">in_place_type</span><span class="o"><</span><span class="n">file_handle</span><span class="o">></span><span class="p">,</span> <span class="s">"hello"</span><span class="p">);</span>
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</code></pre></div>
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<p>Tagged overloading is fine for smaller projects, but for larger code bases:</p>
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<ol>
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<li>It takes longer to type <code>make(std::in_place_type<file_handle>, "hello")</code>,
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and is possibly less intuitive to write,
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than it does <code>make<file_handle>{"hello"}()</code>.</li>
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<li>Compiler error messages are enormously clearer if you encode the permitted
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overloads for construction into the <code>make<file_handle></code> type rather than
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letting a variadic free function fail to resolve an appropriate overload.</li>
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<li>Resolving variadic free function overloads is not constant time for the compiler,
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whereas resolving the type specialisation for <code>make<file_handle></code>
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is constant time. In other words, free functions are <em>expensive</em> on build
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times, whereas fully specialised types are not.</li>
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<li>It actually turns out to be quite useful when writing generic code
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to pass around object constructing factory objects all of which have
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no parameters for their call operator. It becomes, effectively, a
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<em>lazy construction</em> mechanism.</li>
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</ol>
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</div><p><small>Last revised: February 08, 2019 at 22:18:08 UTC</small></p>
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