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<div class="titlepage"><div><div><h1 style="clear: both">TRY is greedy</h1></div></div></div>
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<p><a href="../../../reference/macros/tryv.html" class="api-reference"><code>BOOST_OUTCOME_TRYV(expr)/BOOST_OUTCOME_TRY(expr)</code></a>
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has ‘greedier’ implicit conversion semantics than
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<a href="../../../reference/types/basic_result.html" class="api-reference"><code>basic_result<T, E, NoValuePolicy></code></a>
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. For example, this code won’t compile:</p>
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<div class="highlight"><pre class="chroma"><code class="language-c++" data-lang="c++"><span class="n">outcome</span><span class="o">::</span><span class="n">result</span><span class="o"><</span><span class="kt">int</span><span class="p">,</span> <span class="n">std</span><span class="o">::</span><span class="n">error_code</span><span class="o">></span> <span class="n">test</span><span class="p">(</span><span class="n">outcome</span><span class="o">::</span><span class="n">result</span><span class="o"><</span><span class="kt">int</span><span class="p">,</span> <span class="n">std</span><span class="o">::</span><span class="n">errc</span><span class="o">></span> <span class="n">r</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">r</span><span class="p">;</span> <span class="c1">// you need to use explicit construction here
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</span><span class="c1"></span> <span class="c1">// i.e. return outcome::result<int>(r);
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</span><span class="c1"></span><span class="p">}</span>
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</code></pre></div>
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<p>This is chosen because there is a non-trivial conversion between <code>std::errc</code> and <code>std::error_code</code>,
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so even though that conversion is implicit for <code>std::error_code</code>, Outcome does not expose the
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implicitness here in order to keep the implicit constructor count low (implicit constructors
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add significantly to build times).</p>
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<p>The <code>TRY</code> operation is more greedy though:</p>
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<div class="highlight"><pre class="chroma"><code class="language-c++" data-lang="c++"><span class="n">outcome</span><span class="o">::</span><span class="n">result</span><span class="o"><</span><span class="kt">int</span><span class="p">,</span> <span class="n">std</span><span class="o">::</span><span class="n">error_code</span><span class="o">></span> <span class="n">test</span><span class="p">(</span><span class="n">outcome</span><span class="o">::</span><span class="n">result</span><span class="o"><</span><span class="kt">int</span><span class="p">,</span> <span class="n">std</span><span class="o">::</span><span class="n">errc</span><span class="o">></span> <span class="n">r</span><span class="p">)</span>
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<span class="p">{</span>
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<span class="n">BOOST_OUTCOME_TRY</span><span class="p">(</span><span class="n">r</span><span class="p">);</span> <span class="c1">// no explicit conversion needed
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</span><span class="c1"></span> <span class="k">return</span> <span class="n">r</span><span class="p">.</span><span class="n">value</span><span class="p">();</span>
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<span class="p">}</span>
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</code></pre></div>
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<p>This is because <code>result<int, std::error_code></code> will implicitly construct from anything which
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either <code>int</code> or <code>std::error_code</code> will implicitly construct from. However,
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<code>result<int, std::error_code></code> will not implicitly construct from <code>result<int, std::errc></code>.</p>
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<p>Thus bear this in mind during usage: <code>TRY</code> is greedier for implicit conversions than the Outcome
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types themselves.</p>
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</div><p><small>Last revised: February 03, 2020 at 12:05:24 UTC</small></p>
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