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<div class="titlepage"><div><div><h1 style="clear: both">TRY operations</h1></div></div></div>
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<p>In the implementation of function <code>print_half</code> we have seen the usage of the macro <a href="../../../reference/macros/tryv.html" class="api-reference"><code>BOOST_OUTCOME_TRYV(expr)/BOOST_OUTCOME_TRY(expr)</code></a>
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:</p>
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<div class="highlight"><pre class="chroma"><code class="language-c++" data-lang="c++"><span class="n">BOOST_OUTCOME_TRY</span> <span class="p">(</span><span class="n">i</span><span class="p">,</span> <span class="n">BigInt</span><span class="o">::</span><span class="n">fromString</span><span class="p">(</span><span class="n">text</span><span class="p">));</span>
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</code></pre></div>
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<p>The <code>BOOST_OUTCOME_TRY</code> macro uses C macro overloading to select between two implementations based on the number of
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input parameters. If there is exactly one input parameter i.e. without the <code>i</code>, the control statement is
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roughly equivalent to:</p>
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<div class="highlight"><pre class="chroma"><code class="language-c++" data-lang="c++"><span class="k">auto</span><span class="o">&&</span> <span class="n">__result</span> <span class="o">=</span> <span class="n">BigInt</span><span class="o">::</span><span class="n">fromString</span><span class="p">(</span><span class="n">text</span><span class="p">);</span>
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<span class="k">if</span> <span class="p">(</span><span class="o">!</span><span class="n">__result</span><span class="p">)</span>
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<span class="k">return</span> <span class="n">__result</span><span class="p">.</span><span class="n">as_failure</span><span class="p">();</span>
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</code></pre></div>
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<p>Where <code>__result</code> is a compile time generated unique name. This single argument form is equivalent to
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<code>BOOST_OUTCOME_TRYV(expr)</code>, incidentally.</p>
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<p>If there are between two and eight parameters, this control statement is roughly equivalent to:</p>
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<div class="highlight"><pre class="chroma"><code class="language-c++" data-lang="c++"><span class="k">auto</span><span class="o">&&</span> <span class="n">__result</span> <span class="o">=</span> <span class="n">BigInt</span><span class="o">::</span><span class="n">fromString</span><span class="p">(</span><span class="n">text</span><span class="p">);</span>
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<span class="k">if</span> <span class="p">(</span><span class="o">!</span><span class="n">__result</span><span class="p">)</span>
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<span class="k">return</span> <span class="n">__result</span><span class="p">.</span><span class="n">as_failure</span><span class="p">();</span>
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<span class="k">auto</span><span class="o">&&</span> <span class="n">i</span> <span class="o">=</span> <span class="n">__result</span><span class="p">.</span><span class="n">value</span><span class="p">();</span>
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</code></pre></div>
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<p>So here <code>i</code> as the first C macro parameter is set to the value of any successful result.</p>
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<p>C macro overloads are provided for up to eight arguments. To prevent the
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confounding of the C preprocessor by commas in template specifications causing more than
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eight arguments appearing to the C preprocessor, you should consider wrapping the
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second argument in brackets.</p>
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<hr>
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<h3 id="compiler-specific-extension-boost-outcome-tryx">Compiler-specific extension: <code>BOOST_OUTCOME_TRYX</code></h3>
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<div class="notices note" style="background: url('../../../images/note.png') top left no-repeat padding-box padding-box;">
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<div class="notices heading">note</div>
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<div class="notices message"><p>This macro makes use of a proprietary extension in GCC and clang, and is not
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portable. The macro is not made available on unsupported compilers,
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so you can test for its presence using <code>#ifdef BOOST_OUTCOME_TRYX</code>.</p>
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</div>
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</div>
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<p>GCC and Clang provide an extension to C++ known as
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<a href="https://gcc.gnu.org/onlinedocs/gcc/Statement-Exprs.html" title="GCC docs on statement expressions">statement expressions</a>.
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These make it possible to use a more convenient macro: <code>BOOST_OUTCOME_TRYX</code>, which is an expression. With the above macro, the above declaration of variable <code>i</code> can be rewritten to:</p>
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<div class="highlight"><pre class="chroma"><code class="language-c++" data-lang="c++"><span class="kt">int</span> <span class="n">i</span> <span class="o">=</span> <span class="n">BOOST_OUTCOME_TRYX</span> <span class="p">(</span><span class="n">BigInt</span><span class="o">::</span><span class="n">fromString</span><span class="p">(</span><span class="n">text</span><span class="p">));</span>
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</code></pre></div>
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<p>This has an advantage that you can use it any place where you can put an expression, e.g., in “simple initialization”:</p>
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<div class="highlight"><pre class="chroma"><code class="language-c++" data-lang="c++"><span class="k">if</span> <span class="p">(</span><span class="kt">int</span> <span class="n">i</span> <span class="o">=</span> <span class="n">BOOST_OUTCOME_TRYX</span><span class="p">(</span><span class="n">BigInt</span><span class="o">::</span><span class="n">fromString</span><span class="p">(</span><span class="n">text</span><span class="p">)))</span>
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<span class="n">use_a_non_zero_int</span><span class="p">(</span><span class="n">i</span><span class="p">);</span>
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</code></pre></div>
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<p>or in as a subexpression of a bigger full expression:</p>
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<div class="highlight"><pre class="chroma"><code class="language-c++" data-lang="c++"><span class="kt">int</span> <span class="n">ans</span> <span class="o">=</span> <span class="n">BOOST_OUTCOME_TRYX</span><span class="p">(</span><span class="n">BigInt</span><span class="o">::</span><span class="n">fromString</span><span class="p">(</span><span class="s">"1"</span><span class="p">))</span> <span class="o">+</span> <span class="n">BOOST_OUTCOME_TRYX</span><span class="p">(</span><span class="n">BigInt</span><span class="o">::</span><span class="n">fromString</span><span class="p">(</span><span class="s">"2"</span><span class="p">));</span>
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</code></pre></div>
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</div><p><small>Last revised: February 08, 2019 at 22:18:08 UTC</small></p>
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