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<div class="titlepage"><div><div><h2 class="title" style="clear: both">TRY operations</h2></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/try" class="api-reference"><i class="fa fa-book" aria-hidden="true"></i> BOOST_OUTCOME_TRY</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. If there are between two and eight parameters,
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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. To prevent
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confounding of the C preprocessor, you should always wrap the function invocation in brackets. This is done
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throughout this tutorial.</p>
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<p>Additionally, in GCC and Clang which 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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it is possible to use an alternative 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 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>Macro <code>BOOST_OUTCOME_TRY</code> does two things:</p>
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<ol>
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<li>It extracts a <code>T</code> from <code>result<T></code> (or <code>outcome<T></code>).</li>
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<li>It declares a dependency between statements: If the “tried” statement fails, the next statement is not executed.</li>
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</ol>
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<p>When you are calling functions that return <code>result<void></code> the first of the two things does not make sense. You then have to use macro <code>BOOST_OUTCOME_TRYV</code> instead. Let’s use our function <code>print_half</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="n">outcome</span><span class="o">::</span><span class="n">result</span><span class="o"><</span><span class="kt">void</span><span class="o">></span> <span class="n">test</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">print_half</span><span class="p">(</span><span class="s">"2"</span><span class="p">)));</span>
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<span class="n">BOOST_OUTCOME_TRY</span> <span class="p">((</span><span class="n">print_half</span><span class="p">(</span><span class="s">"X"</span><span class="p">)));</span>
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<span class="n">BOOST_OUTCOME_TRY</span> <span class="p">((</span><span class="n">print_half</span><span class="p">(</span><span class="s">"4"</span><span class="p">)));</span> <span class="c1">// will not execute
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</span><span class="c1"></span> <span class="k">return</span> <span class="n">outcome</span><span class="o">::</span><span class="n">success</span><span class="p">();</span>
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<span class="p">}</span>
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</code></pre></div><a href="https://github.com/ned14/boost-outcome/tree/master/doc/src/snippets/using_result.cpp#L141" class="code-snippet-url" target="_blank">View this code on Github</a></div>
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<p>The first statement will succeed. The second statement will “fail”, i.e. it will return an <em>errored</em> result. This will cause an immediate return and the subsequent statements will be skipped.</p>
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</div><p><small>Last revised: December 05, 2018 at 14:18:52 UTC</small></p>
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