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- <div class="titlepage"><div><div><h1 style="clear: both">Tying it all together</h1></div></div></div>
-
- <p>Firstly let’s alias a more convenient form of <code>status_result</code>:</p>
- <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="p">,</span> <span class="k">class</span><span class="err"> </span><span class="nc">E</span> <span class="o">=</span> <span class="n">outcome_e</span><span class="o">::</span><span class="n">error</span><span class="o">></span>
- <span class="k">using</span> <span class="n">result</span> <span class="o">=</span> <span class="c1">//
- </span><span class="c1"></span><span class="n">outcome_e</span><span class="o">::</span><span class="n">status_result</span><span class="o"><</span><span class="n">T</span><span class="p">,</span> <span class="n">E</span><span class="p">,</span> <span class="n">outcome_e</span><span class="o">::</span><span class="n">policy</span><span class="o">::</span><span class="n">default_status_result_policy</span><span class="o"><</span><span class="n">T</span><span class="p">,</span> <span class="n">E</span><span class="o">>></span><span class="p">;</span>
- </code></pre></div><a href="https://github.com/boostorg/outcome/tree/master/doc/src/snippets/experimental_status_code.cpp#L196" class="code-snippet-url" target="_blank">View this code on Github</a></div>
- <p>(The defaulting of <code>default_result_policy</code> is superfluous, it’s already the default)</p>
- <p>What follows now is very standard Outcome code. Indeed, it would compile
- just fine under standard Outcome with only a few typedefs.</p>
- <div class="code-snippet"><div class="highlight"><pre class="chroma"><code class="language-c++" data-lang="c++"><span class="n">result</span><span class="o"><</span><span class="n">file_handle</span><span class="p">,</span> <span class="n">file_io_error</span><span class="o">></span> <span class="n">open_file</span><span class="p">(</span><span class="k">const</span> <span class="kt">char</span> <span class="o">*</span><span class="n">path</span><span class="p">)</span> <span class="c1">// models throws(file_io_error)
- </span><span class="c1"></span><span class="p">{</span>
- <span class="n">file_handle</span> <span class="n">ret</span><span class="p">(</span><span class="o">::</span><span class="n">fopen</span><span class="p">(</span><span class="n">path</span><span class="p">,</span> <span class="s">"r"</span><span class="p">));</span>
- <span class="k">if</span><span class="p">(</span><span class="n">ret</span><span class="p">)</span>
- <span class="k">return</span> <span class="n">ret</span><span class="p">;</span>
- <span class="k">return</span> <span class="nf">file_io_error</span><span class="p">({</span><span class="n">errno</span><span class="p">,</span> <span class="n">__LINE__</span><span class="p">,</span> <span class="n">__FILE__</span><span class="p">});</span>
- <span class="p">}</span>
- <span class="n">result</span><span class="o"><</span><span class="kt">void</span><span class="o">></span> <span class="n">open_resource</span><span class="p">()</span> <span class="c1">// models throws(std::error)
- </span><span class="c1"></span><span class="p">{</span>
- <span class="k">for</span><span class="p">(;;)</span>
- <span class="p">{</span>
- <span class="n">result</span><span class="o"><</span><span class="n">file_handle</span><span class="p">,</span> <span class="n">file_io_error</span><span class="o">></span> <span class="n">r</span> <span class="o">=</span> <span class="n">open_file</span><span class="p">(</span><span class="s">"some file"</span><span class="p">);</span>
- <span class="k">if</span><span class="p">(</span><span class="n">r</span><span class="p">)</span>
- <span class="k">break</span><span class="p">;</span>
- <span class="n">file_io_error</span> <span class="n">e</span> <span class="o">=</span> <span class="n">r</span><span class="p">.</span><span class="n">error</span><span class="p">();</span>
- <span class="k">if</span><span class="p">(</span><span class="n">e</span> <span class="o">!=</span> <span class="n">outcome_e</span><span class="o">::</span><span class="n">errc</span><span class="o">::</span><span class="n">resource_unavailable_try_again</span><span class="p">)</span>
- <span class="p">{</span>
- <span class="c1">// NOTE this implicitly converts from `file_io_error` to `error` via the
- </span><span class="c1"></span> <span class="c1">// `make_status_code()` free function customisation point defined above.
- </span><span class="c1"></span> <span class="k">return</span> <span class="n">e</span><span class="p">;</span>
- <span class="p">}</span>
- <span class="p">}</span>
- <span class="c1">// success continues here ...
- </span><span class="c1"></span> <span class="k">return</span> <span class="n">outcome_e</span><span class="o">::</span><span class="n">success</span><span class="p">();</span>
- <span class="p">}</span>
- <span class="kt">int</span> <span class="n">main</span><span class="p">(</span><span class="kt">void</span><span class="p">)</span>
- <span class="p">{</span>
- <span class="n">result</span><span class="o"><</span><span class="kt">void</span><span class="o">></span> <span class="n">r</span> <span class="o">=</span> <span class="n">open_resource</span><span class="p">();</span>
- <span class="k">if</span><span class="p">(</span><span class="n">r</span><span class="p">)</span>
- <span class="n">printf</span><span class="p">(</span><span class="s">"Success!</span><span class="se">\n</span><span class="s">"</span><span class="p">);</span>
- <span class="k">else</span>
- <span class="p">{</span>
- <span class="k">auto</span> <span class="n">e</span> <span class="o">=</span> <span class="n">std</span><span class="o">::</span><span class="n">move</span><span class="p">(</span><span class="n">r</span><span class="p">).</span><span class="n">error</span><span class="p">();</span>
- <span class="c1">// A quick demonstration that the indirection works as indicated
- </span><span class="c1"></span> <span class="n">printf</span><span class="p">(</span><span class="s">"Returned error has a code domain of '%s', a message of '%s'</span><span class="se">\n</span><span class="s">"</span><span class="p">,</span> <span class="n">e</span><span class="p">.</span><span class="n">domain</span><span class="p">().</span><span class="n">name</span><span class="p">().</span><span class="n">c_str</span><span class="p">(),</span> <span class="n">e</span><span class="p">.</span><span class="n">message</span><span class="p">().</span><span class="n">c_str</span><span class="p">());</span>
- <span class="n">printf</span><span class="p">(</span><span class="s">"</span><span class="se">\n</span><span class="s">And semantically comparing it to 'errc::no_such_file_or_directory' = %d</span><span class="se">\n</span><span class="s">"</span><span class="p">,</span> <span class="n">e</span> <span class="o">==</span> <span class="n">outcome_e</span><span class="o">::</span><span class="n">errc</span><span class="o">::</span><span class="n">no_such_file_or_directory</span><span class="p">);</span>
- <span class="p">}</span>
- <span class="p">}</span>
- </code></pre></div><a href="https://github.com/boostorg/outcome/tree/master/doc/src/snippets/experimental_status_code.cpp#L201" class="code-snippet-url" target="_blank">View this code on Github</a></div>
- <p>And running this program yields:</p>
- <pre><code>Returned error has a code domain of 'file i/o error domain', a message of 'No such file or directory (c:\users\ned\documents\boostish\outcome\doc\src\snippets\experimental_status_code.cpp:195)'
- And semantically comparing it to 'errc::no_such_file_or_directory' = 1
- </code></pre>
- <h3 id="conclusion">Conclusion</h3>
- <p>Once you get used to <code><system_error2></code> and the fact that any <code>result</code> with
- <code>E = error</code> is always move-only, using experimental Outcome is just like
- using normal Outcome. Except that codegen will be better, custom domains
- are safe to use in headers, semantic comparisons have guaranteed complexity
- bounds, and build times are much reduced.</p>
- <p>What’s not to like? :)</p>
- <p>Finally, if you have feedback on using experimental Outcome which you think
- would be of use to the standards committee when evaluating possible
- implementations of <a href="http://wg21.link/P0709">P0709 <em>Zero overhead exceptions: Throwing values</em></a>,
- please do get in touch! This <strong>especially</strong> includes successful experiences!!!</p>
- </div><p><small>Last revised: February 05, 2019 at 17:14:18 UTC</small></p>
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