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- <div class="titlepage"><div><div><h1 style="clear: both">Phase 2 construction</h1></div></div></div>
- <p>Its phase 2 constructor:</p>
- <div class="code-snippet"><div class="highlight"><pre class="chroma"><code class="language-c++" data-lang="c++"><span class="c1">// Phase 2 static member constructor function, which cannot throw
- </span><span class="c1"></span><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">file_handle</span><span class="o">::</span><span class="n">file</span><span class="p">(</span><span class="n">file_handle</span><span class="o">::</span><span class="n">path_type</span> <span class="n">path</span><span class="p">,</span> <span class="n">file_handle</span><span class="o">::</span><span class="n">mode</span> <span class="n">mode</span><span class="p">)</span> <span class="k">noexcept</span>
- <span class="p">{</span>
- <span class="c1">// Perform phase 1 of object construction
- </span><span class="c1"></span> <span class="n">file_handle</span> <span class="n">ret</span><span class="p">;</span>
- <span class="c1">// Perform phase 2 of object construction
- </span><span class="c1"></span> <span class="kt">int</span> <span class="n">flags</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span>
- <span class="k">switch</span><span class="p">(</span><span class="n">mode</span><span class="p">)</span>
- <span class="p">{</span>
- <span class="k">case</span> <span class="n">mode</span><span class="o">::</span><span class="nl">attr_read</span><span class="p">:</span>
- <span class="k">case</span> <span class="n">mode</span><span class="o">::</span><span class="nl">read</span><span class="p">:</span>
- <span class="n">flags</span> <span class="o">=</span> <span class="n">O_RDONLY</span><span class="p">;</span>
- <span class="k">break</span><span class="p">;</span>
- <span class="k">case</span> <span class="n">mode</span><span class="o">::</span><span class="nl">attr_write</span><span class="p">:</span>
- <span class="k">case</span> <span class="n">mode</span><span class="o">::</span><span class="nl">write</span><span class="p">:</span>
- <span class="n">flags</span> <span class="o">=</span> <span class="n">O_RDWR</span><span class="p">;</span>
- <span class="k">break</span><span class="p">;</span>
- <span class="k">case</span> <span class="n">mode</span><span class="o">::</span><span class="nl">append</span><span class="p">:</span>
- <span class="n">flags</span> <span class="o">=</span> <span class="n">O_APPEND</span><span class="p">;</span>
- <span class="k">break</span><span class="p">;</span>
- <span class="k">default</span><span class="o">:</span>
- <span class="k">return</span> <span class="n">std</span><span class="o">::</span><span class="n">errc</span><span class="o">::</span><span class="n">invalid_argument</span><span class="p">;</span>
- <span class="p">}</span>
- <span class="n">ret</span><span class="p">.</span><span class="n">_fd</span> <span class="o">=</span> <span class="o">::</span><span class="n">open</span><span class="p">(</span><span class="n">path</span><span class="p">.</span><span class="n">u8string</span><span class="p">().</span><span class="n">c_str</span><span class="p">(),</span> <span class="n">flags</span><span class="p">);</span>
- <span class="k">if</span><span class="p">(</span><span class="o">-</span><span class="mi">1</span> <span class="o">==</span> <span class="n">ret</span><span class="p">.</span><span class="n">_fd</span><span class="p">)</span>
- <span class="p">{</span>
- <span class="c1">// Note that if we bail out here, ~file_handle() will correctly not call ::close()
- </span><span class="c1"></span> <span class="k">return</span> <span class="p">{</span><span class="n">errno</span><span class="p">,</span> <span class="n">std</span><span class="o">::</span><span class="n">system_category</span><span class="p">()};</span>
- <span class="p">}</span>
- <span class="k">if</span><span class="p">(</span><span class="o">-</span><span class="mi">1</span> <span class="o">==</span> <span class="o">::</span><span class="n">fstat</span><span class="p">(</span><span class="n">ret</span><span class="p">.</span><span class="n">_fd</span><span class="p">,</span> <span class="o">&</span><span class="n">ret</span><span class="p">.</span><span class="n">_stat</span><span class="p">))</span>
- <span class="p">{</span>
- <span class="c1">// Note that if we bail out here, ~file_handle() will correctly call ::close()
- </span><span class="c1"></span> <span class="k">return</span> <span class="p">{</span><span class="n">errno</span><span class="p">,</span> <span class="n">std</span><span class="o">::</span><span class="n">system_category</span><span class="p">()};</span>
- <span class="p">}</span>
- <span class="c1">// Returning ret directly is an area full of compiler specific behaviour quirks,
- </span><span class="c1"></span> <span class="c1">// so be explicit by wrapping into an initialiser list with embedded move.
- </span><span class="c1"></span> <span class="k">return</span> <span class="p">{</span><span class="n">std</span><span class="o">::</span><span class="n">move</span><span class="p">(</span><span class="n">ret</span><span class="p">)};</span>
- <span class="p">}</span>
- </code></pre></div><a href="https://github.com/boostorg/outcome/tree/master/doc/src/snippets/constructors.cpp#L109" class="code-snippet-url" target="_blank">View this code on Github</a></div>
- <p>The static member function implementing phase 2 firstly calls phase 1
- which puts the object into a legally destructible state. We then
- proceed to implement phase 2 of construction, filling in the various
- parts as we go, reporting via <code>result</code> any failures.</p>
- <div class="notices note" style="background: url('../../../images/note.png') top left no-repeat padding-box padding-box;">
- <div class="notices heading">note</div>
- <div class="notices message"><p>Remember that <code>operator new</code> has a non-throwing form, <code>new(std::nothrow)</code>.</p>
- </div>
- </div>
- <p>For the final return, in theory we could just <code>return ret</code> and
- depending on the C++ version currently in force, it might work
- via move, or via copy, or it might refuse to compile. You can
- of course type lots of boilerplate to be explicit, but this use
- via initialiser list is a reasonable balance of explicitness
- versus brevity, and it should generate minimum overhead code
- irrespective of compiler, C++ version, or any other factor.</p>
- </div><p><small>Last revised: February 08, 2019 at 22:18:08 UTC</small></p>
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