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<div class="titlepage"><div><div><h1 style="clear: both">Auto-throwing filesystem_error</h1></div></div></div>
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<p>Something not mentioned at all until now (and properly described in the next
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section, <a href="../../default-actions/">Default actions</a>) is that Outcome can be
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programmed take various actions when the user tries to observe <code>.value()</code>
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when there is no value, and so on for the other possible state observations.</p>
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<p>Seeing as we are replacing the throwing overload of <code>copy_file()</code> in the
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Filesystem TS with a <code>result</code> returning edition instead, it would make
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sense if an attempt to observe the value of an unsuccessful <code>fs_result</code>
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threw the exact same <code>filesystem_error</code> as the Filesystem TS does.</p>
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<p>Telling Outcome how to throw a <code>filesystem_error</code> with payload of the
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failing paths is easy:</p>
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<div class="code-snippet"><div class="highlight"><pre class="chroma"><code class="language-c++" data-lang="c++"><span class="k">namespace</span> <span class="n">filesystem2</span>
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<span class="p">{</span>
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<span class="c1">// If we would like Outcome to do something other than the default action (see next
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</span><span class="c1"></span> <span class="c1">// section), we can declare this ADL discovered free function to customise what
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</span><span class="c1"></span> <span class="c1">// to do instead.
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</span><span class="c1"></span> <span class="c1">//
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</span><span class="c1"></span> <span class="c1">// Note that rvalue semantics are propagated internally by Outcome, so if the user
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</span><span class="c1"></span> <span class="c1">// called .value() on a rvalued result, failure_info will be moved rather than
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</span><span class="c1"></span> <span class="c1">// copied from the result. That means we can overload once with value semantics,
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</span><span class="c1"></span> <span class="c1">// and not need to overload for lvalue and rvalue situations unless we really feel
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</span><span class="c1"></span> <span class="c1">// we need to for some reason.
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</span><span class="c1"></span> <span class="kr">inline</span> <span class="kt">void</span> <span class="n">outcome_throw_as_system_error_with_payload</span><span class="p">(</span><span class="n">failure_info</span> <span class="n">fi</span><span class="p">)</span>
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<span class="p">{</span>
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<span class="c1">// If the error code is not filesystem related e.g. ENOMEM, throw that as a
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</span><span class="c1"></span> <span class="c1">// standard STL exception.
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</span><span class="c1"></span> <span class="n">outcome</span><span class="o">::</span><span class="n">try_throw_std_exception_from_error</span><span class="p">(</span><span class="n">fi</span><span class="p">.</span><span class="n">ec</span><span class="p">);</span>
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<span class="c1">// Throw the exact same filesystem_error exception which the throwing copy_file()
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</span><span class="c1"></span> <span class="c1">// edition does.
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</span><span class="c1"></span> <span class="k">throw</span> <span class="nf">filesystem_error</span><span class="p">(</span><span class="n">fi</span><span class="p">.</span><span class="n">ec</span><span class="p">.</span><span class="n">message</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">fi</span><span class="p">.</span><span class="n">path1</span><span class="p">),</span> <span class="c1">//
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</span><span class="c1"></span> <span class="n">std</span><span class="o">::</span><span class="n">move</span><span class="p">(</span><span class="n">fi</span><span class="p">.</span><span class="n">path2</span><span class="p">),</span> <span class="n">fi</span><span class="p">.</span><span class="n">ec</span><span class="p">);</span>
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<span class="p">}</span>
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<span class="p">}</span>
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</code></pre></div><a href="https://github.com/boostorg/outcome/tree/master/doc/src/snippets/outcome_payload.cpp#L100" class="code-snippet-url" target="_blank">View this code on Github</a></div>
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<p>Reference documentation for the above functions:</p>
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<ul>
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<li><a href="../../../reference/functions/policy.html">List of builtin <code>outcome_throw_as_system_error_with_payload()</code> overloads</a></li>
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<li><a href="../../../reference/functions/try_throw_std_exception_from_error.html" class="api-reference"><code>void try_throw_std_exception_from_error(std::error_code ec, const std::string &msg = std::string{})</code></a>
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</li>
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</ul>
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<p>Usage of our new “upgraded” Filesystem <code>copy_file()</code> might now be as follows:</p>
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<div class="code-snippet"><div class="highlight"><pre class="chroma"><code class="language-c++" data-lang="c++"> <span class="c1">// Non-throwing use case
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</span><span class="c1"></span> <span class="k">auto</span> <span class="n">o</span> <span class="o">=</span> <span class="n">filesystem2</span><span class="o">::</span><span class="n">copy_file</span><span class="p">(</span><span class="s">"dontexist"</span><span class="p">,</span> <span class="s">"alsodontexist"</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">o</span><span class="p">)</span>
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<span class="p">{</span>
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<span class="n">std</span><span class="o">::</span><span class="n">cerr</span> <span class="o"><<</span> <span class="s">"Copy file failed with error "</span> <span class="o"><<</span> <span class="n">o</span><span class="p">.</span><span class="n">error</span><span class="p">().</span><span class="n">ec</span><span class="p">.</span><span class="n">message</span><span class="p">()</span> <span class="c1">//
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</span><span class="c1"></span> <span class="o"><<</span> <span class="s">" (path1 = "</span> <span class="o"><<</span> <span class="n">o</span><span class="p">.</span><span class="n">error</span><span class="p">().</span><span class="n">path1</span> <span class="o"><<</span> <span class="s">", path2 = "</span> <span class="o"><<</span> <span class="n">o</span><span class="p">.</span><span class="n">error</span><span class="p">().</span><span class="n">path2</span> <span class="o"><<</span> <span class="s">")"</span> <span class="c1">//
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</span><span class="c1"></span> <span class="o"><<</span> <span class="n">std</span><span class="o">::</span><span class="n">endl</span><span class="p">;</span>
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<span class="p">}</span>
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<span class="c1">// Throwing use case
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</span><span class="c1"></span> <span class="k">try</span>
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<span class="p">{</span>
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<span class="c1">// Try to observe the successful value, thus triggering default actions which invokes
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</span><span class="c1"></span> <span class="c1">// our outcome_throw_as_system_error_with_payload() above which then throws filesystem_error
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</span><span class="c1"></span> <span class="c1">// exactly like the Filesystem TS does for its throwing overload.
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</span><span class="c1"></span> <span class="n">filesystem2</span><span class="o">::</span><span class="n">copy_file</span><span class="p">(</span><span class="s">"dontexist"</span><span class="p">,</span> <span class="s">"alsodontexist"</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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<span class="k">catch</span><span class="p">(</span><span class="k">const</span> <span class="n">filesystem2</span><span class="o">::</span><span class="n">filesystem_error</span> <span class="o">&</span><span class="n">e</span><span class="p">)</span>
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<span class="p">{</span>
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<span class="n">std</span><span class="o">::</span><span class="n">cerr</span> <span class="o"><<</span> <span class="s">"Copy file failed with exception "</span> <span class="o"><<</span> <span class="n">e</span><span class="p">.</span><span class="n">what</span><span class="p">()</span> <span class="c1">//
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</span><span class="c1"></span> <span class="o"><<</span> <span class="s">" (path1 = "</span> <span class="o"><<</span> <span class="n">e</span><span class="p">.</span><span class="n">path1</span><span class="p">()</span> <span class="o"><<</span> <span class="s">", path2 = "</span> <span class="o"><<</span> <span class="n">e</span><span class="p">.</span><span class="n">path2</span><span class="p">()</span> <span class="o"><<</span> <span class="s">")"</span> <span class="c1">//
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</span><span class="c1"></span> <span class="o"><<</span> <span class="n">std</span><span class="o">::</span><span class="n">endl</span><span class="p">;</span>
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<span class="p">}</span>
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<span class="k">catch</span><span class="p">(</span><span class="k">const</span> <span class="n">std</span><span class="o">::</span><span class="n">exception</span> <span class="o">&</span><span class="n">e</span><span class="p">)</span>
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<span class="p">{</span>
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<span class="n">std</span><span class="o">::</span><span class="n">cerr</span> <span class="o"><<</span> <span class="s">"Copy file failed with exception "</span> <span class="o"><<</span> <span class="n">e</span><span class="p">.</span><span class="n">what</span><span class="p">()</span> <span class="c1">//
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</span><span class="c1"></span> <span class="o"><<</span> <span class="n">std</span><span class="o">::</span><span class="n">endl</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/boostorg/outcome/tree/master/doc/src/snippets/outcome_payload.cpp#L127" class="code-snippet-url" target="_blank">View this code on Github</a></div>
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</div><p><small>Last revised: February 08, 2019 at 22:18:08 UTC</small></p>
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