mirror of
https://github.com/boostorg/outcome.git
synced 2026-02-18 02:12:09 +00:00
143 lines
12 KiB
HTML
143 lines
12 KiB
HTML
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
|
|
<html><meta http-equiv="Content-Type" content="text/html; charset=utf-8">
|
|
<title>result<> - Boost.Outcome documentation</title>
|
|
<link rel="stylesheet" href="../css/boost.css" type="text/css">
|
|
<meta name="generator" content="Hugo 0.52 with Boostdoc theme">
|
|
<meta name="viewport" content="width=device-width,initial-scale=1.0"/>
|
|
|
|
<link rel="icon" href="../images/favicon.ico" type="image/ico"/>
|
|
<body><div id="boost-common-heading-doc" style="background: #574D74 url('../images/header-bg.png') repeat-x top left;">
|
|
<div class="heading-inner" style="background: url('../images/header-fg.png') no-repeat top left;">
|
|
<div class="heading-placard"></div>
|
|
|
|
<h1 class="heading-title">
|
|
<a href="../">
|
|
<img src="../images/space.png" alt="Boost C++ Libraries" class="heading-logo" />
|
|
<span class="heading-boost">Boost</span>
|
|
<span class="heading-cpplibraries">C++ Libraries</span>
|
|
</a>
|
|
</h1>
|
|
|
|
<p class="heading-quote">
|
|
<q>...one of the most highly
|
|
regarded and expertly designed C++ library projects in the
|
|
world.</q> <span class="heading-attribution">— <a href=
|
|
"http://www.gotw.ca/" class="external">Herb Sutter</a> and <a href=
|
|
"http://en.wikipedia.org/wiki/Andrei_Alexandrescu" class="external">Andrei
|
|
Alexandrescu</a>, <a href=
|
|
"http://safari.awprofessional.com/?XmlId=0321113586" class="external">C++
|
|
Coding Standards</a></span></p>
|
|
</div>
|
|
</div>
|
|
<div id="boost-common-heading-doc-spacer"></div>
|
|
<div class="spirit-nav">
|
|
<a accesskey="p" href="../tutorial.html"><img src="../images/prev.png" alt="Prev"></a>
|
|
<a accesskey="u" href="../tutorial.html"><img src="../images/up.png" alt="Up"></a>
|
|
<a accesskey="h" href="../index.html"><img src="../images/home.png" alt="Home"></a><a accesskey="n" href="../tutorial/result/inspecting.html"><img src="../images/next.png" alt="Next"></a></div><div id="content">
|
|
|
|
<div class="titlepage"><div><div><h2 class="title" style="clear: both">result<></h2></div></div></div>
|
|
<div class="toc"><dl class="toc">
|
|
<dt>
|
|
<dd><dl>
|
|
<dt><a href="#outcome-2-0-namespace">Outcome 2.0 namespace</a></dt>
|
|
<dt><a href="#creating-result">Creating <code>result<></code></a></dt>
|
|
</dl></dd></dt>
|
|
</dl>
|
|
</div>
|
|
|
|
|
|
<h2 id="outcome-2-0-namespace">Outcome 2.0 namespace</h2>
|
|
|
|
<p>It is recommended that you refer to entities from this Outcome 2.0 via the following namespace alias:</p>
|
|
|
|
<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">outcome</span> <span class="o">=</span> <span class="n">BOOST_OUTCOME_V2_NAMESPACE</span><span class="p">;</span>
|
|
</code></pre></div><a href="https://github.com/ned14/boost-outcome/tree/master/doc/src/snippets/using_result.cpp#L8" class="code-snippet-url" target="_blank">View this code on Github</a></div>
|
|
|
|
|
|
<p>As patches and modifications are applied to this library, namespaces get permuted in order
|
|
not to break any backward compatibility. At some point namespace <code>outcome::v2</code> will be defined,
|
|
and this will be the prefered namespace. Until then <code>BOOST_OUTCOME_V2_NAMESPACE</code> denotes the most recently
|
|
updated version, getting closer to <code>outcome::v2</code>.</p>
|
|
|
|
<h2 id="creating-result">Creating <code>result<></code></h2>
|
|
|
|
<p>We will define a function that converts an <code>std::string</code> to an <code>int</code>. This function can fail for a number of reasons;
|
|
if it does we want to communicate the failure reason.</p>
|
|
|
|
<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">int</span><span class="o">></span> <span class="n">convert</span><span class="p">(</span><span class="k">const</span> <span class="n">std</span><span class="o">::</span><span class="n">string</span><span class="o">&</span> <span class="n">str</span><span class="p">)</span> <span class="k">noexcept</span><span class="p">;</span>
|
|
</code></pre></div><a href="https://github.com/ned14/boost-outcome/tree/master/doc/src/snippets/using_result.cpp#L12" class="code-snippet-url" target="_blank">View this code on Github</a></div>
|
|
|
|
|
|
<p>Class template <a href="reference/result/#standardese-outcome_v2_xxx__result-R-S-NoValuePolicy-" class="api-reference"><i class="fa fa-book" aria-hidden="true"></i> result<T, EC, NVP></a>
|
|
|
|
has three template parameters, but the last two have default values. The first
|
|
(<code>T</code>) represents the type of the object returned from the function upon success.
|
|
The second (<code>EC</code>) is the type of object containing information about the reason
|
|
for failure when the function fails. A result object stores either a <code>T</code> or an
|
|
<code>EC</code> at any given moment, and is therefore conceptually similar to <code>variant<T, EC></code>. <code>EC</code> is defaulted to <code>std::error_code</code>. The third parameter (<code>NVP</code>) is called a
|
|
no-value policy. We will cover it later.</p>
|
|
|
|
<p>If both <code>T</code> and <code>EC</code> are trivially copyable, <code>result<T, EC, NVP></code> is also trivially copyable.</p>
|
|
|
|
<p>Now, we will define an enumeration describing different failure situations during conversion.</p>
|
|
|
|
<div class="code-snippet"><div class="highlight"><pre class="chroma"><code class="language-c++" data-lang="c++"><span class="k">enum</span> <span class="k">class</span><span class="err"> </span><span class="nc">ConversionErrc</span>
|
|
<span class="p">{</span>
|
|
<span class="n">Success</span> <span class="o">=</span> <span class="mi">0</span><span class="p">,</span> <span class="c1">// 0 should not represent an error
|
|
</span><span class="c1"></span> <span class="n">EmptyString</span> <span class="o">=</span> <span class="mi">1</span><span class="p">,</span> <span class="c1">// (for rationale, see tutorial on error codes)
|
|
</span><span class="c1"></span> <span class="n">IllegalChar</span> <span class="o">=</span> <span class="mi">2</span><span class="p">,</span>
|
|
<span class="n">TooLong</span> <span class="o">=</span> <span class="mi">3</span><span class="p">,</span>
|
|
<span class="p">};</span>
|
|
|
|
<span class="c1">// all boilerplate necessary to plug ConversionErrc
|
|
</span><span class="c1">// into std::error_code framework
|
|
</span><span class="c1"></span></code></pre></div><a href="https://github.com/ned14/boost-outcome/tree/master/doc/src/snippets/using_result.cpp#L16" class="code-snippet-url" target="_blank">View this code on Github</a></div>
|
|
|
|
|
|
<p>Assume we have plugged it into <code>std::error_code</code> framework, as described in <a href="../../motivation/plug_error_code">this section</a>.</p>
|
|
|
|
<p>One notable effect of such plugging is that <code>ConversionErrc</code> is now convertible to <code>std::error_code</code>.
|
|
Now we can implement function <code>convert</code> as follows:</p>
|
|
|
|
<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">int</span><span class="o">></span> <span class="n">convert</span><span class="p">(</span><span class="k">const</span> <span class="n">std</span><span class="o">::</span><span class="n">string</span><span class="o">&</span> <span class="n">str</span><span class="p">)</span> <span class="k">noexcept</span>
|
|
<span class="p">{</span>
|
|
<span class="k">if</span> <span class="p">(</span><span class="n">str</span><span class="p">.</span><span class="n">empty</span><span class="p">())</span>
|
|
<span class="k">return</span> <span class="n">ConversionErrc</span><span class="o">::</span><span class="n">EmptyString</span><span class="p">;</span>
|
|
|
|
<span class="k">if</span> <span class="p">(</span><span class="o">!</span><span class="n">std</span><span class="o">::</span><span class="n">all_of</span><span class="p">(</span><span class="n">str</span><span class="p">.</span><span class="n">begin</span><span class="p">(),</span> <span class="n">str</span><span class="p">.</span><span class="n">end</span><span class="p">(),</span> <span class="o">::</span><span class="n">isdigit</span><span class="p">))</span>
|
|
<span class="k">return</span> <span class="n">ConversionErrc</span><span class="o">::</span><span class="n">IllegalChar</span><span class="p">;</span>
|
|
|
|
<span class="k">if</span> <span class="p">(</span><span class="n">str</span><span class="p">.</span><span class="n">length</span><span class="p">()</span> <span class="o">></span> <span class="mi">9</span><span class="p">)</span>
|
|
<span class="k">return</span> <span class="n">ConversionErrc</span><span class="o">::</span><span class="n">TooLong</span><span class="p">;</span>
|
|
|
|
<span class="k">return</span> <span class="nf">atoi</span><span class="p">(</span><span class="n">str</span><span class="p">.</span><span class="n">c_str</span><span class="p">());</span>
|
|
<span class="p">}</span>
|
|
</code></pre></div><a href="https://github.com/ned14/boost-outcome/tree/master/doc/src/snippets/using_result.cpp#L35" class="code-snippet-url" target="_blank">View this code on Github</a></div>
|
|
|
|
|
|
<p><code>result<T, EC></code> is convertible from any <code>T2</code> convertible to <code>T</code> as well as any <code>EC2</code> convertible to <code>EC</code>,
|
|
provided that there is no constructability possible in either direction between <code>T</code> and <code>EC</code>. If there is,
|
|
all implicit conversion is disabled, and you will need to use one of the tagged constructors:</p>
|
|
|
|
<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">int</span><span class="o">></span> <span class="n">r</span> <span class="p">{</span><span class="n">outcome</span><span class="o">::</span><span class="n">in_place_type</span><span class="o"><</span><span class="n">std</span><span class="o">::</span><span class="n">error_code</span><span class="o">></span><span class="p">,</span> <span class="n">ConversionErrc</span><span class="o">::</span><span class="n">EmptyString</span><span class="p">};</span>
|
|
<span class="n">outcome</span><span class="o">::</span><span class="n">result</span><span class="o"><</span><span class="kt">int</span><span class="o">></span> <span class="n">s</span> <span class="p">{</span><span class="n">outcome</span><span class="o">::</span><span class="n">in_place_type</span><span class="o"><</span><span class="kt">int</span><span class="o">></span><span class="p">,</span> <span class="mi">1</span><span class="p">};</span>
|
|
</code></pre></div><a href="https://github.com/ned14/boost-outcome/tree/master/doc/src/snippets/using_result.cpp#L85" class="code-snippet-url" target="_blank">View this code on Github</a></div>
|
|
|
|
|
|
<p>Or use helper factory functions:</p>
|
|
|
|
<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">int</span><span class="o">></span> <span class="n">r</span> <span class="o">=</span> <span class="n">outcome</span><span class="o">::</span><span class="n">failure</span><span class="p">(</span><span class="n">ConversionErrc</span><span class="o">::</span><span class="n">EmptyString</span><span class="p">);</span>
|
|
<span class="n">outcome</span><span class="o">::</span><span class="n">result</span><span class="o"><</span><span class="kt">int</span><span class="o">></span> <span class="n">s</span> <span class="o">=</span> <span class="n">outcome</span><span class="o">::</span><span class="n">success</span><span class="p">(</span><span class="mi">1</span><span class="p">);</span>
|
|
</code></pre></div><a href="https://github.com/ned14/boost-outcome/tree/master/doc/src/snippets/using_result.cpp#L93" class="code-snippet-url" target="_blank">View this code on Github</a></div>
|
|
|
|
|
|
|
|
|
|
</div><p><small>Last revised: December 05, 2018 at 14:18:52 UTC</small></p>
|
|
<hr>
|
|
<div class="spirit-nav">
|
|
<a accesskey="p" href="../tutorial.html"><img src="../images/prev.png" alt="Prev"></a>
|
|
<a accesskey="u" href="../tutorial.html"><img src="../images/up.png" alt="Up"></a>
|
|
<a accesskey="h" href="../index.html"><img src="../images/home.png" alt="Home"></a><a accesskey="n" href="../tutorial/result/inspecting.html"><img src="../images/next.png" alt="Next"></a></div></body>
|
|
</html>
|