144 lines
14 KiB
HTML
144 lines
14 KiB
HTML
<!DOCTYPE html>
|
|
<html lang="zh" dir="ltr" class="client-nojs" xmlns="http://www.w3.org/1999/xhtml" xml:lang="zh">
|
|
<head><meta http-equiv="x-ua-compatible" content="ie=edge">
|
|
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
|
|
<title>std::variant<Types...>::valueless_by_exception</title>
|
|
<link rel="stylesheet" href="ext.css" />
|
|
<link rel="stylesheet" href="site_modules.css" />
|
|
</head>
|
|
<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_utility_variant_valueless_by_exception skin-cppreference2 action-view cpp-navbar">
|
|
<div id="cpp-content-base">
|
|
<div id="content"><a id="top"></a>
|
|
<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::variant<Types...>::</span>valueless_by_exception</h1>
|
|
<div id="bodyContent">
|
|
<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-utility.html">utility</a>‎ | <a href="cpp-utility-variant.html">variant</a></span></div>
|
|
<div id="mw-content-text" lang="zh" dir="ltr" class="mw-content-ltr" xml:lang="zh">
|
|
<table class="t-dcl-begin">
|
|
<tbody>
|
|
<tr class="t-dcl t-since-cxx17">
|
|
<td>
|
|
<div><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span> <span class="kw4">bool</span> valueless_by_exception<span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
|
|
</td>
|
|
<td class="t-dcl-nopad"></td>
|
|
<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td>
|
|
</tr>
|
|
<tr class="t-dcl-sep">
|
|
<td></td>
|
|
<td></td>
|
|
<td></td>
|
|
</tr>
|
|
</tbody>
|
|
</table>
|
|
<p>当且仅当 <code>variant</code> 保有值时返回 <code>false</code> 。</p>
|
|
<h3><span class="mw-headline" id=".E6.B3.A8.E8.A7.A3">注解</span></h3>
|
|
<p><code>variant</code> 可以在下列情形中变得无值：</p>
|
|
<ul>
|
|
<li>（保证）在<a href="cpp-utility-variant-operator=.html">移动赋值</a>中，移动初始化所含值时抛出异常</li>
|
|
<li>（可选）在<a href="cpp-utility-variant-operator=.html">复制赋值</a>中，复制初始化所含值时抛出异常</li>
|
|
<li>（可选）在类型更改<a href="cpp-utility-variant-operator=.html">赋值</a>期间，初始化所含值时抛出异常</li>
|
|
<li>（可选）在类型更改 <a href="cpp-utility-variant-emplace.html"><code>emplace</code></a> 期间，初始化所含值时抛出异常</li>
|
|
</ul>
|
|
<p>因为 <code>variant</code> 决不容许分配动态内存，故在这些情况下不可能保留先前的值。</p>
|
|
<p>这还应用到非类类型的 <code>variant</code> ：</p>
|
|
<div dir="ltr" class="mw-geshi" style="text-align: left;">
|
|
<div class="cpp source-cpp">
|
|
<pre class="de1"><span class="kw1">struct</span> S <span class="br0">{</span>
|
|
operator <span class="kw4">int</span><span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">throw</span> <span class="nu0">42</span><span class="sy4">;</span> <span class="br0">}</span>
|
|
<span class="br0">}</span><span class="sy4">;</span>
|
|
<a href="cpp-utility-variant.html"><span class="kw1085">std::<span class="me2">variant</span></span></a><span class="sy1"><</span><span class="kw4">float</span>, <span class="kw4">int</span><span class="sy1">></span> v<span class="br0">{</span><span class="nu0">12</span>.<span class="me1">f</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// OK</span>
|
|
v.<span class="me1">emplace</span><span class="sy1"><</span><span class="nu0">1</span><span class="sy1">></span><span class="br0">(</span>S<span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// v 可能为无值</span></pre></div>
|
|
</div>
|
|
<p>因异常无值的 <code>variant</code> 被当做在非法态： <a href="cpp-utility-variant-index.html"><code>index</code></a> 返回 <a href="cpp-utility-variant-variant_npos.html"><code>variant_npos</code></a> ， <a href="cpp-utility-variant-get.html"><code>get</code></a> 与 <a href="cpp-utility-variant-visit.html"><code>visit</code></a> 抛出 <a href="cpp-utility-variant-bad_variant_access.html"><code>bad_variant_access</code></a> 。</p>
|
|
<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
|
|
<div class="t-example">
|
|
<div class="t-example-live-link"></div>
|
|
<div dir="ltr" class="mw-geshi" style="text-align: left;">
|
|
<div class="cpp source-cpp">
|
|
<pre class="de1"><span class="co2">#include <cassert></span>
|
|
<span class="co2">#include <iostream></span>
|
|
<span class="co2">#include <stdexcept></span>
|
|
<span class="co2">#include <string></span>
|
|
<span class="co2">#include <variant></span>
|
|
|
|
<span class="kw1">struct</span> Demo <span class="br0">{</span>
|
|
Demo<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span>
|
|
Demo<span class="br0">(</span><span class="kw4">const</span> Demo<span class="sy3">&</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">throw</span> <a href="cpp-error-domain_error.html"><span class="kw766">std::<span class="me2">domain_error</span></span></a><span class="br0">(</span><span class="st0">"copy ctor"</span><span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span>
|
|
Demo<span class="sy3">&</span> operator<span class="sy1">=</span> <span class="br0">(</span><span class="kw4">const</span> Demo<span class="sy3">&</span><span class="br0">)</span> <span class="sy1">=</span> <span class="kw1">default</span><span class="sy4">;</span>
|
|
<span class="br0">}</span><span class="sy4">;</span>
|
|
|
|
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
|
|
<span class="br0">{</span>
|
|
<a href="cpp-utility-variant.html"><span class="kw1085">std::<span class="me2">variant</span></span></a><span class="sy1"><</span><a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a>, Demo<span class="sy1">></span> var<span class="br0">{</span><span class="st0">"str"</span><span class="br0">}</span><span class="sy4">;</span>
|
|
<a href="cpp-error-assert.html"><span class="kw774">assert</span></a><span class="br0">(</span>var.<span class="me1">index</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1">==</span> <span class="nu0">0</span><span class="br0">)</span><span class="sy4">;</span>
|
|
<a href="cpp-error-assert.html"><span class="kw774">assert</span></a><span class="br0">(</span>std<span class="sy4">::</span><span class="me2">get</span><span class="sy1"><</span><span class="nu0">0</span><span class="sy1">></span><span class="br0">(</span>var<span class="br0">)</span> <span class="sy1">==</span> <span class="st0">"str"</span><span class="br0">)</span><span class="sy4">;</span>
|
|
<a href="cpp-error-assert.html"><span class="kw774">assert</span></a><span class="br0">(</span>var.<span class="me1">valueless_by_exception</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1">==</span> <span class="kw2">false</span><span class="br0">)</span><span class="sy4">;</span>
|
|
|
|
<span class="kw1">try</span> <span class="br0">{</span>
|
|
var <span class="sy1">=</span> Demo<span class="br0">{</span><span class="nu0">666</span><span class="br0">}</span><span class="sy4">;</span>
|
|
<span class="br0">}</span> <span class="kw1">catch</span> <span class="br0">(</span><span class="kw4">const</span> <a href="cpp-error-domain_error.html"><span class="kw766">std::<span class="me2">domain_error</span></span></a><span class="sy3">&</span> ex<span class="br0">)</span> <span class="br0">{</span>
|
|
<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"1) Exception: "</span> <span class="sy1"><<</span> ex.<span class="me1">what</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
|
|
<span class="br0">}</span>
|
|
<a href="cpp-error-assert.html"><span class="kw774">assert</span></a><span class="br0">(</span>var.<span class="me1">index</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1">==</span> <a href="cpp-utility-variant-variant_npos.html"><span class="kw1090">std::<span class="me2">variant_npos</span></span></a><span class="br0">)</span><span class="sy4">;</span>
|
|
<a href="cpp-error-assert.html"><span class="kw774">assert</span></a><span class="br0">(</span>var.<span class="me1">valueless_by_exception</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1">==</span> <span class="kw2">true</span><span class="br0">)</span><span class="sy4">;</span>
|
|
|
|
<span class="co1">// 现在 var “无值”，此为 var 的初始化过程中引发的异常导致的非法状态。</span>
|
|
|
|
<span class="kw1">try</span> <span class="br0">{</span>
|
|
std<span class="sy4">::</span><span class="me2">get</span><span class="sy1"><</span><span class="nu0">1</span><span class="sy1">></span><span class="br0">(</span>var<span class="br0">)</span><span class="sy4">;</span>
|
|
<span class="br0">}</span> <span class="kw1">catch</span> <span class="br0">(</span><span class="kw4">const</span> <a href="cpp-utility-variant-bad_variant_access.html"><span class="kw1089">std::<span class="me2">bad_variant_access</span></span></a><span class="sy3">&</span> ex<span class="br0">)</span> <span class="br0">{</span>
|
|
<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"2) Exception: "</span> <span class="sy1"><<</span> ex.<span class="me1">what</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
|
|
<span class="br0">}</span>
|
|
|
|
var <span class="sy1">=</span> <span class="st0">"str2"</span><span class="sy4">;</span>
|
|
<a href="cpp-error-assert.html"><span class="kw774">assert</span></a><span class="br0">(</span>var.<span class="me1">index</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1">==</span> <span class="nu0">0</span><span class="br0">)</span><span class="sy4">;</span>
|
|
<a href="cpp-error-assert.html"><span class="kw774">assert</span></a><span class="br0">(</span>std<span class="sy4">::</span><span class="me2">get</span><span class="sy1"><</span><span class="nu0">0</span><span class="sy1">></span><span class="br0">(</span>var<span class="br0">)</span> <span class="sy1">==</span> <span class="st0">"str2"</span><span class="br0">)</span><span class="sy4">;</span>
|
|
<a href="cpp-error-assert.html"><span class="kw774">assert</span></a><span class="br0">(</span>var.<span class="me1">valueless_by_exception</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1">==</span> <span class="kw2">false</span><span class="br0">)</span><span class="sy4">;</span>
|
|
<span class="br0">}</span></pre></div>
|
|
</div>
|
|
<p>可能的输出：</p>
|
|
<div dir="ltr" class="mw-geshi" style="text-align: left;">
|
|
<div class="text source-text">
|
|
<pre class="de1">1) Exception: copy ctor
|
|
2) Exception: std::get: variant is valueless</pre></div>
|
|
</div>
|
|
</div>
|
|
<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
|
|
<table class="t-dsc-begin">
|
|
<tr class="t-dsc">
|
|
<td>
|
|
<div class="t-dsc-member-div">
|
|
<div><a href="cpp-utility-variant-get.html"><span class="t-lines"><span>std::get<span class="t-dsc-small">(std::variant)</span></span></span></a></div>
|
|
<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span></span></div>
|
|
</div>
|
|
</td>
|
|
<td>以给定索引或类型（若类型唯一）读取 <code>variant</code> 的值，错误时抛出异常<br />
|
|
<span class="t-mark">(函数模板)</span></td>
|
|
</tr>
|
|
<tr class="t-dsc">
|
|
<td>
|
|
<div class="t-dsc-member-div">
|
|
<div><a href="cpp-utility-variant-index.html"><span class="t-lines"><span>index</span></span></a></div>
|
|
</div>
|
|
</td>
|
|
<td>返回 <code>variant</code> 所保有可选项的零基下标<br />
|
|
<span class="t-mark">(公开成员函数)</span></td>
|
|
</tr>
|
|
<tr class="t-dsc">
|
|
<td>
|
|
<div class="t-dsc-member-div">
|
|
<div><a href="cpp-utility-variant-bad_variant_access.html"><span class="t-lines"><span>bad_variant_access</span></span></a></div>
|
|
<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span></span></div>
|
|
</div>
|
|
</td>
|
|
<td>非法地访问 <code>variant</code> 的值时抛出的异常<br />
|
|
<span class="t-mark">(类)</span></td>
|
|
</tr>
|
|
</table>
|
|
</div>
|
|
<div class="visualClear"></div>
|
|
</div>
|
|
</div>
|
|
</div>
|
|
</body>
|
|
</html>
|