2022-06-30 10:23:03 +08:00
<!DOCTYPE HTML>
< html lang = "zh" class = "sidebar-visible no-js light" >
< head >
<!-- Book generated using mdBook -->
< meta charset = "UTF-8" >
2022-07-02 13:21:48 +08:00
< title > Item 17:理解特殊成员函数函数的生成 - Effective Modern C++< / title >
2022-06-30 10:23:03 +08:00
<!-- Custom HTML head -->
< meta content = "text/html; charset=utf-8" http-equiv = "Content-Type" >
< meta name = "description" content = "" >
< meta name = "viewport" content = "width=device-width, initial-scale=1" >
< meta name = "theme-color" content = "#ffffff" / >
< link rel = "icon" href = "../favicon.svg" >
< link rel = "shortcut icon" href = "../favicon.png" >
< link rel = "stylesheet" href = "../css/variables.css" >
< link rel = "stylesheet" href = "../css/general.css" >
< link rel = "stylesheet" href = "../css/chrome.css" >
< link rel = "stylesheet" href = "../css/print.css" media = "print" >
<!-- Fonts -->
< link rel = "stylesheet" href = "../FontAwesome/css/font-awesome.css" >
< link rel = "stylesheet" href = "../fonts/fonts.css" >
<!-- Highlight.js Stylesheets -->
< link rel = "stylesheet" href = "../highlight.css" >
< link rel = "stylesheet" href = "../tomorrow-night.css" >
< link rel = "stylesheet" href = "../ayu-highlight.css" >
<!-- Custom theme stylesheets -->
<!-- MathJax -->
< script async type = "text/javascript" src = "https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.1/MathJax.js?config=TeX-AMS-MML_HTMLorMML" > < / script >
< / head >
< body >
<!-- Provide site root to javascript -->
< script type = "text/javascript" >
var path_to_root = "../";
var default_theme = window.matchMedia("(prefers-color-scheme: dark)").matches ? "navy" : "light";
< / script >
<!-- Work around some values being stored in localStorage wrapped in quotes -->
< script type = "text/javascript" >
try {
var theme = localStorage.getItem('mdbook-theme');
var sidebar = localStorage.getItem('mdbook-sidebar');
if (theme.startsWith('"') & & theme.endsWith('"')) {
localStorage.setItem('mdbook-theme', theme.slice(1, theme.length - 1));
}
if (sidebar.startsWith('"') & & sidebar.endsWith('"')) {
localStorage.setItem('mdbook-sidebar', sidebar.slice(1, sidebar.length - 1));
}
} catch (e) { }
< / script >
<!-- Set the theme before any content is loaded, prevents flash -->
< script type = "text/javascript" >
var theme;
try { theme = localStorage.getItem('mdbook-theme'); } catch(e) { }
if (theme === null || theme === undefined) { theme = default_theme; }
var html = document.querySelector('html');
html.classList.remove('no-js')
html.classList.remove('light')
html.classList.add(theme);
html.classList.add('js');
< / script >
<!-- Hide / unhide sidebar before it is displayed -->
< script type = "text/javascript" >
var html = document.querySelector('html');
var sidebar = 'hidden';
if (document.body.clientWidth >= 1080) {
try { sidebar = localStorage.getItem('mdbook-sidebar'); } catch(e) { }
sidebar = sidebar || 'visible';
}
html.classList.remove('sidebar-visible');
html.classList.add("sidebar-" + sidebar);
< / script >
< nav id = "sidebar" class = "sidebar" aria-label = "Table of contents" >
< div class = "sidebar-scrollbox" >
2023-05-06 14:19:14 +08:00
< ol class = "chapter" > < li class = "chapter-item expanded " > < a href = "../Introduction.html" > 简介< / a > < / li > < li class = "chapter-item expanded " > < div > 第一章 类型推导< / div > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../1.DeducingTypes/item1.html" > Item 1:理解模板类型推导< / a > < / li > < li class = "chapter-item expanded " > < a href = "../1.DeducingTypes/item2.html" > Item 2:理解auto类型推导< / a > < / li > < li class = "chapter-item expanded " > < a href = "../1.DeducingTypes/item3.html" > Item 3:理解decltype< / a > < / li > < li class = "chapter-item expanded " > < a href = "../1.DeducingTypes/item4.html" > Item 4:学会查看类型推导结果< / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < div > 第二章 auto< / div > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../2.Auto/item5.html" > Item 5:优先考虑auto而非显式类型声明< / a > < / li > < li class = "chapter-item expanded " > < a href = "../2.Auto/item6.html" > Item 6:auto推导若非己愿， < / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < div > 第三章 移步现代C++< / div > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../3.MovingToModernCpp/item7.html" > Item 7:区别使用()和{}创建对象< / a > < / li > < li class = "chapter-item expanded " > < a href = "../3.MovingToModernCpp/item8.html" > Item 8:优先考虑nullptr而非0和NULL< / a > < / li > < li class = "chapter-item expanded " > < a href = "../3.MovingToModernCpp/item9.html" > Item 9:优先考虑别名声明而非typedefs< / a > < / li > < li class = "chapter-item expanded " > < a href = "../3.MovingToModernCpp/item10.html" > Item 10:优先考虑限域枚举而非未限域枚举< / a > < / li > < li class = "chapter-item expanded " > < a href = "../3.MovingToModernCpp/item11.html" > Item 11:优先考虑使用deleted函数而非使用未定义的私有声明< / a > < / li > < li class = "chapter-item expanded " > < a href = "../3.MovingToModernCpp/item12.html" > Item 12:使用override声明重载函数< / a > < / li > < li class = "chapter-item expanded " > < a href = "../3.MovingToModernCpp/item13.html" > Item 13:优先考虑const_iterator而非iterator< / a > < / li > < li class = "chapter-item expanded " > < a href = "../3.MovingToModernCpp/item14.html" > Item 14:如果函数不抛出异常请使用noexcept< / a > < / li > < li class = "chapter-item expanded " > < a href = "../3.MovingToModernCpp/item15.html" > Item 15:尽可能的使用constexpr< / a > < / li > < li class = "chapter-item expanded " > < a href = "../3.MovingToModernCpp/item16.html" > Item 16:让const成员函数线程安全< / a > < / li > < li class = "chapter-item expanded " > < a href = "../3.MovingToModernCpp/item17.html" class = "active" > Item 17:理解特殊成员函数函数的生成< / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < div > 第四章 智能指针< / div > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../4.SmartPointers/item18.html" > Item 18:对于独占资源使用std::unique_ptr< / a > < / li > < li class = "chapter-item expanded " > < a href = "../4.SmartPointers/item19.html" > Item 19:对于共享资源使用std::shared_ptr< / a > < / li > < li class = "chapter-item expanded " > < a href = "../4.SmartPointers/item20.html" > Item 20:当std::shared_ptr可能悬空时使用std::weak_ptr< / a > < / li > < li class = "chapter-item expanded " > < a href = "../4.SmartPointers/item21.html" > Item 21:优先考虑使用std::make_unique和std::make_shared而非new< / a > < / li > < li class = "chapter-item expanded " > < a href = "../4.SmartPointers/item22.html" > Item 22:当使用Pimpl惯用法， < / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < div > 第五章 右值引用，移动语义，完美转发< / div > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../5.RRefMovSemPerfForw/item23.html" > Item 23:理解std::move和std::forward< / a > < / li > < li class = "chapter-item expanded " > < a href = "../5.RRefMovSemPerfForw/item24.html" > Item 24:区别通用引用和右值引用< / a > < / li > < li class = "chapter-item expanded " > < a href = "../5.RRefMovSemPerfForw/item25.html" > Item 25:对于右值引用使用std::move， < / a > < / li > < li class = "chapter-item expanded " > < a href = "../5
2022-06-30 10:23:03 +08:00
< / div >
< div id = "sidebar-resize-handle" class = "sidebar-resize-handle" > < / div >
< / nav >
< div id = "page-wrapper" class = "page-wrapper" >
< div class = "page" >
< div id = "menu-bar-hover-placeholder" > < / div >
< div id = "menu-bar" class = "menu-bar sticky bordered" >
< div class = "left-buttons" >
< button id = "sidebar-toggle" class = "icon-button" type = "button" title = "Toggle Table of Contents" aria-label = "Toggle Table of Contents" aria-controls = "sidebar" >
< i class = "fa fa-bars" > < / i >
< / button >
< button id = "theme-toggle" class = "icon-button" type = "button" title = "Change theme" aria-label = "Change theme" aria-haspopup = "true" aria-expanded = "false" aria-controls = "theme-list" >
< i class = "fa fa-paint-brush" > < / i >
< / button >
< ul id = "theme-list" class = "theme-popup" aria-label = "Themes" role = "menu" >
< li role = "none" > < button role = "menuitem" class = "theme" id = "light" > Light (default)< / button > < / li >
< li role = "none" > < button role = "menuitem" class = "theme" id = "rust" > Rust< / button > < / li >
< li role = "none" > < button role = "menuitem" class = "theme" id = "coal" > Coal< / button > < / li >
< li role = "none" > < button role = "menuitem" class = "theme" id = "navy" > Navy< / button > < / li >
< li role = "none" > < button role = "menuitem" class = "theme" id = "ayu" > Ayu< / button > < / li >
< / ul >
< button id = "search-toggle" class = "icon-button" type = "button" title = "Search. (Shortkey: s)" aria-label = "Toggle Searchbar" aria-expanded = "false" aria-keyshortcuts = "S" aria-controls = "searchbar" >
< i class = "fa fa-search" > < / i >
< / button >
< / div >
< h1 class = "menu-title" > Effective Modern C++< / h1 >
< div class = "right-buttons" >
< a href = "../print.html" title = "Print this book" aria-label = "Print this book" >
< i id = "print-button" class = "fa fa-print" > < / i >
< / a >
2022-09-08 16:47:52 +08:00
< a href = "https://github.com/CnTransGroup/EffectiveModernCppChinese" title = "Git repository" aria-label = "Git repository" >
< i id = "git-repository-button" class = "fa fa-github" > < / i >
< / a >
2022-06-30 10:23:03 +08:00
< / div >
< / div >
< div id = "search-wrapper" class = "hidden" >
< form id = "searchbar-outer" class = "searchbar-outer" >
< input type = "search" id = "searchbar" name = "searchbar" placeholder = "Search this book ..." aria-controls = "searchresults-outer" aria-describedby = "searchresults-header" >
< / form >
< div id = "searchresults-outer" class = "searchresults-outer hidden" >
< div id = "searchresults-header" class = "searchresults-header" > < / div >
< ul id = "searchresults" >
< / ul >
< / div >
< / div >
<!-- Apply ARIA attributes after the sidebar and the sidebar toggle button are added to the DOM -->
< script type = "text/javascript" >
document.getElementById('sidebar-toggle').setAttribute('aria-expanded', sidebar === 'visible');
document.getElementById('sidebar').setAttribute('aria-hidden', sidebar !== 'visible');
Array.from(document.querySelectorAll('#sidebar a')).forEach(function(link) {
link.setAttribute('tabIndex', sidebar === 'visible' ? 0 : -1);
});
< / script >
< div id = "content" class = "content" >
< main >
< h2 id = "条款十七理解特殊成员函数的生成" > < a class = "header" href = "#条款十七理解特殊成员函数的生成" > 条款十七：理解特殊成员函数的生成< / a > < / h2 >
< p > < strong > Item 17: Understand special member function generation< / strong > < / p >
< p > 在C++术语中，< strong > 特殊成员函数< / strong > 是指C++自己生成的函数。C++98有四个： ， ， ， ， （ ， ） < code > inline< / code > ，它们是非虚的，除非相关函数是在派生类中的析构函数，派生类继承了有虚析构函数的基类。在这种情况下，编译器为派生类生成的析构函数是虚的。< / p >
< p > 但是你早就知道这些了。好吧好吧， ： ， ， ， ， ， ， < / p >
< p > C++11特殊成员函数俱乐部迎来了两位新会员： ： < / p >
< pre > < code class = "language-cpp" > class Widget {
public:
…
Widget(Widget& & rhs); //移动构造函数
Widget& operator=(Widget& & rhs); //移动赋值运算符
…
};
< / code > < / pre >
< p > 掌控它们生成和行为的规则类似于拷贝系列。移动操作仅在需要的时候生成， ， < code > rhs< / code > 参数里面对应的成员移动构造出新non-static部分， （ ） ， < / p >
2022-11-25 13:43:31 +08:00
< p > 现在，当我对一个数据成员或者基类使用移动构造或者移动赋值时，没有任何保证移动一定会真的发生。逐成员移动，实际上，更像是逐成员移动< strong > 请求< / strong > ，因为对< strong > 不可移动类型< / strong > （ ， ） < code > std::move< / code > ，然后函数决议时会选择执行移动还是拷贝操作。< a href = "../5.RRefMovSemPerfForw/item23.html" > Item23< / a > 包括了这个操作的细节。本条款中，简单记住如果支持移动就会逐成员移动类成员和基类成员，如果不支持移动就执行拷贝操作就好了。< / p >
2022-06-30 10:23:03 +08:00
< p > 像拷贝操作情况一样，如果你自己声明了移动操作，编译器就不会生成。然而它们生成的精确条件与拷贝操作的条件有点不同。< / p >
< p > 两个拷贝操作是独立的： ， ， ， ， ， < / p >
< p > 两个移动操作不是相互独立的。如果你声明了其中一个，编译器就不再生成另一个。如果你给类声明了，比如，一个移动构造函数，就表明对于移动操作应怎样实现，与编译器应生成的默认逐成员移动有些区别。如果逐成员移动构造有些问题，那么逐成员移动赋值同样也可能有问题。所以声明移动构造函数阻止移动赋值运算符的生成，声明移动赋值运算符同样阻止编译器生成移动构造函数。< / p >
< p > 再进一步，如果一个类显式声明了拷贝操作，编译器就不会生成移动操作。这种限制的解释是如果声明拷贝操作（构造或者赋值）就暗示着平常拷贝对象的方法（逐成员拷贝）不适用于该类，编译器会明白如果逐成员拷贝对拷贝操作来说不合适，逐成员移动也可能对移动操作来说不合适。< / p >
2022-11-25 13:43:31 +08:00
< p > 这是另一个方向。声明移动操作（构造或赋值）使得编译器禁用拷贝操作。（编译器通过给拷贝操作加上< em > delete< / em > 来保证，参见< a href = "../3.MovingToModernCpp/item11.html" > Item11< / a > 。） （ ： ， ） ， ， ， ， ， ， ， < / p >
2022-06-30 10:23:03 +08:00
< p > 也许你早已听过_Rule of Three_规则。这个规则告诉我们如果你声明了拷贝构造函数， ， ， ， ， （ ） ， （ ） （ ） ， （ ） < em > the big three< / em > ”：拷贝构造，拷贝赋值和析构。< / p >
< p > < em > Rule of Three< / em > 带来的后果就是只要出现用户定义的析构函数就意味着简单的逐成员拷贝操作不适用于该类。那意味着如果一个类声明了析构， ， ， ， ， < / p >
< p > < em > Rule of Three< / em > 规则背后的解释依然有效， ， < / p >
< p > 所以仅当下面条件成立时才会生成移动操作（当需要时）：< / p >
< ul >
< li > 类中没有拷贝操作< / li >
< li > 类中没有移动操作< / li >
< li > 类中没有用户定义的析构< / li >
< / ul >
< p > 有时， ， ， ， （ ） ， ， < code > = default< / code > 就可以表达你想做的：< / p >
< pre > < code class = "language-cpp" > class Widget {
public:
…
~Widget(); //用户声明的析构函数
… //默认拷贝构造函数
Widget(const Widget& ) = default; //的行为还可以
Widget& //默认拷贝赋值运算符
operator=(const Widget& ) = default; //的行为还可以
…
};
< / code > < / pre >
< p > 这种方法通常在多态基类中很有用，即通过操作的是哪个派生类对象来定义接口。多态基类通常有一个虚析构函数，因为如果它们非虚，一些操作（比如通过一个基类指针或者引用对派生类对象使用< code > delete< / code > 或者< code > typeid< / code > ）会产生未定义或错误结果。除非类继承了一个已经是< em > virtual< / em > 的析构函数，否则要想析构函数为虚函数的唯一方法就是加上< code > virtual< / code > 关键字。通常，默认实现是对的，< code > = default< / code > 是一个不错的方式表达默认实现。然而用户声明的析构函数会抑制编译器生成移动操作，所以如果该类需要具有移动性，就为移动操作加上< code > = default< / code > 。声明移动会抑制拷贝生成，所以如果拷贝性也需要支持，再为拷贝操作加上< code > = default< / code > ： < / p >
< pre > < code class = "language-cpp" > class Base {
public:
virtual ~Base() = default; //使析构函数virtual
Base(Base& & ) = default; //支持移动
Base& operator=(Base& & ) = default;
Base(const Base& ) = default; //支持拷贝
Base& operator=(const Base& ) = default;
…
};
< / code > < / pre >
< p > 实际上，就算编译器乐于为你的类生成拷贝和移动操作，生成的函数也如你所愿，你也应该手动声明它们然后加上< code > = default< / code > 。这看起来比较多余， ， ， ， ： < / p >
< pre > < code class = "language-cpp" > class StringTable {
public:
StringTable() {}
… //插入、删除、查找等函数，但是没有拷贝/移动/析构功能
private:
std::map< int, std::string> values;
};
< / code > < / pre >
< p > 假设这个类没有声明拷贝操作，没有移动操作，也没有析构，如果它们被用到编译器会自动生成。没错，很方便。< / p >
< p > 后来需要在对象构造和析构中打日志，增加这种功能很简单：< / p >
< pre > < code class = "language-cpp" > class StringTable {
public:
StringTable()
{ makeLogEntry(" Creating StringTable object" ); } //增加的
~StringTable() //也是增加的
{ makeLogEntry(" Destroying StringTable object" ); }
… //其他函数同之前一样
private:
std::map< int, std::string> values; //同之前一样
};
< / code > < / pre >
< p > 看起来合情合理，但是声明析构有潜在的副作用：它阻止了移动操作的生成。然而，拷贝操作的生成是不受影响的。因此代码能通过编译，运行，也能通过功能（译注：即打日志的功能）测试。功能测试也包括移动功能，因为即使该类不支持移动操作，对该类的移动请求也能通过编译和运行。这个请求正如之前提到的，会转而由拷贝操作完成。它意味着对< code > StringTable< / code > 对象的移动实际上是对对象的拷贝，即拷贝里面的< code > std::map< int, std::string> < / code > 对象。拷贝< code > std::map< int, std::string> < / code > 对象很可能比移动慢< strong > 几个数量级< / strong > 。简单的加个析构就引入了极大的性能问题！对拷贝和移动操作显式加个< code > = default< / code > ，问题将不再出现。< / p >
< p > 受够了我喋喋不休的讲述C++11拷贝移动规则了吧， ， ， ， ： ， < / p >
< p > C++11对于特殊成员函数处理的规则如下： < / p >
< ul >
< li > < strong > 默认构造函数< / strong > ： < / li >
2022-11-25 13:43:31 +08:00
< li > < strong > 析构函数< / strong > ： ； < code > noexcept< / code > （参见< a href = "../3.MovingToModernCpp/item14.html" > Item14< / a > ） ， < / li >
2022-06-30 10:23:03 +08:00
< li > < strong > 拷贝构造函数< / strong > ： ： < em > delete< / em > 的。当用户声明了拷贝赋值或者析构，该函数自动生成已被废弃。< / li >
< li > < strong > 拷贝赋值运算符< / strong > ： ： < em > delete< / em > 的。当用户声明了拷贝构造或者析构，该函数自动生成已被废弃。< / li >
< li > < strong > 移动构造函数< / strong > 和< strong > 移动赋值运算符< / strong > ： ， < / li >
< / ul >
< p > 注意没有“成员函数< strong > 模版< / strong > 阻止编译器生成特殊成员函数”的规则。这意味着如果< code > Widget< / code > 是这样：< / p >
< pre > < code class = "language-cpp" > class Widget {
…
template< typename T> //从任何东西构造Widget
Widget(const T& rhs);
template< typename T> //从任何东西赋值给Widget
Widget& operator=(const T& rhs);
…
};
< / code > < / pre >
2022-11-25 13:43:31 +08:00
< p > 编译器仍会生成移动和拷贝操作（假设正常生成它们的条件满足），即使可以模板实例化产出拷贝构造和拷贝赋值运算符的函数签名。（当< code > T< / code > 为< code > Widget< / code > 时。）很可能你会觉得这是一个不值得承认的边缘情况，但是我提到它是有道理的，< a href = "../5.RRefMovSemPerfForw/item26.html" > Item26< / a > 将会详细讨论它可能带来的后果。< / p >
2022-06-30 10:23:03 +08:00
< p > < strong > 请记住：< / strong > < / p >
< ul >
< li > 特殊成员函数是编译器可能自动生成的函数：默认构造函数，析构函数，拷贝操作，移动操作。< / li >
< li > 移动操作仅当类没有显式声明移动操作，拷贝操作，析构函数时才自动生成。< / li >
< li > 拷贝构造函数仅当类没有显式声明拷贝构造函数时才自动生成，并且如果用户声明了移动操作，拷贝构造就是< em > delete< / em > 。拷贝赋值运算符仅当类没有显式声明拷贝赋值运算符时才自动生成，并且如果用户声明了移动操作，拷贝赋值运算符就是< em > delete< / em > 。当用户声明了析构函数，拷贝操作的自动生成已被废弃。< / li >
< li > 成员函数模板不抑制特殊成员函数的生成。< / li >
< / ul >
< / main >
< nav class = "nav-wrapper" aria-label = "Page navigation" >
<!-- Mobile navigation buttons -->
< a rel = "prev" href = "../3.MovingToModernCpp/item16.html" class = "mobile-nav-chapters previous" title = "Previous chapter" aria-label = "Previous chapter" aria-keyshortcuts = "Left" >
< i class = "fa fa-angle-left" > < / i >
< / a >
< a rel = "next" href = "../4.SmartPointers/item18.html" class = "mobile-nav-chapters next" title = "Next chapter" aria-label = "Next chapter" aria-keyshortcuts = "Right" >
< i class = "fa fa-angle-right" > < / i >
< / a >
< div style = "clear: both" > < / div >
< / nav >
< / div >
< / div >
< nav class = "nav-wide-wrapper" aria-label = "Page navigation" >
< a rel = "prev" href = "../3.MovingToModernCpp/item16.html" class = "nav-chapters previous" title = "Previous chapter" aria-label = "Previous chapter" aria-keyshortcuts = "Left" >
< i class = "fa fa-angle-left" > < / i >
< / a >
< a rel = "next" href = "../4.SmartPointers/item18.html" class = "nav-chapters next" title = "Next chapter" aria-label = "Next chapter" aria-keyshortcuts = "Right" >
< i class = "fa fa-angle-right" > < / i >
< / a >
< / nav >
< / div >
< script type = "text/javascript" >
window.playground_copyable = true;
< / script >
< script src = "../elasticlunr.min.js" type = "text/javascript" charset = "utf-8" > < / script >
< script src = "../mark.min.js" type = "text/javascript" charset = "utf-8" > < / script >
< script src = "../searcher.js" type = "text/javascript" charset = "utf-8" > < / script >
< script src = "../clipboard.min.js" type = "text/javascript" charset = "utf-8" > < / script >
< script src = "../highlight.js" type = "text/javascript" charset = "utf-8" > < / script >
< script src = "../book.js" type = "text/javascript" charset = "utf-8" > < / script >
<!-- Custom JS scripts -->
< / body >
< / html >
