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< title > Item 32:使用初始化捕获来移动对象到闭包中 - Effective Modern C++< / title >
2022-06-30 10:23:03 +08:00
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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" > 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.RRefMovSemPerf
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< h2 id = "条款三十二使用初始化捕获来移动对象到闭包中" > < a class = "header" href = "#条款三十二使用初始化捕获来移动对象到闭包中" > 条款三十二：使用初始化捕获来移动对象到闭包中< / a > < / h2 >
< p > < strong > Item 32: Use init capture to move objects into closures< / strong > < / p >
< p > 在某些场景下，按值捕获和按引用捕获都不是你所想要的。如果你有一个只能被移动的对象（例如< code > std::unique_ptr< / code > 或< code > std::future< / code > ） ， ， （ ） ， < / p >
< p > 但那是C++11的时候。到了C++14就另一回事了， ， ， ， < / p >
2022-11-25 13:43:31 +08:00
< p > 缺少移动捕获被认为是C++11的一个缺点， ， ， ， < strong > 初始化捕获< / strong > （ < em > init capture< / em > ） ， ， ， < a href = "../6.LambdaExpressions/item31.html" > Item31< / a > 说明提醒了你无论如何都应该远离默认捕获模式。（ ， ， ） < / p >
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< p > 使用初始化捕获可以让你指定：< / p >
< ol >
< li > 从lambda生成的闭包类中的< strong > 数据成员名称< / strong > ； < / li >
< li > 初始化该成员的< strong > 表达式< / strong > ； < / li >
< / ol >
< p > 这是使用初始化捕获将< code > std::unique_ptr< / code > 移动到闭包中的方法：< / p >
< pre > < code class = "language-c++" > class Widget { //一些有用的类型
public:
…
bool isValidated() const;
bool isProcessed() const;
bool isArchived() const;
private:
…
};
auto pw = std::make_unique< Widget> (); //创建Widget；
//的有关信息参见条款21
… //设置*pw
auto func = [pw = std::move(pw)] //使用std::move(pw)初始化闭包数据成员
{ return pw-> isValidated()
& & pw-> isArchived(); };
< / code > < / pre >
< p > 高亮的文本包含了初始化捕获的使用（译者注：高亮了“< code > pw = std::move(pw)< / code > ”），“< code > =< / code > ”的左侧是指定的闭包类中数据成员的名称，右侧则是初始化表达式。有趣的是，“< code > =< / code > ”左侧的作用域不同于右侧的作用域。左侧的作用域是闭包类，右侧的作用域和< em > lambda< / em > 定义所在的作用域相同。在上面的示例中，“< code > =< / code > ”左侧的名称< code > pw< / code > 表示闭包类中的数据成员，而右侧的名称< code > pw< / code > 表示在< em > lambda< / em > 上方声明的对象，即由调用< code > std::make_unique< / code > 去初始化的变量。因此，“< code > pw = std::move(pw)< / code > ”的意思是“在闭包中创建一个数据成员< code > pw< / code > ，并使用将< code > std::move< / code > 应用于局部变量< code > pw< / code > 的结果来初始化该数据成员”。< / p >
< p > 一般来说，< em > lambda< / em > 主体中的代码在闭包类的作用域内，因此< code > pw< / code > 的使用指的是闭包类的数据成员。< / p >
< p > 在此示例中，注释“设置< code > *pw< / code > ”表示在由< code > std::make_unique< / code > 创建< code > Widget< / code > 之后，< em > lambda< / em > 捕获到指向< code > Widget< / code > 的< code > std::unique_ptr< / code > 之前，该< code > Widget< / code > 以某种方式进行了修改。如果不需要这样的设置，即如果< code > std::make_unique< / code > 创建的< code > Widget< / code > 处于适合被< em > lambda< / em > 捕获的状态，则不需要局部变量< code > pw< / code > ，因为闭包类的数据成员可以通过< code > std::make_unique< / code > 直接初始化：< / p >
< pre > < code class = "language-c++" > auto func = [pw = std::make_unique< Widget> ()] //使用调用make_unique得到的结果
{ return pw-> isValidated() //初始化闭包数据成员
& & pw-> isArchived(); };
< / code > < / pre >
< p > 这清楚地表明了， ， ， < strong > 通用< em > lambda< / em > 捕获< / strong > （ < em > generalized lambda capture< / em > ）。< / p >
< p > 但是， ？ < / p >
< p > 请记住，< em > lambda< / em > 表达式只是生成一个类和创建该类型对象的一种简单方式而已。没什么事是你用< em > lambda< / em > 可以做而不能自己手动实现的。 那么我们刚刚看到的C++14的示例代码可以用C++11重新编写， ： < / p >
< pre > < code class = "language-c++" > class IsValAndArch { //“is validated and archived”
public:
using DataType = std::unique_ptr< Widget> ;
explicit IsValAndArch(DataType& & ptr) //条款25解释了std::move的使用
: pw(std::move(ptr)) {}
bool operator()() const
{ return pw-> isValidated() & & pw-> isArchived(); }
private:
DataType pw;
};
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auto func = IsValAndArch(std::make_unique< Widget> ())();
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< / code > < / pre >
< p > 这个代码量比< em > lambda< / em > 表达式要多， ， ， < / p >
< p > 如果你坚持要使用< em > lambda< / em > （ ， ） ， ： < / p >
< ol >
< li > < strong > 将要捕获的对象移动到由< code > std::bind< / code > 产生的函数对象中；< / strong > < / li >
< li > < strong > 将“被捕获的”对象的引用赋予给< em > lambda< / em > 。< / strong > < / li >
< / ol >
< p > 如果你熟悉< code > std::bind< / code > ，那么代码其实非常简单。如果你不熟悉< code > std::bind< / code > ，那可能需要花费一些时间来习惯它，但这无疑是值得的。< / p >
< p > 假设你要创建一个本地的< code > std::vector< / code > ， ， ， ： < / p >
< pre > < code class = "language-c++" > std::vector< double> data; //要移动进闭包的对象
… //填充data
auto func = [data = std::move(data)] //C++14初始化捕获
{ /*使用data*/ };
< / code > < / pre >
< p > 我已经对该代码的关键部分进行了高亮：要移动的对象的类型（< code > std::vector< double> < / code > ），该对象的名称（< code > data< / code > ）以及用于初始化捕获的初始化表达式（< code > std::move(data)< / code > ） ， ： < / p >
< pre > < code class = "language-c++" > std::vector< double> data; //同上
… //同上
auto func =
std::bind( //C++11模拟初始化捕获
[](const std::vector< double> & data) //译者注：本行高亮
{ /*使用data*/ },
std::move(data) //译者注：本行高亮
);
< / code > < / pre >
< p > 如< em > lambda< / em > 表达式一样，< code > std::bind< / code > 产生函数对象。我将由< code > std::bind< / code > 返回的函数对象称为< strong > bind对象< / strong > （ < em > bind objects< / em > ）。< code > std::bind< / code > 的第一个实参是可调用对象，后续实参表示要传递给该对象的值。< / p >
2022-11-25 13:43:31 +08:00
< p > 一个bind对象包含了传递给< code > std::bind< / code > 的所有实参的副本。对于每个左值实参， ， ， （ < code > std::move< / code > 的结果，请参见< a href = "../5.RRefMovSemPerfForw/item23.html" > Item23< / a > ），因此将< code > data< / code > 移动构造到绑定对象中。这种移动构造是模仿移动捕获的关键， < / p >
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< p > 当“调用”bind对象（ ） ， < code > std::bind< / code > 的可调用对象。在此示例中，这意味着当调用< code > func< / code > （ ） ， < code > func< / code > 中所移动构造的< code > data< / code > 副本将作为实参传递给< code > std::bind< / code > 中的< em > lambda< / em > 。< / p >
< p > 该< em > lambda< / em > 与我们在C++14中使用的< em > lambda< / em > 相同，只是添加了一个形参< code > data< / code > 来对应我们的伪移动捕获对象。此形参是对bind对象中< code > data< / code > 副本的左值引用。（这不是右值引用，因为尽管用于初始化< code > data< / code > 副本的表达式（< code > std::move(data)< / code > ）为右值，但< code > data< / code > 副本本身为左值。）因此，< em > lambda< / em > 将对绑定在对象内部的移动构造的< code > data< / code > 副本进行操作。< / p >
< p > 默认情况下，从< em > lambda< / em > 生成的闭包类中的< code > operator()< / code > 成员函数为< code > const< / code > 的。这具有在< em > lambda< / em > 主体内把闭包中的所有数据成员渲染为< code > const< / code > 的效果。但是， < code > data< / code > 副本不是< code > const< / code > 的，因此，为了防止在< em > lambda< / em > 内修改该< code > data< / code > 副本，< em > lambda< / em > 的形参应声明为reference-to-< code > const< / code > 。 如果将< em > lambda< / em > 声明为< code > mutable< / code > ，则闭包类中的< code > operator()< / code > 将不会声明为< code > const< / code > ，并且在< em > lambda< / em > 的形参声明中省略< code > const< / code > 也是合适的：< / p >
< pre > < code class = "language-c++" > auto func =
std::bind( //C++11对mutable lambda
[](std::vector< double> & data) mutable //初始化捕获的模拟
{ /*使用data*/ },
std::move(data)
);
< / code > < / pre >
< p > 因为bind对象存储着传递给< code > std::bind< / code > 的所有实参的副本， ， < em > lambda< / em > 生成的闭包副本，这是它的第一个实参。 因此闭包的生命周期与bind对象的生命周期相同。 这很重要， ， < / p >
< p > 如果这是你第一次接触< code > std::bind< / code > ， ， < / p >
< ul >
< li > 无法移动构造一个对象到C++11闭包， < / li >
< li > 在C++11中模拟移动捕获包括将对象移动构造进bind对象， < em > lambda< / em > 。< / li >
< li > 由于bind对象的生命周期与闭包对象的生命周期相同， < / li >
< / ul >
< p > 作为使用< code > std::bind< / code > 模仿移动捕获的第二个示例，这是我们之前看到的在闭包中创建< code > std::unique_ptr< / code > 的C++14代码： < / p >
< pre > < code class = "language-c++" > auto func = [pw = std::make_unique< Widget> ()] //同之前一样
{ return pw-> isValidated() //在闭包中创建pw
& & pw-> isArchived(); };
< / code > < / pre >
< p > 这是C++11的模拟实现： < / p >
< pre > < code class = "language-c++" > auto func = std::bind(
[](const std::unique_ptr< Widget> & pw)
{ return pw-> isValidated()
& & pw-> isArchived(); },
std::make_unique< Widget> ()
);
< / code > < / pre >
2022-11-25 13:43:31 +08:00
< p > 具备讽刺意味的是，这里我展示了如何使用< code > std::bind< / code > 解决C++11 < em > lambda< / em > 中的限制，因为在< a href = "../6.LambdaExpressions/item34.html" > Item34< / a > 中，我主张使用< em > lambda< / em > 而不是< code > std::bind< / code > 。但是， < code > std::bind< / code > 可能有用，这就是其中一种。 （ ， < code > auto< / code > 形参等特性使得这些情况不再存在。）< / p >
2022-06-30 10:23:03 +08:00
< p > < strong > 请记住：< / strong > < / p >
< ul >
< li > 使用C++14的初始化捕获将对象移动到闭包中。< / li >
< li > 在C++11中， < code > std::bind< / code > 的方式来模拟初始化捕获。< / li >
< / ul >
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