在C ++ 11 lambda表达式中使用超出范围的变量 [英] using out of scope variables in C++11 lambda expressions

问题描述

我正在玩C ++ 11。我想知道为什么会这样：

I'm playing with C++11 for fun. I'm wondering why this happens:

//...
std::vector<P_EndPoint> agents;
P_CommunicationProtocol requestPacket;
//...
bool repeated = std::any_of(agents.begin(), agents.end(),
                    [](P_EndPoint i)->bool 
                    {return requestPacket.identity().id()==i.id();});

编译会因以下错误而终止：

Compilation terminates with this error:

error: 'requestPacket' has not been declared

已声明较早的代码。我尝试了 :: requestPacke ，它也无法正常工作。

Which is declared earlier in code. I tried ::requestPacke and it doesn't worked too.

如何在内部使用外部范围变量Lambda函数？

How can I use an external scope variable inside a lambda function?

推荐答案

您需要变量，按值（使用 [=] 语法）

You need to capture the variable, either by value (using the [=] syntax)

bool repeated = std::any_of(agents.begin(), agents.end(),
                    [=](P_EndPoint i)->bool                          
                    {return requestPacket.identity().id()==i.id();});

或通过引用（使用 [&] 语法）

or by reference (using the [&] syntax)

bool repeated = std::any_of(agents.begin(), agents.end(),
                    [&](P_EndPoint i)->bool 
                    {return requestPacket.identity().id()==i.id();});

请注意，正如@aschepler指出的那样，未捕获具有静态存储持续时间的全局变量，仅捕获了函数级变量：

Note that as @aschepler points out, global variables with static storage duration are not captured, only function-level variables:

#include <iostream>

auto const global = 0;

int main()
{
    auto const local = 0;

    auto lam1 = [](){ return global; }; // global is always seen
    auto lam2 = [&](){ return local; }; // need to capture local

    std::cout << lam1() << "\n";
    std::cout << lam2() << "\n";
}

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