using out of scope variables in C++11 lambda expressions

Question

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();});

Compilation terminates with this error:

error: 'requestPacket' has not been declared

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

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

Answer 1

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();});

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();});

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";
}