What's a good way to capture member variable by value in c++11? [duplicate]

Question

struct myclass {
    myclass(){}
    myclass(int qx):z(qx){   }
    std::function<void()> create() {
        auto px = [z](){
            std::cout << z << std::endl;
        };
        return px;
    }
    int z;
};

myclass my;
my.z = 2;
auto func = my.create();
func();
my.z = 3;
func();

This code will compile in gcc 4.6.3, which will do the correct thing to make a copy of member variable z, and both print will get 2. In gcc 4.8.2 this doesn't compile anymore..

error: 'this' was not captured for this lambda function

I wonder why this functionality was removed as it was quite useful.

Answer 1

I don't think you can do this directly in C++11 (see @Nawaz's comment for a workaround). However, C++14 helps exactly in this instance, via generalized lambda captures:

auto px = [v = this->z]() // capture by value just ONE member variable
{ 
        std::cout << v << std::endl;
};

Example:

#include <functional>
#include <iostream>

struct myclass {
    myclass(): z{} {}
    myclass(int qx): z(qx) {   }
    std::function<void()> create() {
        auto px = [v = this->z]() {
            std::cout << v << std::endl;
        };
        return px;
    }
    int z;
};

int main()
{
    myclass my;
    my.z = 2;
    auto func = my.create();
    func();
    my.z = 3;
    func();
}

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