Why can't a data member be in a lambda capture list

Question

I have a class foo that has bar as a member variable.

In another member function of the class, I'm writing a lambda function:

[bar](void){}

But I can't include bar in the capture list. Why is that?

Answer 1

Only objects with automatic storage duration can be captured by a lambda in C++11 (ie local variables and function parameters). If you want the effect of capturing a non-static class data member, you can either capture the this pointer as in Danvil's answer :

auto f = [this]{ std::cout << a << std::endl; };

or cache the data member's value in a local variable and capture that:

auto a = this->a;
auto f = [a]{ std::cout << a << std::endl; };

which will be much more concise in C++14:

auto f = [a = this->a]{ std::cout << a << std::endl; };

The choice between these two options depends on whether you want to store the value a has right now or if you want to retrieve the value a has when the lambda is called . Note that in the case where this is captured you must ensure that the lifetime of the pointer object encloses the lifetime of the lambda a call to the lambda after the object is destroyed has undefined behavior. The more simple case that captures a copy of a is completely self-contained and has no such lifetime issues.

Answer 2

You capture class members by saying this in the capture list. This has nothing to do with the fact that the member is const .

---

Example:

#include <iostream>

struct Foo
{
    const int a = 0;
    int b;
    Foo() : b{42} {
        auto f = [this]() { std::cout << a << " " << b << std::endl; };
//                ^^^^ 
        f();
    }
};

int main() {
    Foo x;
}