Avoid memory allocation with std::function and member function

Question

This code is just for illustrating the question.

#include 
struct MyCallBack {
    void Fire() {
    }
};

int main()
{
    MyCallBack cb;
             


        

Answer 1

As an addendum to the already existent and correct answer, consider the following:

MyCallBack cb;
std::cerr << sizeof(std::bind(&MyCallBack::Fire, &cb)) << "\n";
auto a = [&] { cb.Fire(); };
std::cerr << sizeof(a);

This program prints 24 and 8 for me, with both gcc and clang. I don't exactly know what bind is doing here (my understanding is that it's a fantastically complicated beast), but as you can see, it's almost absurdly inefficient here compared to a lambda.

As it happens, std::function is guaranteed to not allocate if constructed from a function pointer, which is also one word in size. So constructing a std::function from this kind of lambda, which only needs to capture a pointer to an object and should also be one word, should in practice never allocate.