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

Many std::function implementations will avoid allocations and use space inside the function class itself rather than allocating if the callback it wraps is "small enough" and has trivial copying. However, the standard does not require this, only suggests it.

On g++, a non-trivial copy constructor on a function object, or data exceeding 16 bytes, is enough to cause it to allocate. But if your function object has no data and uses the builtin copy constructor, then std::function won't allocate. Also, if you use a function pointer or a member function pointer, it won't allocate.

While not directly part of your question, it is part of your example. Do not use std::bind. In virtually every case, a lambda is better: smaller, better inlining, can avoid allocations, better error messages, faster compiles, the list goes on. If you want to avoid allocations, you must also avoid bind.