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

Run this little hack and it probably will print the amount of bytes you can capture without allocating memory:

#include 
#include 
#include 

void h(std::function&& f, void* g)
{
  f(g);
}

template
void do_test()
{
  size_t a[number_of_size_t];
  std::memset(a, 0, sizeof(a));
  a[0] = sizeof(a);

  std::function g = [a](void* ptr) {
    if (&a != ptr)
      std::cout << "malloc was called when capturing " << a[0] << " bytes." << std::endl;
    else
      std::cout << "No allocation took place when capturing " << a[0] << " bytes." << std::endl;
  };

  h(std::move(g), &g);
}

int main()
{
  do_test<1>();
  do_test<2>();
  do_test<3>();
  do_test<4>();
}

With gcc version 8.3.0 this prints

No allocation took place when capturing 8 bytes.
No allocation took place when capturing 16 bytes.
malloc was called when capturing 24 bytes.
malloc was called when capturing 32 bytes.