How can I store a lambda expression as a field of a class in C++11?

Question

I\'d like to create a class where the client can store a lambda expression like []() -> void {} as a field of the class, but I can\'t figure out how to do so

Answer 1

In case of void () simple function pointer can be used (declaration syntax is void (*pointer_name) ();). Lambda with empty capture block is implicitly convertible to function pointer with same signature. And std::function have runtime and size (std::function is at least three times larger) overhead.

struct S
{
  void (*f_p)() {}; // `{}` means `= nullptr`;
};

int main()
{
    S s { [] { std::cout << "Lambda called\n"; }};

    s.f_p();

    S s2;
    if (s2.f_p) // check for null
        s.f_p();

    s2.f_p = [] { std::cout << "Lambda2 called\n"; };
    s2.f_p();

    s2.f_p = std::terminate; // you can use regular functions too

    s2.f_p();
}

Output

Lambda called
Lambda2 called
terminate called without an active exception