covariance and contravariance in std::function

std_function_variance.cpp

// Covariance and contra-variance in std::function
// See
// https://quuxplusone.github.io/blog/2019/01/20/covariance-and-contravariance/

#include <functional>

struct B {};
struct D : B {};

// std::function is covariant on return types.
//  B -----> B(void)
//  ^          ^
//  |          |
//  D -----> D(void)

B give_b();
D give_d();

std::function<B(void)> co = give_d;
// std::function<D(void)> contra_bad = give_b;

// std::function is contravariant on parameters.
//  B -----> void(B)
//  ^          |
//  |          V
//  D -----> void(D)

void take_b(B);
void take_d(D);

std::function<void(D)> contra = take_b;
// std::function<void(B)> co_bad = take_d;