#include <iostream>

#include <type_traits>

class E { public: template<class T> E(T&&) { } };

int main()

{

class A {};

class B : public A {};

class C {};

class D { public: operator C() { return c; } C c; };

bool b2a = std::is_convertible<B*, A*>::value;

bool a2b = std::is_convertible<A*, B*>::value;

bool b2c = std::is_convertible<B*, C*>::value;

bool d2c = std::is_convertible<D, C>::value;

// 完美转发构造函数使类能从任何类型转换

bool everything2e = std::is_convertible<A, E>::value; //< B, C, D 等

std::cout << std::boolalpha;

std::cout << b2a << '\n';

std::cout << a2b << '\n';

std::cout << b2c << '\n';

std::cout << d2c << '\n';

std::cout << '\n';

std::cout << everything2e << '\n';

}