Basic data member detection in C++17

data_member_detection.cpp

#include <type_traits>

#define HAS_MEMBER(member) \
template <typename T, typename = void> \
struct has_##member : std::false_type {}; \
template <typename T> \
struct has_##member<T, std::void_t<decltype(std::declval<T>().member)>> : std::true_type {};

struct WithBla {
    int bla;
};

struct NoBla {
};

HAS_MEMBER(bla)

#include <iostream>

struct MyStruct {
    template<typename A, std::enable_if_t<has_bla<A>::value, int> = 0>
    MyStruct(A& par) {
        std::cout << "To bla: " << par.bla << '\n';
    }

    template<typename A, std::enable_if_t<!has_bla<A>::value, int> = 0>
    MyStruct(A& par) {
        std::cout << "or not to bla?" << '\n';
    }
};

int main(int argc, char *argv[])
{
    WithBla wb{42};
    NoBla nb;

    MyStruct a(wb);
    MyStruct b(nb);

    return 0;
}