Check if member exists via std::enable_if (SFINAE)

memberDetector.cpp

#include <iostream>
#include <string>
#include <type_traits>
#include <vector>

using StringVec = std::vector<std::string>;

namespace
{

struct Fallback{};
struct TryExistenceCheckOverloadFirst : public Fallback{};

template<typename T> struct MemberDetector : public std::true_type {};

template<typename T, typename U,
    std::enable_if_t<
        MemberDetector<decltype(T::refs)>::value and
        std::is_same<decltype(T::refs), StringVec>::value
    , int> = 0>
void mapMultipleRefs(T obj, std::vector<U>& acc, TryExistenceCheckOverloadFirst)
{
    std::cout << "detected plural refs (new meta)" << std::endl;
}
template<typename T, typename U>
void mapMultipleRefs(T obj, std::vector<U> acc, Fallback)
{
    std::cout << "detected NO plural refs (deprecated)" << std::endl;
}

template<typename T, typename U,
    std::enable_if_t<
        MemberDetector<decltype(T::ref)>::value and
        std::is_same<decltype(T::ref), std::string>::value
    , int> = 0>
void mapSingleRef(T obj, std::vector<U>& acc, TryExistenceCheckOverloadFirst)
{
    std::cout << "detected singular ref (deprecated)" << std::endl;
}
template<typename T, typename U>
void mapSingleRef(T obj, std::vector<U>& acc, Fallback)
{
    std::cout << "detected NO singular ref (new meta)" << std::endl;
}

}

struct A0
{
    std::string ref;
};
struct A1
{
    std::string ref;
    std::vector<std::string> refs;
};
struct A2
{
    std::vector<std::string> refs;
};
struct B
{};

int main()
{
    std::vector<B> acc;

    ::mapSingleRef(A0{}, acc, TryExistenceCheckOverloadFirst{});
    ::mapMultipleRefs(A0{}, acc, TryExistenceCheckOverloadFirst{});

    ::mapSingleRef(A1{}, acc, TryExistenceCheckOverloadFirst{});
    ::mapMultipleRefs(A1{}, acc, TryExistenceCheckOverloadFirst{});

    ::mapSingleRef(A2{}, acc, TryExistenceCheckOverloadFirst{});
    ::mapMultipleRefs(A2{}, acc, TryExistenceCheckOverloadFirst{});

    return 0;
}