decltype on a member that might not exist

September 22, 2023

I am looking for a way to define a variable with a type depending on the type of the member of a type my class is templated on, with the additional caveat that the member variable might not exist. As I only access the variable in constexpr blocks if the member does exist, I don’t care about declaring it or what type it is otherwise. However, I learned that typename conditional<hasMember<T>, decltype(T::member), int> does not work, as the not-used branch still needs to compile.

Here is an example of what I was hoping to get to work:

#include <iostream>
using namespace std;

struct X{};
struct Y{string member;};

template<class T>
concept hasMember = requires (T t) {t.member;};

template<class T>
struct A{
    void hi(T a) {
            // stuff
           typename conditional<hasMember<T>, decltype(T::member), int /*member is unused if hasMember<T> is false, so I don't care what type it is or if it exists*/>::type member;
           
           if constexpr (hasMember<T>){
               member = a.member;
           }
            // stuff
           if constexpr (hasMember<T>){
               std::cout << member<< std::endl;
           }
    };
};

int main() {
    X x;
    Y y{"hi"};
    
    // Does not compile 
    // A<X> ax;
    // ax.hi(x);
    
    // Compiles and runs fine
    A<Y> ay;
    ay.hi(y);

    return 0;
}

>Solution :

You could create an old-school type trait to get the type instead:

template<class T, class O>
struct member_or {
    static O test(...);

    template<class U>
    static auto test(U) -> decltype(U::member);

    using type = decltype(test(std::declval<T>()));
};
template<class T, class O>
using member_or_t = member_or<T, O>::type;

Your A implementation could then be:

template <class T>
struct A {
    void hi(T a) {
        // stuff
        member_or_t<T, int> member;    // <- now no problem

        if constexpr (hasMember<T>) {
            member = a.member;
        }
        // stuff
        if constexpr (hasMember<T>) {
            std::cout << member << std::endl;
        }
    };
};