How to check if a member name (variable or function) exists in a class, with or without specifying type?

Question

This Q is an extension of:
Is it possible to write a C++ template to check for a function\'s existence?

Is there any utility which will help to find:

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Answer 1

With std::experimental::is_detected and std::experimental::disjunction you could do this:

//check for a type member named foo
template 
using foo_type_t = typename T::foo;

//check for a non-type member named foo
template 
using foo_non_type_t = decltype(&T::foo);

template 
using has_foo = disjunction,
                            is_detected>;

Then you would use has_foo::value in whatever you want.

The above will work for more than just types and member functions, but you could easily constrain it by using traits like std::is_member_function_pointer and std::is_member_object_pointer if you like.

To supply your optional argument, you could use the std::experimental::is_detected_exact helper.

Live Demo

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Note that if you take the implementations of the above traits from the pages I linked, you can use this with C++14. A minor change to the disjunction code will let you use it in C++11.