has_public_data_member, has_public_member_function?

[baryluk]

It can be probably implemented in a library, not in the base standard, but I found no way to implement it (using get_public_member_functions), after spending 3 hours trying.

I want just:

  if constexpr (std::reflect::has_public_member_function<reflect(declype(a))>("foo")) {
     a.foo(5);
  }

or something equivalently short.

D language has this using:

  static if (__traits(hasMember, a, "foo")) {
    a.foo(x);
  }

and

  static if (__traits(compiles, a.foo(0))) {
    a.foo(x);
  }

(there is also a library helper for the first one:

  static if (std.traits.hasMember!(typeof(a), "foo")) {
    a.foo(x);
  }

)

See https://dlang.org/spec/traits.html#hasMember and https://dlang.org/phobos/std_traits.html#hasMember

This can be done in C++ with contepts, but the problem with concept is that it must be first defined out of the place it is used, making the usage harder:

template <class T>
concept HasFoo = requires (T t) { t.foo(0); };

/// some other code


  if constexpr (HasFoo<decltype(a)>) {
    a.foo(x);
  }

which is far from acceptable. This is because concepts cannot be defined locally in functions.

So I believe this deserves specialization, or at least a defined utility to extract by name from ObjectSequence (in case of overloaded functions in a aggregate type, would be fine to emit compile error).

[baryluk]

Actually maybe this could work:

  if constexpr (requires(T& t) {t.foo(0);}) {
    a.foo(x);
  }

or even

  if constexpr (requires{a.foo(0);}) {
    a.foo(x);
  }

or inline using requires requires