decltype(*this) equivalent outside function body

Question

Is it possible to write a trait, which results the type of the class it is used in? How to implement get_class in the example below?

class Foo {
    typedef get_class::type type; // type = Foo now
}; 

Note: I have to write a macro, which expands in the class body, used for multiple classes, so I cannot simply write 'typedef Foo type;'

Use case:

I have a reflectable(...) macro which generates infrastructure to iterate over members, visit them and look them up using their names:

class Foo 
{
    friend std::ostream &operator<<(std::ostream &, const Foo&);
    reflectable(
         (int) bar,
         (double) baz
    )
}

reflectable(...) should be a macro so I can get the types and member names separately as strings to build maps for the lookup.

I would like all reflectable class to be streamable, but if I put my reflectable() macro to private section, I have to add the friend declaration to the class. I would like to move it to the macro as:

friend std::ostream &operator<<(std::ostream &, const get_class::type&);

Answer 1

I'm sorry, but I'm pretty sure that in standard C++ there is no way around you having to pass in the class name to the macro one way or another. For some failed attempts, see the end of this answer.

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That said, I think your best bet is to slightly change your design. Instead of

friend std::ostream &operator<<(std::ostream &, const get_class::type&);

provide a public member function

void stream_to (std::ostream &) const {
  // Implementation
}

together with a free function template

template<typename T, typename std:: enable_if<has_stream_to<T>::value>::type * = nullptr>
std::ostream &operator<<(std::ostream & s, T const & t) {
  t.stream_to(s);
  return s;
}

in your libraries/programs namespace. (Note: I'll add the trait has_stream_to later, for the impatient search for "detect member function" within C++ tag)

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Approaches that don't work:

• Use a pointer to member function, extract the class type with a template function. Reason: &foo inside of a class with a member function foo won't give a member function pointer to foo. The syntax is required (by the standard) to be &C::foo (where C is the class name...).

• Use a pointer to data member, extract the class type with a template function. Same reason as above.

• Use a dedicated member function that returns *this, deduce the return type to get the class type. Reason: Need an instance to call this function on.

• Use a pointer to a static member function. Reason: Such pointers are same as pointers to free functions, the class name cannot be deduced from them.

• Use the this pointer in a member initialisation. Reason: type of member needs to encode class type somehow, but auto is not allowed for non static data members.