Question

The g++ compiler complains with this error when I declare a friend thusly:

friend MyClass; 

instead of

friend class MyClass; 

Why should the class keyword be required? (the Borland C++ compiler, BTW, does not require it.)

Couldn’t the compiler simply look-up MyClass in the symbol table and tell it was declared as a class? (it is obviously doing the look-up anyway because it complains when MyClass it not declared)

It is not like it is making a forward declaration of the class: I still have to have either declared the class above or at least have forward declared it.

It would make sense to me (would be great actually) if

friend class MyClass; 

makes a forward declaration if needed, otherwise it just seems like syntactic salt to me.

I have been merrily using friend statements without the class or struct keyword with no compiler complaints for almost 20 years. Is this something fairly new?

Answer 1

I was surprised about this (and as a result deleted a previous incorrect answer). The C++03 standard says in 11.4:

An elaborated-type-specifier shall be used in a friend declaration for a class.

Then to make sure there’s no misunderstanding, it footnotes that with:

The class-key of the elaborated-type-specifier is required.

GCC is the only compiler that I have that complains about the missing class-key, but it looks like other compilers are letting us get away with something non-standard…

Now as for the rationale – you’d have to ask someone who knows more about compilers (or standards) than I do.