a class-key must be declared when declaring a friend

c++ friend
Roger Nelson picture Roger Nelson · Mar 18, 2009 · Viewed 14.3k times · Source

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

Michael Burr picture Michael Burr · Mar 18, 2009

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.

Related questions

When should you use 'friend' in C++?

I have been reading through the C++ FAQ and was curious about the friend declaration. I personally have never used it, however I am interested in exploring the language. What is a good example of using friend? Reading the FAQ …

Read More
Error with multiple definitions of function

I am trying to relearn C++ after taking an intro course a few years ago and I’m having some basic problems. My current problem occurs when trying to use a friend function. Here is my code in 2 files. First: // …

Read More
Is there a way to simulate the C++ 'friend' concept in Java?

I would like to be able to write a Java class in one package which can access non-public methods of a class in another package without having to make it a subclass of the other class. Is this possible?

Read More