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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?
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Friend Keyword in C++ But, to declare any class as a friend class, you do it with the friend keyword. You can use the friend keyword to any class to declare it as a friend class. This keyword enables any class to access private and protected members of other classes and functions.
A function or class can't declare itself as a friend of any class. In a class definition, use the friend keyword and the name of a non-member function or other class to grant it access to the private and protected members of your class. In a template definition, a type parameter can be declared as a friend .
You can declare an entire class as a friend. Suppose class F is a friend of class A . This means that every member function and static data member definition of class F has access to class A .
We can also make a function of one class as a friend of another class. We do this in the same way as we make a function as a friend of a class. The only difference is that we need to write class_name :: in the declaration before the name of that function in the class whose friend it is being declared.
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.
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