"friend Foo" instead of "friend class Foo"?

[Peter]

Hi.

I remember a code snippet from "Thinking in C++" where "class" and
"struct" keywords were missing from a friend declaration. The example
was more or less like this (it came from a chapter about nested
classes and "friend" keyword):

struct Boo
{
struct Foo;
friend Foo; // neither "struct" nor "class" after "friend"
struct Foo{};
};

Tested with an older version of g++ it resulted in the following
errors:

"error: a class-key must be used when declaring a friend"
"error: friend declaration does not name a class or function"

However, Comeau handled the above fine. Which result is standard
compliant in this case? I think an elaborated specifier is usually
required in friend declarations, but I read somewhere there's an
exception in presence of forward declarations like in the code above.
What are the precise rules? When exactly can "struct/class" keywords
be omitted in friend declarations and how do the most recent versions
of g++ and MSVC handle this (do they obey the standard or not)?

 

[Victor Bazarov]

I remember a code snippet from "Thinking in C++" where "class" and
"struct" keywords were missing from a friend declaration. The example
was more or less like this (it came from a chapter about nested
classes and "friend" keyword):

struct Boo
{
struct Foo;
friend Foo; // neither "struct" nor "class" after "friend"
struct Foo{};
};

Tested with an older version of g++ it resulted in the following
errors:

"error: a class-key must be used when declaring a friend"
"error: friend declaration does not name a class or function"

However, Comeau handled the above fine. Which result is standard
compliant in this case? I think an elaborated specifier is usually
required in friend declarations, but I read somewhere there's an
exception in presence of forward declarations like in the code above.
What are the precise rules? When exactly can "struct/class" keywords
be omitted in friend declarations and how do the most recent versions
of g++ and MSVC handle this (do they obey the standard or not)?

[class.friend]/3 allows it to be a simple-type-specifier. IOW, there
does not have to be 'class' or 'struct' there, if the type has already
been declared (like in the case of yours).

V

 

[Öö Tiib]

I remember a code snippet from "Thinking in C++" where "class" and
"struct" keywords were missing from a friend declaration. The example
was more or less like this (it came from a chapter about nested
classes and "friend" keyword):


struct Boo
{
struct Foo;
friend Foo; // neither "struct" nor "class" after "friend"
struct Foo{};
};

Looks like valid code.

Note that the nested class 'Boo::Foo' (as member class) has currently
access to other members of 'Boo'. IOW that 'friend Foo;' does not grant
it anything that it does not already have.