Declaring classes in function declarations

C++ allows you to declare classes in function declarations. The following code is fully standards-compliant:

void foo (class A*){}

A* g_a;

int main() {
    A* a;
    foo(a);
}

I thought this was pretty odd and interesting, so went searching for the applicable rules from the standard. Here is the breakdown on how this works.

class A or struct A is called an elaborated-type-specifier.

[…] If the elaborated-type-specifier is introduced by the class-key and this lookup does not find a previously declared type-name, or if the elaborated-type-specifier appears in a declaration with the form:

class-key attribute-specifier-seqopt identifier ;

the elaborated-type-specifier is a declaration that introduces the class-name as described in 3.3.2.

The point of declaration of a class first declared in an elaborated-type-specifier is as follows:

• […]
• for an elaborated-type-specifier of the form

class-key identifier

if the elaborated-type-specifier is used in the decl-specifier-seq or parameter-declaration-clause of a function defined in namespace scope, the identifier is declared as a class-name in the namespace that contains the declaration; otherwise, except as a friend declaration, the identifier is declared in the smallest namespace or block scope that contains the declaration.

Don’t do this.