Question

Motivated by the discussion

The grammar for C++ classes is defined as

class-key identifier *[opt]* base-clause *[opt]* (Italics are mine)

This to me means that the class name is option and we can have unnamed classes in C++.

So, is the following well-formed?

struct X{
   struct{
      int x;
      int y;
   };
};

int main(){}

VS: error C2467: illegal declaration
of anonymous ‘struct’

Comeau online: error: declaration does
not declare anything
struct{

GCC(ideone): Compiles fine

Any thoughts?

Answer 1

No, it is not well-formed. You cannot derive the language syntax from these grammar statements alone. The extra requirements are given in the text of the standard also have to be taken into account. In this case that would be

7 Declarations

…

3 In a simple-declaration, the optional init-declarator-list can be
omitted only when declaring a class
(clause 9) or enumeration (7.2), that
is, when the decl-specifier-seq
contains either a class-specifier, an
elaboratedtype-specifier with a
class-key (9.1), or an enum-specifier.
In these cases and whenever a
class-specifier or enum-specifier is
present in the decl-specifier-seq, the
identifiers in these specifiers are
among the names being declared by the
declaration (as class-names,
enum-names, or enumerators, depending
on the syntax). In such cases, and
except for the declaration of an
unnamed bit-field (9.6), the
decl-specifier-seq shall introduce one
or more names into the program, or
shall redeclare a name introduced by a
previous declaration.

The last sentence is the one that matters in this case

The "optional" part is only there to allow declarations like

struct { int x; } s;
typedef struct { int x, y; } Point;

The first one which declares a class type with no linkage and variable s of that type. Note that types with no linkage cannot be used to declare a variable with linkage, meaning that such declaration cannot be used in namespace scope.

Your example is ill-formed, but this would be legal

struct X {
   struct {
      int x;
      int y;
   } point;
};

Also, nameless class syntax is used to declare anonymous unions (although I’m a bit puzzled by the fact that 7/3 does not mention anonymous unions).