4.7 Using Anonymous struct Declarations

An anonymous struct declaration is a declaration that declares neither a tag for the struct, nor an object or typedef name. Anonymous structs are not allowed in C++.

The -features=extensions option allows the use of an anonymous struct declaration, but only as member of a union.

The following code is an example of an invalid anonymous struct declaration that compiles when you use the -features=extensions option.

union U {
  struct {
    int a;
    double b;
  };  // invalid: anonymous struct
  struct {
    char* c;
    unsigned d;
  };  // invalid: anonymous struct
};

The names of the struct members are visible without qualification by a struct member name. Given the definition of U in this code example, you can write:

U u;
u.a = 1;

Anonymous structs are subject to the same limitations as anonymous unions.

Note that you can make the code valid by giving a name to each struct, such as:

union U {
  struct {
    int a;
    double b;
  } A;
  struct {
    char* c;
    unsigned d;
  } B;
};
U u;
U.A.a = 1;