CODEWITHSUNDEEP
c++classc++11unions
user751747
user751747

Reputation: 1093

a point from ISO document: based on Anonymous Unions

Point from ISO C++ Standard: section 9.5, para 4, line 1:

      "A union for which objects or pointers are declared is not
       an anonymous union."

Example : 

 struct X {
        union {
               int i;
               double d;
             } ;
        int f () { return i;}      
       };
   int main() { return 0; }

IAm expecting an error from this example according to the above point

but GCC,SUN compiler CC,EDG,etc are not showing error

iam expecting this error// error : cannot directly access "i"

please ..conform above example program is correct are wrong

Upvotes: 3

Views: 101

Answers (2)

Nicol Bolas
Nicol Bolas

Reputation: 473352

To add to what Alf is saying, the purpose of the anonymous union language in the C++ spec is to allow scoping of the unions members. If you have a named union inside a struct:

struct X {
    union {
        int i;
        double d;
    } varname;
};

i is not a member of the struct X. i is a member of varname, which itself is a member of struct X.

However, if the union doesn't have a member variable declared, then i would have to be accessed directly as a member of X. This can only work if the union has no name (no variables are declared with the union definition). Such unions are called "anonymous unions".

Upvotes: 3

Cheers and hth. - Alf
Cheers and hth. - Alf

Reputation: 145259

This would make the union not anonymous:

struct X {
    union {
        int i;
        double d;
    } *p;
    int f () { return i;}      // !Nyet.
};

Cheers & hth.,

Upvotes: 4

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