Classes can be anonymous — that is, they can be declared without an identifier. This is useful when you replace a class name with a typedef name, as in the following:

typedef struct  
{  
    unsigned x;  
    unsigned y;  
} POINT;  

Anonymous classes are also useful when you want a reference to a class member to appear as though it were not contained in a separate class, as in the following:

struct PTValue  
{  
    POINT ptLoc;  
    union  
    {  
        int  iValue;  
        long lValue;  
    };  
};  
  
PTValue ptv;  

In the preceding code, iValue can be accessed using the object member-selection operator (.) as follows:

int i = ptv.iValue;  

Anonymous classes are subject to certain restrictions. (For more information about anonymous unions, see Unions.) Anonymous classes:

• Cannot have a constructor or destructor.

• Cannot be passed as arguments to functions (unless type checking is defeated using ellipses).

• Cannot be returned as return values from functions.

A Microsoft C extension allows you to declare a structure variable within another structure without giving it a name. These nested structures are called anonymous structures. C++ does not allow anonymous structures.

You can access the members of an anonymous structure as if they were members in the containing structure.

// anonymous_structures.c  
#include <stdio.h>  
  
struct phone  
{  
    int  areacode;  
    long number;  
};  
  
struct person  
{  
    char   name[30];  
    char   gender;  
    int    age;  
    int    weight;  
    struct phone;    // Anonymous structure; no name needed  
} Jim;  
  
int main()  
{  
    Jim.number = 1234567;  
    printf_s("%d\n", Jim.number);     
}  
//Output: 1234567