Anonymous Class Types

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;

Note

The use of anonymous classes shown in the previous example is useful for preserving compatibility with existing C code. In some C code, the use of typedef in conjunction with anonymous structures is prevalent.

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.

Anonymous structs

Microsoft Specific

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

END Microsoft Specific