Anonymous Class Types
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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