Why are structure names different from their typedef names?

Raymond Chen

In Windows header files, many structures are declared like this:

typedef struct tagXYZ {
 ...
} XYZ;
typedef struct _XYZ {
 ...
} XYZ;
/* there are other variations, too */

Why is the structure name different from typedef name?

This is a holdover from very early versions of the C language where structure tags, union tags, and typedefs were kept in the same namespace. Consequently, you couldn’t say typedef struct XYZ { ... } XYZ;. At the open brace, the compiler registers XYZ as a structure tag name, and then when XYZ appears a second time, you get a redeclaration error. The standard workaround for this was to make the structure tag name a minor modification of the typedef name, most typically by putting the word tag in front.

The C language standardization process separated the structure and typename name spaces, so this workaround is no longer necessary, but it doesn’t hurt either. Besides, even if new structures followed the typedef struct XYZ { ... } XYZ; pattern, you would just have people asking, “Why do some structures in winuser.h use the tagXYZ pattern and others use the XYZ pattern? Why can’t it just be consistent?”

Next time, why you also don’t see the pattern typedef struct { ... } XYZ very much either.

Raymond Chen

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