Preprocessor Directive Question

[zb0S216C]

In the sys/types.h header, I've noticed some preprocessor directive defines( #define ) that haven't been assigned a value. Here's what I'm on about if you don't already:

#define __need_wchar_t
#define __need_size_t
#define __need_ptrdiff_t  

If a preprocessor define directive hasn't been assigned a value, what does it do? What can I use it for?

[filipe]

You use it to check if some condition is present at compile time:

#ifdef __need_wchar_t
// do something related to the need for wchar_t
#endif 

You also use that form of define for include guards:

#ifndef MY_HEADER_H
#define MY_HEADER_H
// ...
#endif 

which allows you to avoid including the same header more than once into a translation unit.

[quirkyusername]

#defines arent always used to define constants, doing this just defines those values, perhaps so that they can be used for conditional compilation. For example when I make a project in visual studio, running on windows, and check the project properties under c++ -> preprocessor, _DEBUG and WIN32 are defined. when in release NDEBUG and WIN32 are defined. You can define your own things too then check whether they're defined

#ifdef _DEBUG
....
#endif 

so that the project is compiled differently depending on the solution configuration or platform

[zb0S216C]

Thanks for your replies.