I have spotted something in C header files what I can't figure out what is for. For example in file bits/socket.h
there is an enumeration type enum __socket_type
, but after every enumerator there is a define macro which defines the same. Example:
enum __socket_type
{
SOCK_STREAM = 1,
#define SOCK_STREAM SOCK_STREAM
...
};
I have been unable to find out what this is for. Please enlighten me. I don't even know how to form right question for querying google nor this site search box.
A prepreprocessor macro will never expand recursively, so what such a #define
does is leave the name in place whereever it is used. Such things are useful when you want to have a preprocessor feature test.
#ifdef SOCK_STREAM
..
#endif
can be used to conditionally compile some code afterwards.
Edit: So this combines the cleaner approach of enumerations (implicit values without collisions and scoping) with preprocessor tests.
The only thing I can think of is because people see a constant in all-caps, say NUM_FILES
, they'll think it's a macro and are tempted to write this:
#ifdef NUM_FILES
Now normally this would fail, but if you write #define NUM_FILES NUM_FILES
it behaves as a macro for the preprocessor and IDE's and as an enum for the code itself.
I would suspect it's for IDEs or other tools to understand that a symbol is defined in some way.
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