CODEWITHSUNDEEP
c
truf
truf

Reputation: 3111

Preprocessor keywords inside templates in C

I would like to define a template like this:

 #define DECLARE_MY_STRUCT(name) \
 #ifndef MY_STRUCT_DECLARED \
 struct my_##name##_struct { \
   double var; \
 }; \
 #define MY_STRUCT_DECLARED \
 #endif

This would allow me to use DECLARE_MY_STRUCT template wherever I want and not get "my_struct was already defined" error.

Unfortunately, gcc treats #ifndef as its preprocessor directives instead of part of template declaration and fails to build such code. Any way to workaround this? ( Except for using

 #ifndef MY_X_STRUCT_DECLARED
  DECLARE_MY_STRUCT(X)
 #endif

as there may be a lot of different struct names. )

Upvotes: 1

Views: 50

Answers (1)

Jens
Jens

Reputation: 72639

The C Standard does not allow nested pre-processor directives, so this is impossible. The # in #ifndef would be treated as a stringizing #.

Upvotes: 2

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