Jens Gustedt's Blog

August 22, 2013

testing compile time constness and null pointers with C11’s _Generic

Filed under: C11, C99, language, P99, preprocessor, syntax — Jens Gustedt @ 13:23

Sometimes in C it is useful to distinguish if an expression is an “integral constant expression” or a “null pointer constant”. E.g for an object that is allocated statically, only such expressions are valid initializers. Usually we are able to determine that directly when writing an initializer, but if we want to initialize a more complicated struct with a function like initializer macro, with earlier versions of C we have the choice:

  • Use a compiler extension such as gcc’s __builtin_constant_p
  • We’d have to write two different versions of such a macro, one for static allocation and one for automatic.

In the following I will explain how to achieve such a goal with C11’s _Generic feature. I am not aware of a C++ feature that provides the same possibilities. Also, this uses the ternary operator (notably different in C and C++), so readers that merely come from that community should read the following with precaution.

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