In C/C++, is there a directive similar to #ifndef for typedefs?

Question

If I want to define a value only if it is not defined, I do something like this :

#ifndef THING #define THING OTHER_THING #endif 

What if THING is a typedef'd identifier, and not defined? I would like to do something like this:

#ifntypedef thing_type typedef uint32_t thing_type #endif 

The issue arose because I wanted to check to see if an external library has already defined the boolean type, but I'd be open to hearing a more general solution.

Answer 1

There is no such thing in the language, nor is it needed. Within a single project you should not have the same typedef alias referring to different types ever, as that is a violation of the ODR, and if you are going to create the same alias for the same type then just do it. The language allows you to perform the same typedef as many times as you wish and will usually catch that particular ODR (within the same translation unit):

typedef int myint; typedef int myint;       // OK: myint is still an alias to int //typedef double myint;  // Error: myint already defined as alias to int 

If what you are intending to do is implementing a piece of functionality for different types by using a typedef to determine which to use, then you should be looking at templates rather than typedefs.