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

Answer 1

The problem is actually real PITA, because some APIs or SDKs redefine commonly used things. I had issue that header files for a map processing software (GIS) were redefining TRUE and FALSE (generally used by windows SDK)keywords to integer literals instead of true and false keywords ( obviously, that can break SOMETHING). And yes, famous joke "#define true false" is relevant.

define would never feel a typedef or constant declared in C\C++ code because preprocessor doesn't analyze code, it only scans for # statements. And it modifies code prior to giving it to syntax analyzer. SO, in general, it's not possible.

https://msdn.microsoft.com/en-us/library/5xkf423c.aspx?f=255&MSPPError=-2147217396 That one isn't portable so far, though there were known request to implement it in GCC. I think, it also counts as "extension" in MSVC. It's a compiler statement, not a preprocessor statement, so it will not "feel" defined macros, it would detect only typedefs outside of function body. "full type" there means that it will react on full definition, ignoring statements like "class SomeClass;". Use it at own risk.

Edit: apparently it also supported on MacOS now and by Intel comiler with -fms-dialect flag (AIX\Linux?)