How to check if fixed width integers are defined

To determine if a fixed-width integer type is provided, you can check if either of the corresponding [U]INT*_MAX or [U]INT*_MIN macros is defined.

// may be necessary for your C++ implementation
#define __STDC_LIMIT_MACROS 
#include <cstdint>

#ifdef INT32_MAX
// int32_t must be available to get here
int32_t some32bitIntVariable;
#endif

Per 7.20 Integer types <stdint.h>, paragraph 4 of the C11 standard (note the bolded parts):

For each type described herein that the implementation provides, <stdint.h> shall declare that typedef name and define the associated macros. Conversely, for each type described herein that the implementation does not provide, <stdint.h> shall not declare that typedef name nor shall it define the associated macros.

C++ inherits the C implementation via <cstdint>. See <cstdint> vs <stdint.h> for some details. Also see What do __STDC_LIMIT_MACROS and __STDC_CONSTANT_MACROS mean? for details on __STDC_LIMIT_MACROS.

Thus, if int32_t is available, INT32_MAX and INT32_MIN must be #define'd. Conversely, if int32_t is not available, neither INT32_MAX nor INT32_MIN are allowed to be #define'd.

Note though, as @NicolBolas stated in another answer, it may not be necessary to actually check.

Broadly speaking... you don't.

If you need to use the fixed-sized integer types, then that means that you explicitly need those types to be of their specific sizes. That is, your code will be non-functional if you cannot get integers of those sizes. So you should just use them; if someone uses your code on a compiler that lacks said types, then your code will not compile. Which is fine, because your code wouldn't have worked if it did compile.

If you don't actually need fixed-sized integers but simply want them for some other reason, then use the int_least_* types. If the implementation can give you exactly that size, then the least_* types will have that size.