How to define a type that can hold a uintptr_t or a uint32_t without a union?

How can I define a type uintptr such that it can hold at least a uint32_t and a uintptr_t, without resorting to a union?

Were sizeof() allowed in the preprocessor, this would accomplish what I want:

#include <inttypes.h>
#if sizeof(uint32_t) > sizeof(uintptr_t)  // unlikely, but the standard allows it
    typedef uint32_t uintptr;  
#else
    typedef uintptr_t uintptr;
#endif

It is extremely likely that uint32_t will be smaller if not equal to uintptr_t but the standard makes no guarantee. That said, such a platform would be very rare, so for now I've solved this by just having the following:

static_assert(sizeof(uint32_t) <= sizeof(uintptr_t), "Yikes");
typedef uintptr_t uintptr;

Solution

There's no need for the preprocessor to get such an alias. It's a simple use case for the standard library's type traits

using uintptr = std::conditional_t<(sizeof(uint32_t) > sizeof(uintptr_t)),
                                   uint32_t, uintptr_t>;