Is there a portable way to know if uintptr_t is defined in stdint.h?

Preamble: I want to convert a pointer to an integer type, e.g. to check alignment. uintptr_t seems to be the correct type, but it is guaranteed only in C, not in C++ (or C++11)

For the following code:

#include <stdint.h>
#ifndef I_WONDER_IF_UINTPR_T_IS_DEFINED
  typedef unsigned long uintptr_t;
#endif

template <typename T>
bool isAligned(unsigned char* p) ///Checks alignment with respect to some data type
{
   return !((uintptr_t) p % sizeof(T));
}

template bool isAligned<uint16_t>(unsigned char* p);
template bool isAligned<uint32_t>(unsigned char* p);
template bool isAligned<uint64_t>(unsigned char* p);

2 questions:

• Is there a magic and guaranteed word that I can use where I put I_WONDER_IF_UINTPR_T_IS_DEFINED?
• Should I just use unsigned long and forget about it?

Generated assembly (when uintptr_t available)

Note 1: I am aware than in C++11 I should use alignof in place of sizeof
Note 2: I am aware of this discussion: <cstdint> vs <stdint.h>

If you really want to work with an implementation not providing uintptr_t when including <stdint.h>, consider using uintmax_t instead, and a static_assert for suitability (you already established you might be on a goofy setup, so it might be too small):

#include <stdint.h>
static_assert(sizeof(uintmax_t) >= sizeof(void*), "need a bigger unsigned type.");

// Add code using `uintmax_t` to round-trip data-pointers here

There are two dis-advantages though:

1. uintmax_t might not be uintptr_t, important for overload-resolution and linking.
2. uintmax_t might be bigger than neccessary.