unsigned long long conflict with uint64_t? [duplicate]

Question

We use template specialization for some type parameter like

class my_template_class<uint64_t M>: public my_template_class_base<uint64_t> {
 ....
}

class my_template_class<unsigned long long,M>: public my_template_class_base<unsigned long long> {
 ....
}

This is working perfectly with 64-bit compilation with gcc. While when we try the 32 bit mode, it reports "previous definition" for above two classes.

So unsigned long long is the same as uint64_t in the 32-bit compilation but not in 64-bit compliation?

The compilation difference is the CXX flag -m32 and -m64

Answer 1

So unsigned long long is the same as uint64_t in the 32-bit compilation but not in 64-bit compilation?

Yes.

In 32-bit mode, most likely long is 32 bits and long long is 64 bits. In 64-bit mode, both are probably 64 bits.

In 32-bit mode, the compiler (more precisely the <stdint.h> header) defines uint64_t as unsigned long long, because unsigned long isn't wide enough.

In 64-bit mode, it defines uint64_t as unsigned long.

It could have defined it as unsigned long long in both modes. The choice is arbitrary; all that's required is that it has to be a 64-bit type.

In general, each of the integer types defined in <stdint.h> is a typedef for some predefined type with the appropriate characteristics. You can't assume that any of them are distinct from the predefined types.

Answer 2

This is from stdint.h for GCC 4.8:

#if __WORDSIZE == 64
typedef unsigned long int   uint64_t;
#else
__extension__
typedef unsigned long long int  uint64_t;
#endif

So:

So unsigned long long is the same as uint64_t in the 32bit compliation but not in 64 bit compliation?

Yes.