The types long long and unsigned long long are standard C and standard C++ types each with at least 64 bits. All compilers I’m aware of provide these types, except possibly when in a -pedantic mode but in this case int64_t or uint64_t won’t be available with pre-C++ 2011 compilers, either. On all of the systems <stdint.h> is available, too. That is, as far as I can tell it doesn’t matter much how you spell the type. The main goal of <stdint.h> is to provide the best match for a specific number of bits. If you need at least 64 bit but you also want to take advantage of the fasted implementation of such a type, you’d use int_least64_t or uint_least64_t from <stdint.h> or <cstdint> (in case of the latter, the names are defined in namespace std).