Единственная гарантия, что у вас есть:
sizeof(char) == 1
sizeof(char) <= sizeof(short) <= sizeof(int) <= sizeof(long) <= sizeof(long long)
// FROM @KTC. The C++ standard also has:
sizeof(signed char) == 1
sizeof(unsigned char) == 1
// NOTE: These size are not specified explicitly in the standard.
// They are implied by the minimum/maximum values that MUST be supported
// for the type. These limits are defined in limits.h
sizeof(short) * CHAR_BIT >= 16
sizeof(int) * CHAR_BIT >= 16
sizeof(long) * CHAR_BIT >= 32
sizeof(long long) * CHAR_BIT >= 64
CHAR_BIT >= 8 // Number of bits in a byte
Также см .: . long
гарантировано, что оно должно быть не менее 32 бит?