Why isn't there int128_t?
C++CC++ Standard-LibraryC Standard-LibraryC++ Problem Overview
A number of compilers provide 128-bit integer types, but none of the ones I've used provide the typedefs int128_t. Why?
As far as I recall, the standard
- Reserves
int128_tfor this purpose - Encourages implementations that provide such a type to provide the typedef
- Mandates that such implementations provide an
intmax_tof at least 128 bits
(and, I do not believe I've used an implementation that actually conforms to that last point)
C++ Solutions
Solution 1 - C++
I'll refer to the C standard; I think the C++ standard inherits the rules for <stdint.h> / <cstdint> from C.
I know that gcc implements 128-bit signed and unsigned integers, with the names __int128 and unsigned __int128 (__int128 is an implementation-defined keyword) on some platforms.
Even for an implementation that provides a standard 128-bit type, the standard does not require int128_t or uint128_t to be defined. Quoting section 7.20.1.1 of the N1570 draft of the C standard:
> These types are optional. However, if an implementation provides > integer types with widths of 8, 16, 32, or 64 bits, no padding bits, > and (for the signed types) that have a two’s complement > representation, it shall define the corresponding typedef names.
C permits implementations to defined extended integer types whose names are implementation-defined keywords. gcc's __int128 and unsigned __int128 are very similar to extended integer types as defined by the standard -- but gcc doesn't treat them that way. Instead, it treats them as a language extension.
In particular, if __int128 and unsigned __int128 were extended integer types, then gcc would be required to define intmax_t and uintmax_t as those types (or as some types at least 128 bits wide). It does not do so; instead, intmax_t and uintmax_t are only 64 bits.
This is, in my opinion, unfortunate, but I don't believe it makes gcc non-conforming. No portable program can depend on the existence of __int128, or on any integer type wider than 64 bits. And changing intmax_t and uintmax_t would cause serious ABI compatibility problems.