Representing 128-bit numbers in C++

user_118058 0
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What's the best way to represent a 128-bit number in C++? It should behave as closely to the built-in numeric types as possible (i.e. support all the arithmetic operators, etc).

I was thinking of building a class that had 2 64 bit or 4 32 bit numbers. Or possibly just creating a 128 bit block of memory and doing everything myself.

Is there some easier/more standard way, or something that I'm less likely to screw up when implementing it myself? :)

It would also be nice if it could be extended to 256-bit, 512-bit, etc...

math c++
11 Answers

EDIT: when I first wrote this boost::multiprecision::uint128_t wasn't a thing yet. Keeping this answer for historical reasons.

I've made a uint128 class before, you can check it out at: http://www.codef00.com/code/uint128.h.

It is dependent on boost for automatically providing all of the variants of the math operators, so it should support everything a native unsigned int type does.

There are some minor extensions to built in types such as initializing it with a string like this:

uint128_t x("12345678901234567890");

There is a convenience macro which works similary to the ones in C99 which you can use like this:

uint128_t x = U128_C(12345678901234567890);
Evan_Teran_134300

Look into other libraries that have been developed. Lots of people have wanted to do this before you. :D

Try bigint C++

user_899890

Don't reinvent the wheel -- I'm positive other people have already solved this problem, although I can't name any solutions off the top of my head. GMP can surely solve your problem, although it's overkill for fixed-size integers, and it's also a little cumbersome to use (it's a C library, not C++).

Adam_Rosenfield_95300

You may want to try GMP

Burkhard_128600
user_235280

You might be better off with an infinite-precision integer class, rather than a sequence of increasing size. Some languages (like Common Lisp and IIRC Python) have them natively. I'm not sure offhand what's available for C++; last I looked there wasn't a Boost version.

David_Thornley_141480

The cairo graphics library has two files which implement portable 128-bit integer arithmetic: cairo-wideint-private.h, cairo-wideint.c. We include just these two in our project to get 128-bits.

user_4978260

In Visual Studio C++ there is a type FLOAT128 that is used to represent 128-bit integers. It is implemented as:

#if defined(_M_IA64) && !defined(MIDL_PASS)
__declspec(align(16))
#endif
typedef struct _FLOAT128 {
    __int64 LowPart;
    __int64 HighPart;
} FLOAT128;

so I'm not sure about what math operations are implemented for it

user_903160
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