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...
Look into other libraries that have been developed. Lots of people have wanted to do this before you. :D
Try bigint C++
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);