Is floating point addition commutative in C++?

Question

For floating point values, is it guaranteed that a + b == b + a ?

I believe this is guaranteed in IEEE754, however the C++ standard does not specify tha

Answer 1

The C++ standard very specifically does not guarantee IEEE 754. The library does have some support for IEC 559 (which is basically just the IEC's version of the IEEE 754 standard), so you can check whether the underlying implementation uses IEEE 754/IEC 559 though (and when it does, you can depend on what it guarantees, of course).

For the most part, the C and C++ standards assume that such basic operations will be implemented however the underlying hardware works. For something as common as IEEE 754, they'll let you detect whether it's present, but still don't require it.