What is the difference between ! ( x < y ) and x >= y in C++?

Question

Going through EASTL, I stumbled across a peculiar line of code. The following link shows the file with the line number of interest at 1870.

https://github.com/paulhodge/EASTL/blob/master/include/EASTL/algorithm.h

The code at that line is if(!(value < *i)). The comment says that "we always express value comparisons in terms of < or ==" without any explanation as to why this is so. There are also a few other areas where the same comment is placed but without any explanation.

Is there any benefit whatsoever to writing a comparison like that (maybe some context that I am overlooking)? If not, why did the author of EASTL deliberately wrote it in this particular fashion and even took the care to comment about it? Is consistency the only reason here?

Answer 1

It means you only need to provide < and == for container value types. It also means you reduce the amount of variability for those types (as all the algorithms use !(a<b) to mean a>=b and !(a==b) for a!=b); otherwise, you could have >= and != return inconsistent results.

Answer 2

In C++, you can overload the < operator so that it behaves differently than the opposite of >=, so they are not guaranteed to be equivalent.

Additionally, in any IEEE floating-point implementation, NaN < NaN is false, but so is NaN >= NaN, so !(NaN < NaN) is true even though NaN >= NaN is false.