Question

I don’t have any good reference about std::numeric_limits<T>, but sites on the net says that std::numeric_limits<T>::epsilon() will return the difference between 1 and the smallest number after it. As far as I know about the int type, the next numer after 1 is 2, so epsilon should be 1. But it’s 0 (Linux, g++ 4.4.5). What is the reasoning behind that?

I know that in practice epsilon() is only useful for floating point types, I’m just nitpicking.

Answer 1

The standard says ([numeric.limits.members])

Meaningful for all floating point types.

The standard also says ([numeric.special])

many values are only required to be meaningful under certain conditions (for example, epsilon() is only meaningful if is_integer is false). Any value that is not “meaningful” shall be set to 0 or false.

Since the value is not “meaningful”, the value 0 is required by the standard as an indication that it is not a meaningful value.