### 612. numeric_limits::is_modulo insufficiently defined

**Section:** 21.3.4.1 [numeric.limits.members] **Status:** CD1
**Submitter:** Chris Jefferson **Opened:** 2006-11-10 **Last modified:** 2016-02-10

**Priority: **Not Prioritized

**View all other** issues in [numeric.limits.members].

**View all issues with** CD1 status.

**Discussion:**

18.2.1.2 55 states that "A type is modulo if it is possible to add two
positive numbers together and have a result that wraps around to a
third number that is less".
This seems insufficient for the following reasons:

- Doesn't define what that value received is.
- Doesn't state the result is repeatable
- Doesn't require that doing addition, subtraction and other
operations on all values is defined behaviour.

*[
Batavia: Related to
N2144.
Pete: is there an ISO definition of modulo? Underflow on signed behavior is undefined.
]*

*[
Bellevue: accept resolution, move to ready status.
Does this mandate that is_modulo be true on platforms for which int
happens to b modulo? A: the standard already seems to require that.
]*

**Proposed resolution:**

Suggest 21.3.4.1 [numeric.limits.members], paragraph 57 is amended to:

A type is modulo if, ~~it is possible to add two positive numbers
and have a result that wraps around to a third number that is less.~~
given any operation involving +,- or * on values of that type whose value
would fall outside the range `[min(), max()]`, then the value returned
differs from the true value by an integer multiple of `(max() - min() +
1)`.