Programmers and computer scientists call the action of coercing a floating-point or double to an integer truncation. The verb is to truncate.

trunc

Truncate value

Rounds x toward zero, returning the nearest integral value that is not larger in magnitude than x.

C++ reference

Perhaps your C++ function could be a Boolean called isTruncated.

Programmers and computer scientists call the action of coercing a floating-point or double to an integer truncation. The verb is to truncate.

trunc

Truncate value

Rounds x toward zero, returning the nearest integral value that is not larger in magnitude than x.

C++ reference

Perhaps your C++ function could be a Boolean called isTruncated.

EDIT: Mathematics Mathematicians call the function that returns the largest integer less than or equal to a number with or without decimals floor.

In mathematics and computer science, the floor function is the function that takes as input a real number x and gives as output the greatest integer less than or equal to x...

Wikipedia: Floor and ceiling functions

The word floored means overwhelmed in most contexts. However, in your computer programming context, you might consider floored (as an alternative to remainderless or mantiassalessness).