Will my code safely check for an underflow?

Question

I wrote the following C++ code to check for an underflow. Not sure this is a good practice or not.

#include <limits>
#include <iostream>

int main()
{
    float d = 1.e-29;
    std::cout<<"d: "<<d<<" underflow? "<<(d<std::numeric_limits<float>::min())<<std::endl;
    d = 1.e-59;
    std::cout<<"d: "<<d<<" underflow? "<<(d<std::numeric_limits<float>::min())<<std::endl;
}

The printouts are

d: 1e-29 underflow? 0
d: 0 underflow? 1

just thinking logically, this would, by definition, **fail** to detect any underflow. — The Paramagnetic Croissant, Apr 24 '14 at 21:38
Logically speaking, how could any ``float`` ever be less than ``std::numeric_limits::min()``? That's a contradiction. — aruisdante, Apr 24 '14 at 21:38
@HailiangZhang How could any `float` be less than the _minimum float value_? — yizzlez, Apr 24 '14 at 21:42
The code looks OK to me. min() is the minimum positive value, so 0.0f is < to min(). — Rémi, Apr 24 '14 at 21:43
@Rémi: It's quite obviously not okay. You physically can't detect an underflow/overflow _after_ it's occurred, like this. It doesn't matter what `std::numeric_limits::min()` returns you. — Lightness Races in Orbit, Apr 24 '14 at 21:46

score 3 · Answer 1 · answered Apr 24 '14 at 21:46

3

You cannot check for underflow or overflow after a variable has been assigned a value by comparing it with the standard limits.

Simple example:

int v1 = INT_MAX;
int v2 = INT_MAX;
int v3 = v1 * v2;

The expression v1 * v2 results in an overflow. However the value of v3 will still be a valid int.

You have to performs checks prior to v1 * v2 to determine whether it will result in an overflow.

answered Apr 24 '14 at 21:46

R Sahu

204,454
14
159
270

This is true for integers, but float underflow should be testable on the result. – Rémi Apr 24 '14 at 21:48
"the value of v3 will still be a valid int." - no, because the overflow invoked undefined behavior. it may be anything (including a trap representation). – The Paramagnetic Croissant Apr 25 '14 at 06:39

Will my code safely check for an underflow?

1 Answers1