Issue with std::numeric_limits<T>::min() with float/double values

Question

While writing some code to ensure user input is valid I came across an issue with std::numeric_limits<T>::min(); where T is a floating point type i.e double/float etc.

Using -std::numeric_limits<double>::max(); allows me to get the real minimum value but you would expect std::numeric_limits<double>::min(); to return that.

Why does std::numeric_limits<double>::min(); not return the smallest possible value of these types and instead force us to use std::numeric_limits::lowest or -std::numeric_limits<double>::max();?

You're comparing a number against the minimum of that type to see if it's less than that. That doesn't make sense because if it's less than the minimum, the minimum must be that number and then it's no longer less than the minimum. — Qaz
– Qaz, Commented Mar 27, 2014 at 22:59

Community · Accepted Answer · 2017-05-23 10:31:49Z

1

Yes, using -max() will give lowest value.

In C++11 there is also std::numeric_limits::lowest that is consistent for reals and ints. http://en.cppreference.com/w/cpp/types/numeric_limits/lowest

See also How to workaround the inconsistent definition of numeric_limits<T>::min()?

edited May 23, 2017 at 10:31

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answered Mar 27, 2014 at 23:00

Anycorn

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4 Comments

const_ref Over a year ago

Thanks, I gathered as much. Im still interested to know why ::min in this case doesnt return what you would expect however. Any idea?

Anycorn Over a year ago

I don't know the exact rationale, but C++ follows what FLT_MIN is set to. I guess it is a matter of minimum floating being an infinity and not a particular finite number.

const_ref Over a year ago

I guess that makes some sense but it seems slightly misleading with using the term min I guess.

Anycorn Over a year ago

I know. Got bitten by the same problem.

4pie0 · Accepted Answer · 2014-03-27 23:23:36Z

0

Why is this?

This is because std::numeric_limits<>::min() returns implementation defined FLT_MIN, DBL_MIN or INT_MIN. In this regard behavior of this method is consistent. But the return value ofstd::numeric_limits<double>::min(): DBL_MIN has slightly different meaning than INT_MIN. This is the smallest value that double can represent. There might be value greater than 0 but double can't represent it.

why does using min() not do the same thing in this instance?

The rationale behind this is that you can query <limits> for this value to check for this possible underflow specific to floating point arithmetic.

You can use

std::numeric_limits<double>::lowest()

to query for a lowest negative value.

edited Mar 27, 2014 at 23:23

answered Mar 27, 2014 at 23:14

4pie0

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