What's the recommended workaround if numeric_limits<double>::has_infinity is false?
Question
I need to check a double value for infinity in a C++ app on Linux. On most platforms this works by comparing with std::numeric_limits<double>::infinity()
. However, on some old platforms (RedHat 9 for example, with gcc 3.2.2) this is not available, and std::numeric_limits<double>::has_infinity
is false there.
What workaround would you recommend for those platforms?
Solution 3
Ok, I have now resorted to using the INFINITY
and NAN
macros on that particular machine - seems to work fine. They come from math.h
.
OTHER TIPS
If you're using IEEE 754 arithmetic, as you almost certainly are, infinities are well defined values and have defined outcomes for all arithmetic operations. In particular,
infinity - infinity = NaN
Positive and negative infinity and NaN
values are the only values for which this is true. NaNs are special "not-a-number" values used to indicate domain errors of functions, e.g. sqrt(-1)
. Also:
NaN != NaN
NaN
s are the only values for which this is true.
Therefore:
bool is_infinite(double x) {
double y = x - x;
return x == x && y != y;
}
will return true if and only if x
is either positive or negative infinity. Add a test for x > 0
if you only want to check for positive infinity.
For most cases, std::numeric_limits::max() can be a valid replacement for std::numeric_limits::infinity().
You would have to be more careful about using it, however. With overflows I believe you would have to manually detect the overflow and set the result to the max explicitly if an overflow is detected.