Modeling infinity for the largest double value

Question

The question is about modeling infinity in C++ for the double data type. I need it in a header file, so we cannot use functions like numeric_limits

Answer 1

Not sure why you can't use std::numeric_limits in a header file. But there is also this carried over from ANSI C:

#include <cfloat>

DBL_MAX

Answer 2

Wouldn't this work?

const double infinity =  1.0/0.0;

Answer 3

I thought the answer was "42.0" ;)

This article might be of interest:

http://www.cygnus-software.com/papers/comparingfloats/comparingfloats.htm

Or this:

http://www.cplusplus.com/reference/clibrary/cfloat/

MAXimum Maximum finite representable floating-point number:

FLT_MAX  1E+37
DBL_MAX  1E+37
LDBL_MAX 1E+37  

Answer 4

From Wikipedia:

0x 7ff0 0000 0000 0000   = Infinity
0x fff0 0000 0000 0000   = −Infinity

Answer 5

numeric_limits functions are all constexpr so they work just fine as compile time constants (assuming you're using the current version of C++). So std::numeric_limits<double>::infinity() ought to work in any context.

Even if you're using an older version, this will still work anywhere that you don't require a compile time constant. It's not clear from your question if your use really needs a compile time constant or not; just being in a header doesn't necessarily require it.

If you are using an older version, and you really do need a compile time constant, the macro INFINITY in cmath should work for you. It's actually the float value for infinity, but it can be converted to a double.

Answer 6

Maybe in your C++ environment you have float.h, see http://www.gamedev.net/topic/392211-max-value-for-double-/ (DBL_MAX)