Modeling infinity for the largest double value

问题:

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.

Is there a defined constant that represents the largest value?

回答1:

floating point numbers(such as doubles) can actually hold positive and negative infinity. The constant INFINITY should be in your math.h header.

Went standard diving and found the text:

4 The macro INFINITY expands to a constant expression of type float representing positive or unsigned infinity, if available; else to a positive constant of type float that overflows at translation time.

In Section 7.12 Mathematics <math.h>

---

Then of course you have the helper function isinf to test for infinity(which is also in math.h).

7.12.3.3 The isinf macro

int isinf(real-floating x);

Description: The isinf macro determines whether its argument value is an infinity (positive or negative). First, an argument represented in a format wider than its semantic type is converted to its semantic type. Then determination is based on the type of the argument.

Returns: The isinf macro returns a nonzero value if and only if its argument has an infinite value.

回答2:

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.

回答3:

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

回答4:

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

回答5:

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  

回答6:

From Wikipedia:

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

回答7:

DBL_MAX can be used. This is found in float.h as follows

    #define DBL_MAX         1.7976931348623158e+308 /* max value */

回答8:

#include <cmath>
...
double d = INFINITY;

You can find INFINITY defined in <cmath> (math.h):

A constant expression of type float representing positive or unsigned infinity, if available; else a positive constant of type float that overflows at translation time.

回答9:

Wouldn't this work?

const double infinity =  1.0/0.0;