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From: "Al Grant" <tnarga@arm.REVERSE-NAME.com>
Newsgroups: comp.std.c++
Subject: Re: How to test for NaNs and infinites?
Date: Mon, 11 Jun 2001 17:13:18 GMT
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"Bill Gibbons" <bill@gibbons.org> wrote in message
news:bill-1006011032170001@adsl-64-172-170-154.dsl.snfc21.pacbell.net...
> The most portable way I know is:
>
>     bool IsNaN(double d) {
>         return (d != d);
>     }
>
>     bool IsInfinity(double d) {
>         static const double PositiveInfinity = 1.0 / 0.0;
>         static const double NegativeInfinity = -1.0 / 0.0;
>         return (d == PositiveInfinity || d == NegativeInfinity);
>     }
>
> You might get compile-time warnings about divide by zero, but this
approach
> should work on any system which supports IEEE floating point.

Even if you have dynamically turned off FP exception handling,
the compiler will probably have generated code which works
if exceptions are handled, i.e. which does the division the
first time through IsInfinity (by testing a flag).
You can avoid this expense by making the constants global.

Alternatively:

  bool IsInfinity(double d)
  {
      return (d == d && (d - d) != 0.0);
  }



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