So given the following function:
  bool IsZero(const double d)
  {
    return d == 0.0;
  }

May I conclude that C++ doesn't even guarantee that the following three
calls to IsZero return true?
  IsZero(0.0);
  const double dZeroConst = 0.0;
  IsZero(dZeroConst);
  double dZeroVar = 0.0;
  IsZero(dZeroVar);

If so, in which cases is the use of the operator == for floating point
numbers guaranteed safe?

If any non-trivial use of == for floating points is unsafe, why is it still
part of the language???


James Kuyper wrote:
>
> Niels Dekker wrote:
> >
> > Given the following MAGIC_NUMBER macro and IsMagic function:
> >
> >   #define MAGIC_NUMBER 1.23456789E30
> >
> >   bool IsMagic(const double d)
> >   {
> >     return d == MAGIC_NUMBER;
> >   }
> >
> > Does C++ guarantee that the following function call returns true?
> >   IsMagic(MAGIC_NUMBER);
[...]
> As a practical matter, you can safely compare for equality integers
> smaller than the smallest integer that can't be represented exactly, but
> the C++ standard doesn't even say that.


Niels Dekker
ndekker "at" nki "dot" nl

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