Re: [Cppunit-devel] assertDoubleEquals, NaN & Microsoft Visual Studio 6

[CppUnit d. m. l. <cpp...@li...>]

CppUnit development mailing list wrote:
> Hi Baptiste,
>
> I'm unclear on what problem you are solving here.  My guess is that
>
>     CPPUNIT_ASSERT_DOUBLES_EQUAL( nan, nan, 1.0 )
>
> was passing when it should in fact fail.  Is that the case?

Yes. I should have put this first. All the following unit tests were 
failing:
  CPPUNIT_ASSERT_ASSERTION_FAIL( CPPUNIT_ASSERT_DOUBLES_EQUAL( nan, 0.0, 
1.0 ) );
  CPPUNIT_ASSERT_ASSERTION_FAIL( CPPUNIT_ASSERT_DOUBLES_EQUAL( nan, nan, 
1.0 ) );
  CPPUNIT_ASSERT_ASSERTION_FAIL( CPPUNIT_ASSERT_DOUBLES_EQUAL( nan, inf, 
1.0 ) );
  CPPUNIT_ASSERT_ASSERTION_FAIL( CPPUNIT_ASSERT_DOUBLES_EQUAL( inf, nan, 
1.0 ) );

This was caused by the fact that
       equal = expected == actual;
always evaluated to true whenever expected or actual was a NaN.

> Also: why did you make the following change in
> examples/cppunittest/TestAssertTest.cpp?  What does the comment mean?
>
> -  CPPUNIT_ASSERT_ASSERTION_FAIL( CPPUNIT_ASSERT_DOUBLES_EQUAL( nan,
> 0.0, 1.0 ) );
> +  CPPUNIT_ASSERT_ASSERTION_FAIL( CPPUNIT_ASSERT_DOUBLES_EQUAL( nan,
> 0.0, 1.0 ) ); // this one fails

Ignore the comment, I forgot to remove it. It's done now.

I hope it's clearer now.
Baptiste.

> Quoting CppUnit development mailing list
> <cpp...@li...>:
>
>> This seems to confirm that MSVC6 code generator is bug (does not
>> respect IEEE-754 standard in spite of what the doc claim).
>>
>> To work-around this, I added an explicit test for NaN value when
>> testing non-finite value for equalities. Here is the function:
>> // To work around this, a NaN is assumed to be detected if no strict
>> ordering is found.
>> inline bool floatingPointIsUnordered( double x )
>> {
>>    // x != x will detect a NaN on conformant platform
>>    // (2.0 < x  &&  x < 1.0) will detect a NaN on non conformant
>>    platform: // => no ordering can be found for x.
>>    return  (x != x) ||  (2.0 < x  &&  x < 1.0);
>> }
>
> I don't understand why this works for you.  According to the standard
> (see e.g. page 8 of
> http://www.cs.berkeley.edu/~wkahan/ieee754status/ieee754.ps) all
> comparisons with NaN are false except "x != x", which is true.  You
> state that MSVC6 gets "x != x" wrong.  Does it also get "x < y" wrong
> and return true?
>
>
>> I'm not an expert on floating-point manipulation & NaN. Does any one
>> see anything wrong on the above implementation or the comments?
>
> The section of the paper referenced above also states that all
> comparisons with NaN, except == and !=, are *invalid* and thus signal.
>  So the code you wrote is going to cause a problem if the FPU signals
> are enabled.
>
>
> If we need this function, can we rename it floatingPointIsNan()?  Then
> I can create suitable automakery to use the standard isnan() on
> systems that have it.  On that note, does MSVC6 have something like
> isnan(), maybe called _isnan()?
> http://msdn2.microsoft.com/en-us/library/tzthab44(VS.80).aspx
>
>
> Cheers,
> -Steve
>
>
>
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