Re: [boost] Boost Test Library: general floating point comparisons

From: John Maddock (john_at_[hidden])
Date: 2007-02-19 05:05:27

Gennadiy Rozental wrote:
> Ok. How about
>
> BOOST_CHECK_PRC_DIFF
> BOOST_CHECK_REL_DIFF
> BOOST_CHECK_EPS_DIFF
>
> How about
>
> BOOST_CHECK_ABS_DIFF
>
> for absolute difference comparison?

Oh please no.... too cute and cryptic by half, they're not self-explanatory 
at all.

> Wouldn't you were the one who advocated against epsilon() usage?

Was I?  I'm not sure :-)

Certainly you need to be careful with it: numeric_limits<long 
double>::epsilon() is meaningless on MAC OSX for example.  But that's a good 
reason to do it right in one place :-)

>>> I guess I could introduce these synonym if that  will make everyone
>>> happy, but I personally still prefer shorter names.
>>
>> The main question then would be whether the synonyms would have
>> readable error messages: if they still refer to percentages folks
>> will get confused again :-)
>
> What kind of error message would you like for each tool to produce?
>
> Here is an example of BOOS_CHECK_CLOSE produce:
>
> difference between v1{1.23456e-10} and v2{1.234571e-10} exceeds
> 0.0001%

Which is fine if the tolerance is expressed as a percent, but not otherwise: 
the error quoted needs to be expressed in the same units as are passed to 
the macro, otherwise you'll just confuse folks again.

HTH, John.