Re: [boost] is_nan

From: Johan Råde (rade_at_[hidden])
Date: 2006-07-26 06:22:44

John Maddock wrote:
> Howard Hinnant wrote:
>> On Jul 25, 2006, at 1:45 PM, Howard Hinnant wrote:
>>
>>> namespace Mine
>>> {
>>>
>>> struct Person {};
>>>
>>> struct Female : public Person {};
>>>
>>> bool isnormal(const Person&);
>>>
>>> }
>>>
>>> int main()
>>> {
>>>      using namespace boost;  // for boost::bind (just as an example)
>>>      Mine::Female Jane;
>>>      bool b = isnormal(Jane);
>>> }
>> If this code doesn't make you nervous (because of the using
>> directive), the related code below might:
>>
>> namespace Mine
>> {
>>
>> struct sense_of_humor {};
>>
>> struct Person : private boost::optional<sense_of_humor> {};
>>
>> struct Female : public Person {};
>>
>> bool isnormal(const Person&);
>>
>> bool foo()
>> {
>>     Female Jane;
>>     return isnormal(Jane);
>> }
>>
>> }  // Mine
>>
>> int main()
>> {
>>     Mine::foo();
>> }
>>
>> Again, isnormal will get hijacked by boost::isnormal(T) if it is in
>> the translation unit (and in namespace boost and not somehow
>> constrained).
> 
> Won't bool isnormal(const Person&) be the better overload and therefore the 
> one called?  The worst I can see happening is an "ambiguous overload" error.
> 
> BTW "isnorm" (note the spelling) should be in boost::math:: not boost:: so 
> the risk is somewhat reduced.
> 
>> template <class T>
>> typename enable_if
>> <
>>     std::numeric_limits<T>::is_specialized,
>>     bool
>>>>> type
>> isnormal(T t) {...}
> 
> Sigh, yes I can see the point of that, I'm fighting against it because it 
> would break *my* code: it uses numeric types (NTL::RR) for which 
> std::numeric_limits<> support is not appropriate, 'cos the precision is not 
> a compile time constant.  I could use another traits class that defaults to 
> std::numeric_limits<T>::is_specialized but provides a backdoor for other 
> types I guess.
> 
> John. 
> 
> _______________________________________________
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> 

The functions isnorm etc do not have to be in namespace boost.
Put them in some nested namespace, such as boost::fp_classification.
Then nobody is likely to bring them into scope unknowingly.

--Johan Råde