Re: [boost] SafeInt code proposal

Subject: Re: [boost] SafeInt code proposal
From: Bo Persson (bop_at_[hidden])
Date: 2009-06-23 17:56:52

Jean-Francois Bastien wrote:
> Niels,
>
>> Correct me if I'm wrong :)
>>
>> Well... I do appreciate your pedanticism, but I think you're
>> wrong.  All
>> the compilers I tried accept the function calls to the inline
>> friend
>> function, operator-(safe_int,safe_int), including MSVC 2008 SP1,
>> Comeau (www.comeaucomputing.com/tryitout), and g++ 4.1.2, run at
>> http://codepad.org/dB9IbQJ7
>>
>> You might want to have a look at [temp.inject] ("Friend names
>> declared
>> within a class template"). Quoting the latest C++0x Working Draft,
>> www.open-std.org/jtc1/sc22/wg21/docs/papers/2009/n2857.pdf,
>> 14.7.5/2 (slightly reformatted):
>>
>> "As with non-template classes, the names of namespace-scope friend
>> functions of a class template specialization are not visible
>> during an
>> ordinary lookup unless explicitly declared at namespace scope
>> (11.4).
>> Such names may be found under the rules for associated classes
>> (3.4.2). [ Example:
>>
>>    template<typename T> struct number {
>>      number(int);
>>    friend number gcd(number x, number y) { return 0; };
>>    };
>>
>>    void g() {
>>    number<double> a(3), b(4);
>>    a = gcd(a,b); // finds gcd because number<double> is an
>>    // associated class, making gcd visible
>>    // in its namespace (global scope)
>>    b = gcd(3,4); // ill-formed; gcd is not visible
>>    }
>> -end example ]
>
> I might very well be wrong, that part of the standard has always
> been a bit shady to me and had a ring of "pre-standard stuff". That
> being said compilers aren't the best way to verify standard
> compliance ;)
>

I believe that the friend becomes visible once the template is 
instantiated for a specific type. Therefore there is a gcd  for 
number<double> visible in the example above, but not for any other 
types.

Bo Persson