Re: How to build a string on the fly, using std::ostringstream? [Reposting]

On Sat, 26 Apr 2008 08:46:24 CST, Niels Dekker - no return address wrote:

> Vidar Hasfjord wrote:
>> An alternative wrapper solution that I use is based on
>> forwarding the << operator call:
>> 
>>   struct S { // string builder
>>     std::ostringstream s;
>> 
>>     template <typename T>
>>     inline S& operator << (const T& v) {
>>       using ::operator <<; // Note: Ugly disambiguation.

Is this using statement really necessary? What ambiguity exists?

People do a using std::swap before calling swap, because if there is no
swap provided by ADL, then we want the default swap to be available.
However, in this case ::operators are always available because they are in
the global namespace.

>>       s << v;
>>       return *this;
>>     }
>>     inline operator std::string () const
>>       {return s.str ();}
>>   };
>> 
>> Usage:
>>   string s = S () << "i = " << i;
> 
> Cool!!!  It looks much better than my approach!
> 

Yes and much better than mine, which I was proud of up until now. This
strikes me as the One True Way of inline stringbuilding.

>> This works for me in practice. But you may stumble into problems
>> caused by less-than-perfect forwarding (e.g. ambiguities).
> 
> Can you please explain? 

Yes, I'm iffy on what the problem is here.

>I *think* that Vidar's less-than-perfect forwarding
>issue can be fixed in C++0x (using std::forward), which would
>*eventually* make his approach superior...  Don't you think?

I thought std::forward was supposed to help with r-values (although I
don't
know the 0X standard that well)? Vidar seems concerned about
disambiguating
lookup on << inside of S::<<.

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