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

Tony Guinness wrote:
> The operator<< that is called here...
>>    std::string s = (std::ostringstream() << "i = " << i).str();
> is member function std::ostream::operator<<(char const*).  There is no
> case of a tem****ary being bound to a non-const reference here.  It is
> perfectly legal and should compile just fine.

STL****t has it defined as a non-member!

   template <class _Traits>
   inline basic_ostream<char, _Traits>& _STLP_CALL
   operator<<(basic_ostream<char, _Traits>& __os, const char* __s)

Just look at "stl****t/stl/_ostream.h", from
http://www.stl****t.com/archive/STL****t-5.0RC2.tar.gz

> There is a well-known idiom for cir***venting this problem
> (without inventing your wrapper class) by using the knowledge
> that the member-function versions of operator<< all
> return lvalue references; one such member is the one that executes
> free functions whose signatures resemble
> "std::ostream& func(std::ostream&)" as if they were manipulators in
> the operator<< chain (e.g. std::endl, std::flush.) The idiomatic
> approach is to select std::flush for this job as it has no
> discernible side-effects; thus:
> 
>      int i = 42;
>      std::string prefix = "i = ";
>      std::ostringstream() << std::flush << prefix << i;

Nice trick! Actually I didn't know that idiom, but I'm certainly not an
STL expert...

>> Okay, second try, wrapping the ostringstream as follows:
>>
>>   template <typename T> class Wrapper {
>>    T m_data;
>>   public:
>>    T & get() {
>>     return m_data;
>>    }
>>   };
>>
>>   std::string s = (Wrapper<std::ostringstream>().get() << "i = " <<
i).str();
> 
> As explained above, all this is unnecessary.

Well, I still like to use the Wrapper, because it's a more generic way
to get a "tem****ary" lvalue reference. It could also be used for types
that don't sup****t the "<< std::flush" idiom. Instead of using my
Wrapper template, you could of course use Boost's value_initialized:

   std::string s = (boost::value_initialized<std::ostringstream>().data()
     << "i = " << i).str();

Of course my approach still has to compete against the wonderful
StringBuilderFactory::StringBuilder solution by Vidar Hasfjord...

Anyway, thanks for your suggestion!

Kind regards, Niels

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