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

On Apr 23, 12:14 pm, "Niels Dekker - no return address"
<nore...@[EMAIL PROTECTED]
> wrote:
> Does the Standard Library sup****t creating an std::string by means of
> streaming, without having to declare a *named* ostringstream object?  I
tried
> the following:
>
>    #include <sstream>
>    #include <string>
>    int i = 42;
>
>    std::string s = (std::ostringstream() << "i = " << i).str();
>
> Of course it didn't compile, because a tem****ary (std::ostringstream())
cannot
> be passed as non-const reference to a function (operator<<).  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();
>
> Unfortunately it still didn't compile, because operator<< returns an
> std::basic_ostream, instead of an std::ostringstream, and therefore it
doesn't
> have an str() member function.  Now I wonder, is it safe to cast the
stream,
> returned by operator<< to an ostringstream reference?  As follows:
>
>   std::string s =
> static_cast<std::ostringstream&>(Wrapper<std::ostringstream>().get()
>      << "i = " << i).str();
>
> At least, it *seems* to work...   But is it the proper way to do it?

I actually like your workaround, although you should typedef
Wrapper<std::ostringstream> to make it cleaner. I believe that
tem****aries are guaranteed to stick around until the next statement,
so you are good. Oh! also, overload << something like this:

std::string operator<<(basic_ostream& stream, ToStr& tstr) {
  return static_cast<std::ostringstream&>(stream).str();
}

I haven't tested that, and obviously it isn't type safe, but since <<
is left associative you should be able to:

string x = temp_ostringstream().get() << "i = " << i << to_str;

where to_str is the single instance of ToStr.

Pretty nifty eh? There's probably some way to make getting the
tem****ary ostringstream easier, and maybe even making this statically
type safe, but it's late...

iostreams << are pretty lame for formatting syntactically speaking and
pretty much everybody ends up rolling their own formatting utility on
top of it.

Check out:

http://www.boost.org/doc/libs/1_35_0/libs/format/doc/format.html

which is basically a typesafe printf.

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