Construct an object and optionally initialize string content.
  Constructs an object of class ostringstream including the initialization of the associated stringbuf object and the call to its base class' constructor with the stringbuf object as constructor parameter.
  Additionally, in case the second constructor syntax is used, the stream's buffer is initialized with the content of the STL string object str as if a call to member str.

Parameters.

mode

Flags describing the requested i/o mode for the string buffer. This is an object of type ios_base::openmode, it generally consists of a combination of the following flags (member constants):

bit	effect
app	(append) Seek to the end of the stream before each output operation.
ate	(at end) Seek to the end of the stream when opening.
binary	Consider stream as binary rather than text.
in	Allow input operations on a stream.
out	Allow output operations on a stream.
trunc	(truncate) Truncate file to zero when opening.

str

String object to be copied to internal buffer. This is an STL string object.

Return Value.
  none

Example.

// using ostringstream constructors.
#include <iostream>
#include <sstream>
using namespace std;

int main () {

  ostringstream oss (ostringstream::out);

  oss << "This is a test\n";
  cout << oss.str();

  return 0;
}

Basic template member declaration ( basic_ostringstream<charT,traits,Allocator> ):

explicit basic_ostringstream ( openmode mode = out ); explicit basic_ostringstream ( const basic_string<charT,traits,Allocator> * str, openmode mode = out );

See also.
  str
  ostringstream class