cppreference.com > C++ String Streams > str str Syntax:
      #include <sstream>
      void str( string s );
      string str();
The function str() can be used in two ways. First, it can be used to get a copy of the string that is being manipulated by the current stream string. This is most useful with output strings. For example:
      ostringstream stream1;
      stream1 << "Testing!" << endl;
      cout << stream1.str();
Second, str() can be used to copy a string into the stream. This is most useful with input strings. For example:
      istringstream stream1;
      string string1 = "25";
      stream1.str(string1);
str() is also handy when you need to clear the stream so that it can be reused:
      ostringstream stream1;
      stream1 << "Using this once" << endl;
      cout << stream1.str();
      stream1.str("");  // Clear the stream
      stream1 << "Now, using it twice" << endl;
      cout << stream1.str();
Related topics: rdbuf() C++ I/O Streams