Constructing Output Stream Objects

If you use only the predefined cout, cerr, or clog objects, you don’t need to construct an output stream. You must use constructors for:

• File streams
  
  
• String streams

Output File Stream Constructors

You can construct an output file stream in one of three ways:

• Use the default constructor, then call the open member function.

  ofstream myFile; // Static or on the stack
  myFile.open( "filename", iosmode );
  
  ofstream* pmyFile = new ofstream; // On the heap
  pmyFile->open( "filename", iosmode );
  

• Specify a filename and mode flags in the constructor call.

  ofstream myFile( "filename", iosmode );
  

• Specify an integer file descriptor for a file already open for output. You can specify unbuffered output or a pointer to your own buffer.

  int fd = _open( "filename", dosmode );
  ofstream myFile1( fd );  // Buffered mode (default)
  ofstream myFile2( fd, NULL, 0 );  // Unbuffered mode ofstream
  myFile3( fd, pch, buflen);  // User-supplied buffer
  

Output String Stream Constructors

To construct an output string stream, you can use one of two ostrstream constructors. One dynamically allocates its own storage, and the other requires the address and size of a preallocated buffer.

• The dynamic constructor is used like this:

  char* sp;
  ostrstream myString;
  mystring << "this is a test" << ends;
  sp = myString.str();  // Get a pointer to the string
  

  The ends “manipulator” adds the necessary terminating null character to the string.

• The constructor that requires the preallocated buffer is used like this:

  char s[32];
  ostrstream myString( s, sizeof( s ) );
  myString << "this is a test" << ends; // Text stored in s