Constructing Input Stream Objects

If you use only the cin object, you don’t need to construct an input stream. You must construct an input stream if you use:

• File stream
  
  
• String stream

Input File Stream Constructors

There are three ways to create an input file stream:

• Use the void-argument constructor, then call the open member function:

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

• Specify a filename and mode flags in the constructor invocation, thereby opening the file during the construction process:

  ifstream myFile( "filename", iosmode );
  

• Specify an integer file descriptor for a file already open for input. In this case you can specify unbuffered input or a pointer to your own buffer:

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

Input String Stream Constructors

Input string stream constructors require the address of preallocated, preinitialized storage:

char s[] = "123.45";
double amt;
istrstream myString( s );
myString >> amt; // Amt should contain 123.45