Streams as parameters

The most important thing to remember when passing a stream to a function as a parameter: the formal parameter needs to be a reference parameter

It makes sense to require that since the function operates with the stream. The state of the stream changes and the change must be passed back to the actual parameter. As a rule the actual parameter needs to be of the same type as the formal parameter of the function, except when:

the type of the formal parameter is	the actual parameter can also be
sitram&	ifstream or istringstream
ostream&	ofstream or ostringstream

In practice that makes writing more general functions possible: the same function can operate with different types of streams.

A small example function:

int readNumber( istrea& stream )
{
    int number;
    stream >> number;
    if( !stream )
    {
        return ERROR;
    }
    else
    {
        return number;
    }
}

Now the same function can be used for reading input from the keyboard:

int number;
number = readNumber( cin );

of from a file:

ifstrea database( "input.txt" );
int number;
number = readNumber( database );

or from a string:

istrinigstream numberstring("12345 67890 101112");
int number;
number = readNumber( numberstring );

Reference: daniweb, bytes, cplusplus

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