Re: IO-Operator overloading question

From: Jerry Coffin (jcoffin_at_taeus.com)
Date: 07/10/04

  • Next message: James Dennett: "Re: inline assert [C++]" 
    Date: 9 Jul 2004 21:45:53 -0700

    Francis Glassborow <francis@robinton.demon.co.uk> wrote in message news:<95zAgTWNVq7AFwnK@robinton.demon.co.uk>...

    [ ... ]

    > >class B {
    > > virtual std::ostream &write(std::ostream &os) {
    > > // code to write a B to a stream
    > > }
    > >
    > > friend std::ostream &operator<<(std::ostream &os, B const &b) {
    > > return b.write(os);
    > > }
    > >};
    > >
    > >class D : public D {
    > > virtual std::ostream &write(std::ostream &os) {
    > > // code to write a D to a stream.
    > > }
    > >};
    > >
    > >The invocation of operator<< itself isn't virtual, but all it does is
    > >invoke a virtual member function, which gives the same effect.
    >
    > And we have no need to involve a friend declaration.

    It CAN be written without a friend, by making the virtual I/O function
    ('write' in the code above) public. With 'write' private as it is
    above

    With 'write' being private as it is above, the 'friend' is needed --
    the code for operator<< is written inside of the class definition, but
    it's NOT really a class member -- the 'friend' declaration makes it a
    global even though its code is inside of the class definition.

    This is necessary, because as a member of B or D, something like
    'x<<y' would only resolve to using the function when the LEFT operand
    was of type B or D, which it will never be -- it'll always be (some
    descendant of) std::ostream. 

    -- 
    Later,
    Jerry.
The universe is a figment of its own imagination.
  • Next message: James Dennett: "Re: inline assert [C++]"

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