Re: Definition of Pure Virtual Function

From:

"Alex Blekhman" <xfkt@oohay.moc>

Newsgroups:

microsoft.public.vc.language

Date:

Sun, 14 Jan 2007 16:17:25 +0200

Message-ID:

<etW54a#NHHA.4720@TK2MSFTNGP03.phx.gbl>

"Anand Choubey" wrote:

As we read in all C++ Docs that definition of pure virtual
fucntion is given
in driver class only. But following code is compiling and
running but there
is definition of pure virtual function.
Please tell what is purpose of defition of pure virtual
function.
class CA
{
public:
virtual void f() = 0;

};

void CA::f()
{
}

class CB : CA
{
public:
virtual void f();
};

void CB::f()
{
}

Presence of pure virtual function in base class doesn't mean
that base class cannot have definition of such function.
Pure virtual function in base class forces derived class to
provide implementation, otherwise you won't be able to
create an instance of derived class. Usually, base class
provides body for pure virtual function on order to place
there diagnostic messages or debugging code. For example,

void CA::f()
{
    printf("Error! Pure virtual call: %s",
        __FUNCTION__);
    abort();
}

This is what compiler does, more or less with pure virtual
functions, so user gets R6025 run-time error when pure
virtual function is called. See here for additional info:

"Description of the R6025 run-time error in Visual C++"
http://support.microsoft.com/kb/125749

Alex