Re: Function expecting reference to object of Abstract class/structure

From:

"Jon" <jomar@spblocker.net>

Newsgroups:

comp.lang.c++

Date:

Tue, 16 Nov 2010 06:15:53 -0600

Message-ID:

<ibtsll$fki$1@news.eternal-september.org>

dust wrote:

I have a class/stucture A which is abstract as it contains all virtual
member functions.

Virtual functions don't make a class abstract. *Pure* virtual functions
do.

Now I have to call a function(void func(A& client),
but this function is expecting reference to object of type A. Since A
is abstract, I can't create an object, so how can I send an object to
this function? or how do deal with such situation.

It's not really "a situation", it's that you don't understand the
concepts involved. Research the topic "abstract vs. non-abstract" class.
(Some of the younger set like to use "concrete" to mean "non-abstract",
but for those who learned C++ around the time it was invented, "concrete"
has a very different meaning.)

class A // A is abstract
{
  public:
    virtual void DoSomething() = 0; // pure virtual function
};

class B: public A // B is not abstract
{
  public:
    // 'virtual' keyword is optional: compiler will make it virtual
    virtual void DoSomething(){} // provided implementation
};

void func(A& client)
{
    client.DoSomething(); // function from class derived from A will be
called
}

void func2()
{
    B b;
    func(b); // C++ polymorphism machinery makes this work
}