Re: How to use a derived class with constructor

From:

Lionel B <me@privacy.net>

Newsgroups:

comp.lang.c++

Date:

Thu, 10 Jul 2008 16:19:58 +0000 (UTC)

Message-ID:

<g55cre$buf$3@south.jnrs.ja.net>

On Thu, 10 Jul 2008 08:35:30 -0700, Angus wrote:

Hello

I want to use polymorphic feature.

Eg if I had a base Shape class and a derived Circle class I could do
things like this:

  Shape myShape;
  MyCircle* pCircle = &myShape;
  pCircle->SetValues(3,5);
  cout << pCircle->area();

But my derived class has a constructor which gets passed variables.

Eg
Derived(int var1, std::string var2)

So how do I instantiate it?

I tried Derived* pDer(params);

Perhaps you are thinking along the lines of:

class Shape
{
public:
  Shape();
  virtual ~Shape();
  virtual draw() =0; // pure virtual

  ...
};

class Circle : public Shape
{
public:
  Circle(/* arguments */);
  virtual ~Circle();
  virtual draw(); // implements circle drawing functionality

  ...
};

class Rectangle : public Shape
{
public:
  Rectangle(/* arguments */);
  virtual ~Rectangle();
  virtual draw(); // implements rectangle drawing functionality

  ...
};

then:

  Circle circle(3,"foo");
  Shape* pShape1 = &circle;

  Rectangle rectangle(6,7,"bar");
  Shape* pShape2 = &rectangle;

  ...

  pShape1->draw(); // draws circle
  pShape2->draw(); // draws rectangle

You get the idea.

--
Lionel B