Re: Confusion about inheritance and the allocating memory via new

From:

=?ISO-8859-1?Q?Erik_Wikstr=F6m?= <Erik-wikstrom@telia.com>

Newsgroups:

comp.lang.c++

Date:

Sun, 29 Jul 2007 18:39:01 GMT

Message-ID:

<995ri.5017$ZA.2156@newsb.telia.net>

On 2007-07-29 20:26, tharringtonan@netscape.net wrote:

I am compiling the following code, main.cpp, as follows:
gcc main.cpp

I get the following compile error:
main.cpp: In function `int main()':
main.cpp:28: no matching function for call to `CPolygon::area()'

The code is listed below:

#include <iostream>
using namespace std;

class CPolygon {
  public:
    virtual void set_values (int a, int b) { width=a; height=b; };

  protected:
    int width, height;
  };

class CRectangle: public CPolygon {
  public:
    int area () { return (width * height); };
  };

int main ()
{
CPolygon * ppoly1 = new CRectangle;
ppoly1->set_values (4,5);
cout << ppoly1->area() << endl;
return 0;
}

Because a CPolygon does not have a area()-method. Only CRectangle does.
Either declare ppoly1 as a pointer to a CRectangle, or add

virtual int area() = 0;

to CPolygon right under the declaration of set_values().

By the way, set_values is a really bad name, and probable a bad method
as well. Since it has polymorphic behaviour the user can know what
values it will set. Read up on the Liskov substitution principle to see why.

--
Erik Wikstr?m