Re: Confusion about inheritance and the allocating memory via new

=?ISO-8859-1?Q?Erik_Wikstr=F6m?= <>
Sun, 29 Jul 2007 18:39:01 GMT
On 2007-07-29 20:26, 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 {
    virtual void set_values (int a, int b) { width=a; height=b; };

    int width, height;

class CRectangle: public CPolygon {
    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

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