Polymorphism

Hi,
In the following code..

#include<iostream>

using namespace std;

struct game{
virtual void play()
{
cout<<"unknown"<<endl;
}
};

struct tennis :public game{
void play()
{
cout<<"tennis"<<endl;
}
};

void Val(game b)
{
b.play();
}

int main()
{
game *pt=new tennis;
pt->play(); // prints tennis------------>1
(*pt).play(); // prints tennis ---------> 2
Val(*pt); // prints unknown ----------> 3

return 0;
}

The line marked with 1, prints "tennis", I understand.
I expected 2 and 3 to print "unknown" , but 2 prints "tennis" and 3
prints "unknown".
In both the cases i am invoking the method on a object(not on a pointer
or reference) and i do not expect a polymorphic behaviour.
Is my understanding wrong?

Thanks,
Senthil

re: Polymorphism

Senthil wrote:[color=blue]
>
> int main()
> {
> game *pt=new tennis;
> pt->play(); // prints tennis------------>1
> (*pt).play(); // prints tennis ---------> 2
> Val(*pt); // prints unknown ----------> 3
>
> return 0;
> }
>
> The line marked with 1, prints "tennis", I understand.
> I expected 2 and 3 to print "unknown" , but 2 prints "tennis" and 3
> prints "unknown".
> In both the cases i am invoking the method on a object[/color]

pt->play is a syntactic sugar for the expression (*pt).play
i.e., both of them are same as far as compiler is concerend. There is
no object slicing involved.

In third, however, you are passing "by value" which will lead to object
slicing.

re: Polymorphism

Senthil wrote:[color=blue]
> Hi,
> In the following code..
>
> #include<iostream>
>
> using namespace std;
>
> struct game{
> virtual void play()
> {
> cout<<"unknown"<<endl;
> }
> };
>
> struct tennis :public game{
> void play()
> {
> cout<<"tennis"<<endl;
> }
> };
>
> void Val(game b)
> {
> b.play();
> }
>
> int main()
> {
> game *pt=new tennis;
> pt->play(); // prints tennis------------>1[/color]
Correct via base class of tennis object pointed to by pt[color=blue]
> (*pt).play(); // prints tennis ---------> 2[/color]
Correct via base class of tennis object dereferenced from pointer pt[color=blue]
> Val(*pt); // prints unknown ----------> 3[/color]
Correct base clase object passed by value hence no longer part of a
descendant class,(note if it is passed by reference, which I prefer,
tennis would be printed).[color=blue]
>
> return 0;
> }
>
> The line marked with 1, prints "tennis", I understand.
> I expected 2 and 3 to print "unknown" , but 2 prints "tennis" and 3
> prints "unknown".
> In both the cases i am invoking the method on a object(not on a pointer
> or reference) and i do not expect a polymorphic behaviour.
> Is my understanding wrong?
>
> Thanks,
> Senthil
>[/color]
JB