Polymorphism

[Senthil]

Hi,
In the following code..

#include<iostream>

using namespace std;

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

struct tennis ublic 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

 

[Neelesh Bodas]

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

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.

 

[n2xssvv g02gfr12930]

Hi,
In the following code..

#include<iostream>

using namespace std;

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

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

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

int main()
{
game *pt=new tennis;
pt->play(); // prints tennis------------>1

Correct via base class of tennis object pointed to by pt

(*pt).play(); // prints tennis ---------> 2

Correct via base class of tennis object dereferenced from pointer pt

Val(*pt); // prints unknown ----------> 3

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).

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

JB