Polymorphism

[MRKSalman0349]

Hey guys i used polymorphism and Inheritance but still i van not find any i=usage except that it avoids code duplication .... what else could be the use of polymorphism and inheritance plz explain me ..... i'll be thankfull to you....
plz help.....

[hyperfine]

I'm no expert so it'd be good to get a second opinion, but I think a lot of these higher level concepts exist just to avoid code duplication. By that, I mean that polymorphism and inheritance is never necessary. One could always achieve the same thing by writing a large block of simpler code.

Functions are a great example of the same principle. Let's say you have a need to select the greater of two numbers:

int selectgreater(int a, int b)
{
if( a>b )
{
return a;
}
else
{
return b;
}
}

You could write that in your code every time you wanted to use it, but we use functions to avoid unnecessary duplication of the code.

[LB]

It's not about code duplication, it's about not needing to know the actual type of an object to have it do what you want.

[hyperfine]

It's not about code duplication, it's about not needing to know the actual type of an object to have it do what you want.

Not OP, but I'm not sure I get this and I want to understand. By creating an object of a certain derived class, don't you already know which parent classes it belongs to?

[LB]

void f(Base &b)
{
}

Do you know the actual type of "b"? Is it MyDerived? Is it TheirDerived? It doesn't matter.

[hyperfine]

Ah, I see what you mean. Thanks for the example.

[MRKSalman0349]

LB plz give me a clear answer .... im not getting it .....

[MiiNiPaa]

For example let there be Person class and there is a function which reads data from stream:

Person read_person(std::istream& in)
{
    Person p;
    in >> p.name >> p.surname >> p.age;
    return p;
}

It takes a reference to istream. One example of istream object is std::cin: standard input.
There is ifstream class, which is derived from istream. Thanks to inheritance we can do following:

Person x, y;
x = read_person(std::cin); //read data from console

std::ifstream input("persondata.txt");
y = read_person(input); //read data from a file 

Note that our read_person function does not know about file streams existense at all, but can use them. If we want, we can create class network_stream derived from istream and it would work with that function too.

[hyperfine]

I've noticed my usage of classes differs slightly from yours. Is there any significant difference between what you did, and what I typically do? Here's an example:

using namespace std;

class Person
{

string name, surname;
int age;

Person();
void read_person(std::istream& in);
};

Person p;

Person::Person()
{}

void Person::read_person(std::istream& in)
{
in >> name >> surname >> age;
return;
}

Would our two codes achieve the same thing? I've never myself declared a class within a class like you did with:

Person read_person(std::istream& in)
{
Person p;
}

Thanks! (btw, how do I indent lines in code mode here?)

[MiiNiPaa]

btw, how do I indent lines in code mode here?

Spaces. Or copypaste already idented code from your favorite editor.

In my code read_person is not a class member, it is declared as standalone function to decrease class dependencies. Your code would work to and it does the same thing but has slightly different interface:

//My usage:
x = read_person(cin);
//Your
x.read_person(cin);

Both are fine.

[hyperfine]

Oh, I see. I automatically read that as Person::read_person, but obviously it would still have to have a return type. Thanks for the info!