about virtual functions and polymorphism

[Cambalinho]

if we do a virtual functions(polymorphism) why we need re-declare the functions(when we create a new class derived from other)?

[VictorN]

Did you try to define a virtual functions in the derived class without declaring it there?

[zizz]

---

[Cambalinho]

Did you try to define a virtual functions in the derived class without declaring it there?

 class a
{ 
     public:
          virtual void print(){};

};

class b: public a
{ 

};

void b::print()
{
     cout << "hi";
}

error: print not member of class b.
so how can i create polymorphism without re-declare the functions?

[2kaud]

Have a look at this simple example

#include <iostream>
using namespace std;

class a
{
public:
	virtual void print(){ cout << "a print\n"; }

};

class b : public a
{
public:
	virtual void print(){ cout << "b print\n"; }

};

void display(a *cl) { cl->print(); }

int main()
{
	a *pa = new a;
	b *pb = new b;

	display(pa);
	display(pb);
}

Different class pointers are passed to the same function display() and display() calls the appropriate function depending upon the class passed at run time. This is known as late binding (as opposed to early binding which is decided at compile time).

See http://stackoverflow.com/questions/5...ymorphism-in-c
http://www.cplusplus.com/doc/tutorial/polymorphism/
http://www.learncpp.com/cpp-tutorial...rived-objects/

[GCDEF]

 class a
{ 
     public:
          virtual void print(){};

};

class b: public a
{ 

};

void b::print()
{
     cout << "hi";
}

error: print not member of class b.
so how can i create polymorphism without re-declare the functions?

You declared it in a, but implemented it in b. You need to declare and implement it in the base class. If you need to change the behavior in the derived class, you need to declare and implement it again there too.

[Cambalinho]

You declared it in a, but implemented it in b. You need to declare and implement it in the base class. If you need to change the behavior in the derived class, you need to declare and implement it again there too.

so is there anyway for create functions in base, and change the behavior(if i need) in derived class without re-declare them?

[2kaud]

You can create an abstract base class which has at least one pure virtual function (declared but with no body). Then have the definition(s) for the virtual function(s) in the derived classes.
See http://msdn.microsoft.com/en-us/libr...=vs.60%29.aspx
http://stackoverflow.com/questions/1...tual-explained

From my earlier simple example

#include <iostream>
using namespace std;

class a
{
public:
	virtual void print() = 0;

};

class b : public a
{
public:
	virtual void print(){ cout << "b print\n"; }

};

class c : public a
{
public:
	virtual void print(){ cout << "c print\n"; }

};

void display(a *cl) { cl->print(); }

int main()
{
	b *pb = new b;
	c *pc = new c;

	display(pb);
	display(pc);
}

Class a is an abstract class and has no body for the function declaration. Class b and c derived from class a contain the required function body. Note that as class a is an abstract class you can't now instantiate from it. So

a *pa = new a;

is not correct and will generate a compiler error.

However, if you want to be able to instantiate from the base class then you will need to define the function in the base class and to redefine in each derived class where a different function implmentation is neded.

In this example

#include <iostream>
using namespace std;

class b
{
public:
	virtual void print(){ cout << "b print\n"; }

};

class c : public b
{
public:
	virtual void print(){ cout << "c print\n"; }

};

class d : public b
{
public:

};

void display(b *cl) { cl->print(); }

int main()
{
	b *pb = new b;
	c *pc = new c;
	d *pd = new d;

	display(pb);
	display(pc);
	display(pd);
}

although class d is derived from class b, d doesn't have a redefinition of print and so the base definition from class b is used.

[GCDEF]

so is there anyway for create functions in base, and change the behavior(if i need) in derived class without re-declare them?

No, but I don't see why that would be a problem.

[zizz]

so is there anyway for create functions in base, and change the behavior(if i need) in derived class without re-declare them?

What's so horrible about having to specify the signature of the function you override in the derived class?

If you want to minimize the typing you can even put the implementation directly in the definition, like.

class a { 
public:
    virtual void print() {};
};

class b : public a { 
public:
    void print() {cout << "hi";} // implementation in the class definition
};

Here you supply the absolute minimum amount of information that's theoretically possible, namely where the function belongs and what it's supposed to do. No redundancy whatsoever.

Another advantage is that the function becomes a candidate for inlining so this practice can even lead to faster code. This suggests the function shouldn't be too big (even though I'm certain a compiler wouldn't bloat the code by inlining a huge function in zillions of places.)

[OReubens]

if you want to create a new function implementation in the derived class, then you need to redeclare what you'll be reimplementing.

THe only way out of that is not using the normal virtual function system, but creating function pointers members in base (and initialize them in the constructor),
then change the function pointers in derived constructor, they'll have to be global/local scope if you don't want to change the header though.

The above is basically how virtual function works as well, but with all the nice compiler assistance to do the nitty gritty.