inheritance,class,c++

[sajid dayer]

#include<iostream>
using namespace std;

class a
{

public:
void add()
{
cout<<"in A";
}
void fn()
{
add();
}
};
class b: public a
{
public:
void add()
{
cout<<"in b";
}

};
int main()
{
b d;
d.fn();
return 0;

}
// in this code why a::add() invoke but i think b::add() called because d is the type of class b

[Peter87]

If you want to call the version of function a() depending on the dynamic type of the object you should make the function a virtual function.

class a
{
public:
	virtual void add()
	{
		cout<<"in A";
	}
	void fn()
	{
		add();
	}
};

[MiiNiPaa]

Why would b::add invoked and not a::add?

Just for fun, add this in main():

d.a::add();
d.b::add();

Both a::add and b::add are exist at the same time in your class. And as class a does not know that anything is derived from it, all functions of class a uses class a functions only.
To achieve dynamic binding (selection of function depending on real variable type) make add virtual.
https://isocpp.org/wiki/faq/virtual-functions
http://www.learncpp.com/cpp-tutorial/122-virtual-functions/

#include<iostream>

class a
{
public:
    virtual void add()
    { std::cout << "in A"; }

    void fn()
    { add(); }
};

class b: public a
{
public:
    virtual void add() override
    { std::cout << "in b"; }
};

int main()
{
    b d;
    d.fn();
}

http://ideone.com/4Joobo

[sajid dayer]

i know if i make a::add() virtual then b::add() invoke
but want to know here that why b::add() not invoke in this code?

[MiiNiPaa]

because fn is told to call a::add and not b::add. Also it is not told to try to see overridden function in derived classes (what virtual do).

Remember: you might think that both a and b contains function with the same name, but thanks to name mangling those functions have completely different names.

something like

b d;
d.fn();

is compiled into something like

b d;
__a__fn(d);

And said function is like

__a__fn(b& x)
{
    __a__add(x);
}

virtual keyword changes how function is called to something like

__a__fn(b& x)
{
    x.__vtable[__add](x);
}

using saved information from virtual table to select correct function to call

[sajid dayer]

yes i got it
thanks