How virtual function is working in the below code

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#include <stdio.h>
#include <string.h>
using namespace std;

class Base {
public: 
    virtual void gogo(float a){
        printf(" Base :: gogo (int) \n");
    };

    virtual void gogo(char *p){
        printf(" Base :: gogo (int*) \n");
    };
};

class Derived : public Base{
public:
    virtual void gogo(char *p){
        printf(" Derived ::  (int*)");
    };
};

int main(){
    Derived obj;
    obj.gogo(4.2);
}

why the code is giving error. even if the function gogo is present in the base class. but while calling it it gives error

Regarding your second doubt, that is how virtual keyword works. It is used to implement polymorphism.

display1 is only implemented in the base class A. So when virtual is not used for display, obj.display1 ends up calling the display of class A. But when virtual is used for display, it will call display of class B. In this case, the display of class B has overridden the display of class A.

Virtual Functions, What happens if we don't use virtual function in inheritance? A virtual function is a member function which is declared within a base class and is re-defined(Overriden) by a derived class. When you refer to a derived class object using a pointer or a reference to the base class, you can call a virtual function for that object and execute the derived class’s version of the function.

case : If display() is not declared as virtual: obj.display1() will call A::display1() as its not being overridden by class B. As display1() is not virtual it will not be checked for dynamic function resolution (@virtual table) and end up calling A::display().

case : If display() is declared as virtual: For the Call to display(), the object's virtual table (obj : Class B) will be referred for function address and that will lead to the call to B::display().

Virtual Function in C++, A virtual function is a member function which is declared within a base class and is re-defined(Overriden) by a derived class. When you In above code, base class pointer 'bptr' contains the address of object 'd' of derived class. Working of virtual functions(concept of VTABLE and VPTR) Consider the example below:. A virtual function is used to perform late binding as well as dynamic linkage operations by telling the compiler. Therefore, it’s used to point to the base class. Recommended Articles. This is a guide to Virtual Functions in C++. Here we discuss the Introduction and how virtual functions work along with examples and code implementation.

This is a well-known[citation needed] nuance of C++, and has to do with the way multiple overloads interact with inheritance.

Take virtual out of the equation, because it's not relevant here. The way inheritance works is by the way lookup first examines the class you have, then examines the base if no match was found.

In this example you have three separate functions (even though they all share a name (ish)): Base::gogo(float), Base::gogo(char*) and Derived::gogo(char*). You want the compiler to find Base::gogo(float). By calling gogo with a float, you'd expect it to look for gogo(float) in Derived then, not finding it, look for gogo(float) in Base then succeed.

However, instead, it looks for gogo (ignoring the argument type!) in Derived, finds it, then examines all the overloads in that class, finds that there isn't one that takes float, and fails. And that's it.

We say that Derived::gogo(char*) "hides" Base::gogo(float).

This is easily solved, though, with a using declaration:

#include <stdio.h>
#include <string.h>
using namespace std;

class Base {
public: 
    virtual void gogo(float a){
        printf(" Base :: gogo (int) \n");
    };

    virtual void gogo(char *p){
        printf(" Base :: gogo (int*) \n");
    };
};

class Derived : public Base{
public:
    using Base::gogo;  // <--- Here!

    virtual void gogo(char *p){
        printf(" Derived ::  (int*)");
    };
};

int main(){
    Derived obj;
    obj.gogo(4.2);
}

In this way, we essentially create a Derived::gogo(float) with the same contents as Base::gogo(float), without having to write it out again. Consider it a kind of forced inheritance.

Yes, it's a bit weird. But you'll get used to it.

Note that this has nothing to do with virtual.

Virtual function inheritance, What happens when a virtual function is called inside a non-virtual function in C​++ Since there is no virtual function, run time polymorphic behavior doesn't not work in the above code. this -> print(); // called under non - virtual function. }. Perhaps you should read up on how virtual functions and polymorphism work. You cannot inherit in the base from the derived. You cannot inherit in the base from the derived. The virtual function doesn't override a matching virtual function in the base and only affects usage from a Derived* .

15: Polymorphism & Virtual Functions, A virtual function allows derived classes to replace the implementation provided by the base class. Dynamic binding means that the address of the code in a member function Now let's work out a derived class. const; // Covariant Return Types; see below; Circle* create() const; // Covariant Return Types; see below; // . The above program code is the alteration of the program 1, the disp() function is defined with the keyword virtual and now when the obj1 calling the disp() function, the disp() function of the child class B is executed.

What happens when a virtual function is called inside a non-virtual , In this article we will discuss that how virtual functions in C++ works internally while code execution, correct function should be called. If you didn't find what you were looking, then do suggest us in the comments below. A virtual function is a member function in the base class that you expect to redefine in derived classes. Before going into detail, let's build an intuition on why virtual functions are needed in the first place.

Inheritance, The code below shows only public members are accessible from outside of the class: class A { public: int xPublic; protected: int xProtected; private: int xPrivate; };​  Virtual function is the member function of a class that can be overriden in its derived class. It is declared with virtual keyword. Virtual function call is resolved at run-time (dynamic binding) whereas the non-virtual member functions are resolved at compile time (static binding). Describe the virtual function and virtual function table.

Comments
  • your code doesn't compile, there is no definition for C::display(). When I fix this by changing void display() { to void C::display() { your code works as expected.
  • Thanks.. can you clear the second doubt
  • When you call display1() on a B, A::display1() is called which in turn will call A::display() if it isn't overwritten by a class derived from A. When A::display() is not declared virtual it cannot be overwritten.
  • This has nothing to do with virtual calls. It's a lookup problem.
  • The code in the question is now very different than when it was first written.
  • @P.W Does this answer match how the question is now?
  • Yes, I believe so.
  • @P.W Good stuff!
  • Yeah. So what can one do if his answer is downvoted as it no longer applies to the current version of the question.