What is the reason for compilation error here?

Question

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Please experts clear my doubts.
I don't understand why it throws compilation error. These are polymorphic classes, in main I assigned base pointer with a derived class pointer, it should find func_2 as __vptr of base will point to __vtable of derived class.
Or I have some gap in my understanding of __vtable and __vptr.
Please suggest me some good articles to clear my below doubts.

C++

class base
{
  public:
     virtual int func_1()
       {
         cout << "In func_1" ;
       }
};
class deri: public base
{
    public:
        virtual int func_2()
        {
          cout << "in func_2"; 
        }
};

main()
{
 base *ptr = new deri;
 ptr->func_2();//ptr is pointing to derived object
}

What I have tried:

I tried compiling it, it throws compilation error:

C++

main.cpp: In function ‘int main()’:
main.cpp:32:6: error: ‘class base’ has no member named ‘func_2’
 ptr->func_2();
      ^

Posted 22-Nov-18 20:24pm

Member 13913102

Updated 22-Nov-18 21:14pm

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Comments

Member 13913102 23-Nov-18 4:01am

vtable explanation which I learnt is here:
http://www.learncpp.com/cpp-tutorial/125-the-virtual-table/

2 solutions

Solution 2

In order to make polymorphism work the method name (and signature) must be the same. Being ptr a actual deri pointer, you might still access its func_2 method via downcast, but that's not polymorphism:

C++

#include <iostream>
using namespace std;

class base
{
  public:
     virtual void func_1()
       {
         cout << "In func_1 of base.\n" ;
       }
};
class deri: public base
{
    public:
        virtual void func_1() override
        {
          cout << "in func_1 of deri.\n";
        }
        virtual void func_2()
        {
          cout << "in func_2 of deri.\n";
        }

};

int main()
{
 base *ptr = new deri;
 ptr->func_1();//ptr is pointing to derived object
 (dynamic_cast<deri *>(ptr))->func_2(); // this is not polymorphism, though.
}

Posted 22-Nov-18 21:15pm

CPallini

Comments

Member 13913102 23-Nov-18 3:59am

yes I missed on polymorphism, but see this link, it says __vptr will point to __vtable of derived class, and this __vtable will have pointer to func_2, then it must be accessed.
http://www.learncpp.com/cpp-tutorial/125-the-virtual-table/

CPallini 23-Nov-18 4:05am

In fact it is accessible, as my code shows.

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Afzaal Ahmad Zeeshan · Accepted Answer · 2018-11-22T20:44:00

Oh boy, you got polymorphism and type safety all confused up there. What you are expecting to happen is wrong, that is not why a type is inherited. Inheritance works from parent, to child. So a function in parent, is guaranteed—let's ignore the access modifiers in inheritance for a minute—in the child type, but not the other way around. You cannot call a child function from a parent type. Parent type just doesn't know what that function is. So,

C++

class Foo { 
   int a;
   int b;
};

class Bar {
   string name;
};

In the code above, you can do this,

C++

Bar b;
cout << b.a; // a comes from Foo

But you cannot do this,

C++

Foo f;
cout << f.name; // name does not exist

But, in inheritance, polymorphism works just fine. So in your code, if we do this,

C++

class deri : public base
{
public:
    int func_1() override  // <-- let's override, since base has it virtual
    {
        cout << "Derived func_1";
        return 0; // <-- Why did you forget this and didn't complain? 
    }

Now, when you run the same code,

C++

base* base = new deri;
base->func_1();

It would print, "Derived func_1". Why? Because of polymorphism.

I do not want to indulge you in the details of C++ memory model, because all this detail is applied there. Just remember, inheritance works from parent to child, but polymorphism forces that the function calls be done on the actual instance type—in your case new deri;.

Please see the links below, to learn a bit more on this concept,
Polymorphism (computer science) - Wikipedia[^]
Type safety - Wikipedia[^]
Memory model - cppreference.com[^]
c++ - How does inheritance actually work? - Stack Overflow[^]
Friendship and inheritance - C++ Tutorials[^]

What is the reason for compilation error here?

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What is the reason for compilation error here?

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