C++ Inheritance pure virtual functions

Question

my problem is: How do I implement pure virtual functions in an inherited class? It always says i didn't implement the only function, but I tried to do it. So where is my mistake?

My code:

A.h:

class A {
public:
    A();
    virtual std::pair<A*, A*> f1(const A& o) const=0;
    virtual ~A();
};

B.h:

#include "A.h"

class B : public A {
public:
B();
virtual ~B();
virtual std::pair<A*, A*> f1(const A& o);
};

B.cpp:

#include "B.h"

B::B() : A() {}
B::~B() {}
std::pair<A*, A*> B::f1(const A& o) {
   A* K1=new B();
   A* K2=new B();
   return std::make_pair (K1, K2);
}

I get following error:

B.cpp: In member function ‘virtual std::pair<A*, A*> B::f1(const A&)’:
B.cpp:14:16: error: cannot allocate an object of abstract type ‘B’
    A* K1=new B();
                ^
In file included from B.cpp:1:0:
B.h:4:7: note:   because the following virtual functions are pure within ‘B’:
 class B : public A {
       ^
In file included from B.h:1:0,
                 from B.cpp:1:
A.h:10:28: note:    virtual std::pair<A*, A*> A::f1(const A&) const
  virtual std::pair<A*, A*> f1(const A& o) const=0;
                            ^
B.cpp:15:16: error: cannot allocate an object of abstract type ‘B’
    A* K2=new B();
                ^
In file included from B.cpp:1:0:
B.h:4:7: note:   since type ‘B’ has pure virtual functions
 class B : public A {
       ^

Also: What is correct, A* K1=new A(); or new B(); ?

A::f1 is a const method. You forgot the const at the end of the method in class B. — crayzeewulf
– crayzeewulf, Commented Dec 10, 2014 at 18:30

Mooing Duck · Accepted Answer · 2014-12-10 18:32:20Z

3

Lets' put those two side by side and look closer:

A: virtual std::pair<A*, A*> f1(const A& o) const=0; 
B: virtual std::pair<A*, A*> f1(const A& o);

Well, I see a difference, one of these is a const function, and the other is not. That makes them two different functions. Since the const function was never overloaded, B is still abstract, like A, and cannot be instantiated.

A* K1 = new A(); //would give you an A, if A weren't abstract.  Do you want an A or B?
A* K1 = new B(); //gives a B object, stored as a pointer to an A interface.

I also highly recommend using std::unique_ptr where you currently have owning raw pointers. They'll prevent headaches later.

answered Dec 10, 2014 at 18:32

Mooing Duck

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Comments

David G · Accepted Answer · 2014-12-10 18:32:13Z

3

Your override has to match in cv-qualification. You're missing a const here:

std::pair<A*, A*> B::f1(const A& o) /* [const] */

Since it doesn't override and your base class method is pure virtual, your derived class becomes abstract. You can't instantiate objects of abstract type.

You have to add const to both the declaration and definition. Also, to make sure it overrides, use the keyword override:

std::pair<A*, A*> f1(const A& o) override; // error since it does not override

answered Dec 10, 2014 at 18:32

David G

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Comments

Charlie · Accepted Answer · 2014-12-10 18:32:58Z

3

In B, your function needs to be virtual std::pair<A*, A*> f1(const A&) const; or it's a different function, not overriding the one from A.

(If you're using a C++11 compiler, do std::pair<A*, A*> f1(const A&) const override; and it becomes clear to the reader that you intended to override the function.)

answered Dec 10, 2014 at 18:32

Charlie

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