Calling pure virtual function in constructor gives an error

Question

class a //my base class
 {
public:
    a()
    {
        foo();
    }
  virtual void foo() = 0;
};



class b : public a
{
    public:
    void foo()
    {
    }
};

int main()
{
    b obj; //ERROR:  undefined reference to a::foo()
}

Why it gives me error? The pure virtual foo is defined. What do I need to change in my code to make it work? I need pure virtual method from base class be called in its constructor.

score 15 · Accepted Answer · answered Feb 12 '13 at 11:09

15

Calling virtual functions in a constructor is recognised as a bad thing to do.

During base class construction of a derived class object, the type of the object is that of the base class. Not only do virtual functions resolve to the base class, but the parts of the language using runtime type information (e.g., dynamic_cast (see Item 27) and typeid) treat the object as a base class type.

So your instantiation of b invokes the a constructor. That calls foo(), but it's the foo() on a that gets called. And that (of course) is undefined.

answered Feb 12 '13 at 11:09

Brian Agnew

268,207
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I see. I will change the design. – user1873947 Feb 12 '13 at 11:12
would making class a abstract solve the issue? – Falaque Feb 12 '13 at 11:14
1

@Falaque - I don't believe so. See the linked article by Scott Meyers – Brian Agnew Feb 12 '13 at 11:19
@user1873947 the Class 'a' (see the example in question). – Falaque Feb 12 '13 at 12:02
@BrianAgnew can you or anyone else suggest a better pattern/design? – Falaque Feb 12 '13 at 12:03

score 2 · Answer 2 · answered Feb 12 '13 at 11:41

2

Quoted from a book "Let Us C++"by Yashwant Kanetkar

It is always the member function of the current class , is called.That is , the virtual mechanism doesn't work within the constructor

So, the foo() of class a gets called. Since it is declared pure virtual, it will report an error

answered Feb 12 '13 at 11:41

Govind Balaji

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It'll report an error... or compile fine but cause UB, crashing _if you're lucky_. Either way, don't do it. – underscore_d Jul 03 '16 at 00:40

Hui Zheng · Answer 3 · 2013-02-12T11:36:28.643

1

foo function is called in class a's constructor, and at that time, object b has not been fully constructed yet, hence it's foo implementation is unavailable.

Quoted from "Effective C++":

Don't call virtual functions during construction or destruction, because such calls will never go to a more derived class than that of the currently executing constructor or destructor

edited Feb 12 '13 at 11:36

answered Feb 12 '13 at 11:11

Hui Zheng

10,084
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Calling pure virtual function in constructor gives an error

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