Why output is showing errors

Question

This is a code fro diamond tree problem of multiple inheritance and according to me this code is cool but it is showing some error on compilation ..help me to figure the error

#include<iostream>
using namespace std;

class A  //A Diamond tree problem
{
  int x;
public:
  A(int i) { x = i; }
  void print() { cout << x; }
};

class B: virtual public A
{
public:
  B():A(10) {  }
};

class C:  virtual public A 
{
public:
  C():A(20) {  }
};

class D: public B, public C{
};

int main()
{
    D d;
    d.print();
    return 0;
}

It gives an error because A needs to have a default constructor. Or you need to add a constructor to D which calls the A constructor. — Some programmer dude
– Some programmer dude, Commented May 27, 2013 at 12:31

Mike Seymour · Accepted Answer · 2013-05-27 12:54:14Z

8

It would be useful to see the error:

In constructor ‘D::D()’:
error: no matching function for call to ‘A::A()’

When using virtual inheritance, the virtual base class must be initialised by the most derived class. In this case, that is D; so in order to be able to instantiate D, it must initialise A:

class D: public B, public C
{
public:
    D():A(42) {}
};

Alternatively, you could provide A with a default constructor. Declaring any constructor will prevent the compiler from implicitly generating one for you.

edited May 27, 2013 at 12:54

answered May 27, 2013 at 12:32

Mike Seymour

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2 Comments

sethi Over a year ago

Can you explain a little more? Wouldn't compiler provide a default constructor? is it a specific behavior of virtual inheritance

Mike Seymour Over a year ago

@bapusethi: You only get an implicit default constructor if you don't declare any constructors of your own. Declaring A(int) will prevent that. That's true for all classes, regardless of virtual inheritance.

billz · Accepted Answer · 2013-05-27 12:52:15Z

3

You need to provide default construct for D and call A in member initialize list:

class D: public B, public C{
public:
   D():A(30){}
};

Or you could provide a default A constructor

A():x(0) {}

edited May 27, 2013 at 12:52

answered May 27, 2013 at 12:31

billz

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