CODEWITHSUNDEEP
c++constructormultiple-inheritance
PolarPenguin
PolarPenguin

Reputation: 13

How do I avoid explicitly constructing everything inherited in an initializer list in C++?

When I construct an object D I need to include the constructors for A, B, and C in the initializer list. Is there any way to make it so that I don't need all three in the initializer list or not?

If I try to initialize D using only a constructor for B I get an error because I don't have a default constructor for A or C. If I add a default constructor for A and C I get issues with "i" being reinitialized with no value. 

#include <iostream>
using namespace std;

class A 
{
    int i;
public:
    A(int ii) :
        i(ii)
    {}

    ~A() { }

    int getI() { return i; }
};

class B : public virtual A
{
public:
    B(int ii) :
        A(ii)
    { }

    ~B() { }
};

class C : public virtual A 
{
public:
    C(int ii) :
        A(ii)
    { }

    ~C() { }
};

class D : public B, public C 
{
public:
    D(int ii) :
        A(ii), B(ii), C(ii)
    { }
    ~D() { }

 };

int main() 
{
    D d(45);
    cout << d.getI() << endl;
}

Upvotes: 1

Views: 75

Answers (3)

J.R.
J.R.

Reputation: 1978

How about adding a default value to the A, B, C constructors, i.e.

A(int ii=0) :

Upvotes: 0

R Sahu
R Sahu

Reputation: 206577

If you add default constructors to A, B, and C, the implmentation of D becomes a bit simpler.

#include <iostream>
using namespace std;

class A 
{
   int i;
   public:
   A() : i(0) {}
   A(int ii) : i(ii) {}

   ~A() { }

   int getI() { return i; }
};

class B : public virtual A
{
   public:
      B() { }
      B(int ii) : A(ii) { }

      ~B() { }
};

class C : public virtual A 
{
   public:
      C() { }
      C(int ii) : A(ii) { }

      ~C() { }
};

class D : public B, public C 
{
   public:

      // Relies on default constructors of the other classes.
      D() { }

      // Relies on the default constructors of B and C.
      D(int ii) : A(ii) { }
      ~D() { }

};

int main() 
{
   D d1(45);
   D d2;
   cout << d1.getI() << endl;
   cout << d2.getI() << endl;
}

Upvotes: 1

Lightness Races in Orbit
Lightness Races in Orbit

Reputation: 385144

I'm afraid not. With virtual inheritance, your most-derived class must initialise the virtual base.

Read more here.

Upvotes: 0

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