diamond shaped multiple inheritance pattern

Question

Below is a diamond problem faced in multiple inheritance,

class Base {
 public:
  Base() {
    cout << "Empty Base constructor " << endl; 
  }
  Base(const string & strVar) { 
    m_strVar = strVar; 
    cout << m_strVar << endl;
  }
  virtual ~Base() {
    cout << "Empty Base destructor " << endl;
  }
  virtual const string & strVar() const { 
   return m_strVar; 
  }

  string m_strVar;
};

class Derived1: public virtual Base {
 public:
  Derived1() {
    cout << "Empty Derived1 constructor " << endl; 
  }
  Derived1(const string & strVar) : Base(strVar) {
    cout << " Derived1 one arg constructor" << endl;
  }
  ~Derived1() {
    cout << "Empty Derived1 destructor " << endl;
  }
};

class Derived2: public virtual Base {
 public:
  Derived2() {
    cout << "Empty Derived2 constructor " << endl; 
  }
  Derived2(const string & strVar) : Base(strVar) {
    cout << "Derived2 one arg constructor" << endl;
  }
  ~Derived2() {
    cout << "Empty Derived2 destructor " << endl;
  }
};

class Derived: public Derived1, public Derived2 {
 public:
  Derived(const string & strVar) : Derived1(strVar), Derived2(strVar) {
    cout << "Derived Constructor " << endl; 
  }
  ~Derived() {
    cout << "Empty Derived destructor " << endl;
  }
};

int main() {
  Derived derObj ("Print this if you can ! "); 
}

the output i get is

Empty Base constructor
Derived2 one arg constructor
Derived1 one arg constructor
Derived Constructor
Empty Derived destructor
Empty Derived2 destructor
Empty Derived1 destructor
Empty Base destructor

i wonder why my derObj parameter i.e "Print this if you can" is not printed and output is not like

Empty Base constructor
Derived2 one arg constructor
Print this if you can!
Derived1 one arg constructor
Derived Constructor
Empty Derived destructor
Empty Derived2 destructor
Empty Derived1 destructor
Empty Base destructor

Matthieu M. Matthieu M. · Accepted Answer · 2012-02-16T09:29:51

This has to do with virtual inheritance.

When a class is inherited virtually, it is the responsability of the most derived class in the hierarchy to call its constructor: here Derived.

Since Base is default constructible and you did not precise anything, Derived invokes the default constructor of Base.

If you want the string to be printed, use:

Derived(const string & strVar) : Base(strVar), Derived1(strVar), Derived2(strVar)
{
  std::cout << "Derived Constructor\n"; 
}

You could remove the default constructor to let the compiler diagnose the issue, though not all compilers give very helpful messages.

diamond shaped multiple inheritance pattern

1 Answers