diamond shaped multiple inheritance pattern
-
30-04-2021 - |
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
La solution
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.
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