Why is the default constructor of the virtual base class called in a virtual inheritance hierarchy?

Virtual Inheritance and Default Constructor Invocation

In an inheritance hierarchy involving virtual inheritance, the default constructor of the virtual base class can be called unexpectedly. Consider the following code:

class grandmother {
public:
    grandmother() { // Default constructor
        std::cout << "grandmother (default)" << std::endl;
    }
    grandmother(int attr) { // Parameterized constructor
        std::cout << "grandmother: " << attr << std::endl;
    }
};

class mother: virtual public grandmother {
public:
    mother(int attr) : grandmother(attr) {
        std::cout << "mother: " << attr << std::endl;
    }
};

class daughter: virtual public mother {
public:
    daughter(int attr) : mother(attr) {
        std::cout << "daughter: " << attr << std::endl;
    }
};

int main() {
  daughter x(0);
}

When an instance of the daughter class is created, the output is:

grandmother (default)
mother: 0
daughter: 0

Despite the existence of a parameterized constructor in the grandmother class, the default constructor is invoked. Why is this happening?

Invocation of Virtual Base Class Constructor

In virtual inheritance, the virtual base class's constructor is directly called by the most derived class's constructor. In this case, the daughter constructor directly calls the grandmother constructor.

Since the mother class does not explicitly call the grandmother constructor in its initialization list, the default constructor is used. To correctly invoke the desired constructor, the daughter constructor should be modified to:

daughter(int attr) : grandmother(attr), mother(attr) { ... }

By explicitly calling the grandmother(attr) constructor in the initialization list, the correct constructor is used, and the output becomes:

grandmother: 0
mother: 0
daughter: 0

The above is the detailed content of Why is the default constructor of the virtual base class called in a virtual inheritance hierarchy?. For more information, please follow other related articles on the PHP Chinese website!