Why Does Virtual Inheritance Trigger the Default Constructor in C ?

Understanding Virtual Inheritance in C : Why Default Constructor is Called

In the realm of object-oriented programming, inheritance plays a pivotal role in code organization and code reuse. However, with virtual inheritance, certain behaviors can be unexpected. This article aims to clarify why the default constructor is called in the context of virtual inheritance, a specific type of inheritance in C .

Virtual Inheritance: A Brief Introduction

Virtual inheritance is used to resolve ambiguity in multiple inheritance scenarios. It allows multiple derived classes to inherit from a common base class without causing diamond-shaped inheritance issues. In such cases, the virtual base class is inherited only once, and its constructor is called directly by the most derived class.

The Dilemma: Default Constructor Invocation

Consider the following code snippet:

class grandmother {
public:
    grandmother() { cout << "grandmother (default)" << endl; }
    grandmother(int attr) { cout << "grandmother: " << attr << endl; }
};

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

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

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

When executing this code, we encounter a puzzling behavior. Instead of calling the grandmother(int) constructor for the daughter class, as one might expect, the default constructor for the grandmother class is triggered. This raises the question: why is the default constructor called?

Unraveling the Mystery: The Virtual Inheritance Effect

The key to understanding this behavior lies in the nature of virtual inheritance. When virtual inheritance is employed, the virtual base class's constructor is called directly by the most derived class's constructor. In this scenario, since daughter is the most derived class, its constructor is responsible for invoking the grandmother constructor.

However, since no explicit call to the grandmother(int) constructor is specified in the initialization list for the daughter constructor, the default constructor is invoked instead. To rectify this, one would need to explicitly call the desired grandmother constructor as follows:

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

This modification ensures that the correct constructor for the grandmother class is invoked.

Conclusion

In the realm of virtual inheritance, understanding the mechanics behind constructor invocation is essential for avoiding unexpected behavior. By understanding that the virtual base class's constructor is called directly by the most derived class's constructor, one can anticipate and control the flow of constructor calls. This knowledge empowers developers to design robust and well-structured inheritance hierarchies in C .

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