Why Does Virtual Inheritance Cause Constructor Initialization Errors in C ?

Virtual Inheritance and Constructor Initialization

Virtual inheritance is a technique used in C to resolve the ambiguity of multiple inheritance. When a class inherits from multiple base classes, which have the same member, virtual inheritance ensures that only a single copy of that member is created, reducing memory overhead.

However, virtual inheritance introduces a unique situation when it comes to constructor initialization. In the example provided, we have three classes: Base, A, and B, where both A and B virtually inherit from Base. Class C then inherits from both A and B.

The Problem

In the constructor of class C, an error occurs during the initialization of Base. The compiler cannot match the function call to Base() because C does not directly inherit from Base. So why does this error occur?

Understanding Virtual Base Class Initialization

Virtual base classes have a different initialization process compared to non-virtual base classes. When a virtual base class is inherited, it is the responsibility of the most derived class to initialize it. In this case, class C is the most derived class, and it must initialize the virtual base class Base.

Since Base does not have a direct instance of C, it cannot be initialized using the A or B constructors. Instead, Base must be initialized using its default constructor. However, the code provided does not include the default constructor for Base.

The Solution

To resolve this issue, you need to define a default constructor in the Base class, like this:

class Base {
public:
  Base() = default; // Default constructor added
  Base(Base* pParent);
  /* implements basic stuff */
};

This default constructor will be used to initialize the Base virtual member variable in class C.

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