Why Do Virtual Functions Behave Unexpectedly During C Object Initialization?

Understanding Inheritance and Object Initialization in C for Virtual Function Access

In C , inheritance enables the creation of derived classes that inherit properties and behaviors from base classes. However, the initialization order of objects in C can impact how virtual functions behave.

Question:

When an object is constructed in C with a base class having a virtual function, why might that function behave unexpectedly?

Answer:

The key concept here is the order of initialization for base and derived classes. In C , base classes are constructed before derived classes.

Explanation:

In the provided example:

• The base class defines a virtual function value() that returns 0 by default.
• derived inherits from base and overrides the value() function to return 1.
• When derived is instantiated, it first constructs the base part of the object.
• However, at this point, the object is not yet a complete derived instance, as the derived class constructor hasn't been executed yet.
• Therefore, when the base constructor calls value(), it invokes the base version of the function rather than the overridden derived version, resulting in an output of 0.

To rectify this behavior:

To ensure that the derived class's value() function is called during object construction, you should initialize the base class explicitly with the address of the derived class instance:

derived example;
base(&example).value()

By passing the address of the derived object, you instruct the base constructor to call the value() function on the derived object, effectively "maturing" the object into its complete derived form.

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