Why is \'= 0\' Used to Initialize Pure Virtual Functions in C ?

Explaining the Pure Virtual Function Initialization

In C , pure virtual functions are declared using the syntax "virtual void fun() = 0;". This unconventional initialization with "0" has prompted the question of its purpose and necessity.

Background: Virtual Functions and Vtables

Virtual functions allow classes to inherit methods with varying implementations based on the object's type. C uses virtual tables (vtables) to store the addresses of these methods for each object. When a function is called, the vtable entry for that function points to the appropriate method implementation.

The Importance of "0" Initialization

The initialization of a pure virtual function with "0" is not essential for the implementation of vtables. However, Bjarne Stroustrup, the creator of C , chose this syntax because it provided a convenient way to accomplish two important goals:

1. Distinguishing Pure Virtual Functions

The "0" initialization signals that a function is pure virtual, meaning it must be overridden in derived classes. This prevents the use of the pure virtual function directly in the base class and enforces proper polymorphism.

2. Avoiding Ambiguity

Without a dedicated syntax for pure virtual functions, confusion could arise when trying to define a function with a default value of 0. The use of "0" exclusively for pure virtual functions clarifies their purpose and provides a clear distinction from regular functions.

Note: Initializing a pure virtual function with "0" does not set the corresponding vtable entry to NULL. The implementation of vtables varies across different compilers and may involve other techniques for handling pure virtual functions.

The above is the detailed content of Why is \'= 0\' Used to Initialize Pure Virtual Functions in C ?. For more information, please follow other related articles on the PHP Chinese website!