Why Do "Pure Virtual Function Call" Errors Happen in Constructors and Destructors?

Unveiling the Enigma of "Pure Virtual Function Call" Crashes

In the realm of programming, encountering crashes伴随着 "pure virtual function call" errors can be perplexing, leaving developers scratching their heads. This article aims to shed light on the root causes behind these elusive errors.

The Abstract Nature of Pure Virtual Functions

Pure virtual functions are a fundamental aspect of object-oriented programming, enabling the declaration of functions in abstract classes that must be implemented in derived classes. By design, pure virtual functions have no implementation in the abstract class.

The Culprit: Constructor and Destructor Calls

However, the trouble arises when a virtual function call is attempted from within a constructor or destructor. In such cases, an inherent limitation comes into play: virtual function calls during object initialization or destruction are disallowed. The reason behind this restriction lies in the incomplete state of the derived class object during these phases of its existence.

Base Class Invocations

As a consequence of this prohibition, a virtual function call from a constructor or destructor results in the invocation of the base class version of the function. This presents a problem if the pure virtual function has no base class implementation, which is often the case in abstract classes.

An Illustrative Example

To illustrate the issue, consider the following C code:

class Base {
public:
    Base() { reallyDoIt(); }
    void reallyDoIt() { doIt(); } // DON'T DO THIS
    virtual void doIt() = 0;
};

class Derived : public Base {
    void doIt() {}
};

int main() {
    Derived d; // Triggers "pure virtual function call" error
}

In this example, the reallyDoIt() function in the Base class calls the doIt() function. However, since doIt() is a pure virtual function, the base class has no implementation for it. Consequently, when the Derived class object is created, it attempts to call the doIt() function from the constructor, leading to the dreaded "pure virtual function call" error.

Additional Resources

For a deeper dive into this topic, please refer to the insightful articles by Raymond Chen on the subject:

• [Raymond Chen's Blog: "Pure Virtual Functions and the Vptr" (Part 1)](https://devblogs.microsoft.com/oldnewthing/2007/12/18/pure-virtual-functions-and-the-vptr/)
• [Raymond Chen's Blog: "Pure Virtual Functions and the Vptr" (Part 2)](https://devblogs.microsoft.com/oldnewthing/2007/12/20/pure-virtual-functions-and-the-vptr-part-deux/)

The above is the detailed content of Why Do "Pure Virtual Function Call" Errors Happen in Constructors and Destructors?. For more information, please follow other related articles on the PHP Chinese website!