Why Do 'Pure Virtual Function Call' Crashes Occur in Constructors and Destructors?

Unveiling the Enigma of "Pure Virtual Function Call" Crashes

When encountering the cryptic error message "pure virtual function call," users may wonder how programs manage to compile despite the inability to instantiate objects of abstract classes. This issue stems from attempts to invoke virtual functions within constructors or destructors.

The Compiler's Dilemma:

During object creation, a constructor is responsible for initializing the object's state. Conversely, a destructor handles object destruction. However, virtual function calls cannot be made during these stages as the derived class object is either incomplete or non-existent.

In such cases, the compiler invokes the base class version of the virtual function. However, since pure virtual functions lack an implementation in the base class, this results in the infamous "pure virtual function call" error.

A Code Snippet to Illustrate:

Consider the following C example:

class Base {
public:
    Base() {
        reallyDoIt(); // INCORRECT, doesn't work
    }
    void reallyDoIt() { doIt(); } // INCORRECT, doesn't work
    virtual void doIt() = 0; // Pure virtual function
};

class Derived : public Base {
    void doIt() {} // Overrides pure virtual function
};

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

When instantiating the Derived class, the program attempts to call reallyDoIt() from the constructor, which in turn invokes doIt(). Since doIt() is a pure virtual function, there is no implementation in the base class, leading to the fatal error.

Conclusion:

To prevent "pure virtual function call" crashes, it is crucial to avoid making virtual function calls within constructors or destructors. This ensures that only methods specific to the derived class are called during object creation and destruction, preventing invocations of undefined functions in the base class.

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