Why Do 'Pure Virtual Function Call' Errors Cause Program Crashes?

Understanding "Pure Virtual Function Call" Crashes

In programming, "pure virtual function call" errors can sometimes cause programs to crash abruptly. These errors arise when an attempt is made to call a pure virtual function during object construction or destruction.

Pure Virtual Functions

A pure virtual function is a member function of an abstract class that has only a declaration but no implementation. It forces derived classes to implement their own versions of the function.

Constructor and Destructor Limitations

Constructor and destructor functions are called during object initialization and destruction, respectively. During these special functions, virtual function calls are prohibited because:

• Constructors cannot access derived class members that may not yet be constructed.
• Destructors cannot access derived class members that may already be destroyed.

Source of Crashes

"Pure virtual function call" crashes occur when:

• You explicitly call a pure virtual function from a constructor or destructor.
• You indirectly call a pure virtual function through a function that is called from a constructor or destructor.

Example

Consider this 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(void)
{
    Derived d;  // This will cause "pure virtual function call" error
}

In this example, the call to reallyDoIt() in the Base constructor attempts to call the pure virtual function doIt(). Since the constructor is being called, the derived class object has not yet been constructed, and a "pure virtual function call" error occurs.

Resolution

To avoid these crashes, avoid calling pure virtual functions from constructors or destructors. Instead, ensure that derived classes provide their own implementations before calling the pure virtual functions.

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