How to Deal With a Pure Virtual Function Error Easily

If you can’t cope with a pure virtual function error, be quick to get acquainted with the sample below. From this sample, you will know how to solve such errors. This explanation was prepared by an IT expert from AssignmentShark, a service that helps students with their studies. What kind of assistance do we actually provide? First of all, you can find many articles on educational topics which can help you to solve your problems with studying. Secondly, we are able to help you with your specific task.

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Pure Virtual Function Errors

How does the mechanism of virtual functions work? Usually, it is implemented using “vtbl” (virtual table), a table with pointers to functions. Each instance of the class that contains at least one virtual function is a pointer to __vtbl table for its class. In the case with abstract classes and a pure virtual function, the pointer is still there, but it points on the standard handler __pure_virtual_func_called (), which leads to this error. But how can we call it, as a direct attempt will be caught at the compilation stage?

#include <iostream>

class Basis

{

public:

Basis() { init(); }

~Basis() {}

virtual void log() = 0;

private:

void init() { log(); }

};

class Derivative: public Basis

{

public:

Derivative () {}

~ Derivative () {}

virtual void log() { std::cout << ” Derivative created” << std::endl; }

};

int main(int argc, char* argv[])

{

Derivative a;

return 0;

}

 

Let’s examine what happens when we initiate an instance of a class-descendant object that contains vtbl.

Step 1. Construct a base part of the upper level:

a) Set pointer __vtbl to the vtbl of the parent class;

b) Construct the base class instance variables;

c) Implement the body of the base class constructor.

Step 2. Bequest part (s):

a) Change the __vtbl pointer to vtbl of the descendant class;

b) Construct descendant class variables;

c) Implement the body of the descendant class constructor.

It is easy to guess that when the object of the class derivative will be created, during the implementation of the base class constructor, it will still be considered as the base class, and its vtbl will still be the base class vtbl. Usually, compilers don’t detect this beforehand and the error occurs only on the runtime stage.

So as the conclusion, we may say that it is better to avoid calling virtual functions in constructors and destructors.

 

Thanks for your attention!

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