Pure virtual destructor in C++

3.1
Do not be anxious about anything, but in everything, 
by prayer and petition, with thanksgiving, present 
your requests to god. 
Philippians 4:6 (Bible)

Can a destructor be pure virtual in C++?
Yes, it is possible to have pure virtual destructor. Pure virtual destructor are legal in standard C++ and one of the most important thing is that if class contains pure virtual destructor it is must to provide a function body for the pure virtual destructor. This seems strange that how a virtual function is pure if it requires a function body? But destructors are always called in the reverse order of the class derivation. That means derived class destructor will be invoked first & then base class destructor will be called. If definition for the pure virtual destructor is not provided then what function body will be called during object destruction? Therefore compiler & linker enforce existence of function body for pure virtual destructor.
Consider following program:

#include <iostream>
class Base
{
public:
    virtual ~Base()=0; // Pure virtual destructor
};

class Derived : public Base
{
public:
    ~Derived()
    {
        std::cout << "~Derived() is executed";
    }
};

int main()
{
    Base *b=new Derived();
    delete b;
    return 0;
}

The linker will produce following error in the above program.

test.cpp:(.text$_ZN7DerivedD1Ev[__ZN7DerivedD1Ev]+0x4c): 
undefined reference to `Base::~Base()' 

Now if the definition for the pure virtual destructor is provided then the program compiles & runs fine.

#include <iostream>
class Base
{
public:
    virtual ~Base()=0; // Pure virtual destructor
};
Base::~Base()
{
    std::cout << "Pure virtual destructor is called";
}

class Derived : public Base
{
public:
    ~Derived()
    {
        std::cout << "~Derived() is executed\n";
    }
};

int main()
{
    Base *b = new Derived();
    delete b;
    return 0;
}

Output:

~Derived() is executed
Pure virtual destructor is called

It is important to note that class becomes abstract class when it contains pure virtual destructor. For example try to compile the below program.

#include <iostream>
class Test
{
public:
    virtual ~Test()=0; // Test now becomes abstract class
};
Test::~Test() { }

int main()
{
    Test p;
    Test* t1 = new Test;
    return 0;
}

The above program fails in compilation & shows following error messages.
[Error] cannot declare variable ‘p’ to be of abstract type ‘Test’
[Note] because the following virtual functions are pure within ‘Test’:
[Note] virtual Test::~Test()
[Error] cannot allocate an object of abstract type ‘Test’
[Note] since type ‘Test’ has pure virtual functions

Related Articles :
Constructors in C++
Destructors in C++
Virtual Destructor

Sources:
http://www.bogotobogo.com/cplusplus/virtualfunctions.php
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2005/n1905.pdf

This article is contributed by Meet Pravasi. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above

GATE CS Corner    Company Wise Coding Practice

Recommended Posts:



3.1 Average Difficulty : 3.1/5.0
Based on 25 vote(s)










Writing code in comment? Please use ide.geeksforgeeks.org, generate link and share the link here.