Virtual Destructor

 

　　Deleting a derived class object using a pointer to a base class that has a non-virtual destructor results in undefined behavior. To correct this situation, the base class should be defined with a virtual destructor.
　　Source: https://www.securecoding.cert.org/confluence/display/cplusplus/OOP34-CPP.+Ensure+the+proper+destructor+is+called+for+polymorphic+objects

　　For example, following program results in undefined behavior. Although the output of following program may be different on different compilers, when compiled using Dev-CPP, it prints following.
　　Constructing base
　　Constructing derived
　　Destructing base

// A program without virtual destructor causing undefined behavior
#include<iostream>

using namespace std;

class base 
{
public:
    base()     
    { 
        cout<<"Constructing base \n"; 
    }
    ~base()
    { 
        cout<<"Destructing base \n"; 
    }     
};

class derived: public base 
{
public:
    derived()     
    { 
        cout<<"Constructing derived \n"; 
    }
    ~derived()
    { 
        cout<<"Destructing derived \n"; 
    }
};

int main(void)
{
    derived *d = new derived();  
    base *b = d;
    delete b;
    getchar();
    return 0;
}

 

　　Making base class destructor virtual guarantees that the object of derived class is destructed properly, i.e., both base class and derived class destructors are called.

　　For example, following program prints:
　　Constructing base
　　Constructing derived
　　Destructing derived
　　Destructing base

// A program without virtual destructor causing undefined behavior
#include<iostream>

using namespace std;

class base 
{
public:
    base()     
    { 
        cout<<"Constructing base \n"; 
    }
    virtual ~base()
    { 
        cout<<"Destructing base \n"; 
    }     
};

class derived: public base 
{
public:
    derived()     
    { 
        cout<<"Constructing derived \n"; 
    }
    ~derived()
    { 
        cout<<"Destructing derived \n"; 
    }
};

int main(void)
{
    derived *d = new derived();  
    base *b = d;
    delete b;
    getchar();
    return 0;
}

 

　　As a guideline, any time you have a virtual function in a class, you should immediately add a virtual destructor (even if it does nothing). This way, you ensure against any surprises later.

 

 

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　　2013-11-26　　21:06:36