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

 1 // A program without virtual destructor causing undefined behavior
 2 #include<iostream>
 3
 4 using namespace std;
 5
 6 class base
 7 {
 8 public:
 9     base()
10     {
11         cout<<"Constructing base \n";
12     }
13     ~base()
14     {
15         cout<<"Destructing base \n";
16     }
17 };
18
19 class derived: public base
20 {
21 public:
22     derived()
23     {
24         cout<<"Constructing derived \n";
25     }
26     ~derived()
27     {
28         cout<<"Destructing derived \n";
29     }
30 };
31
32 int main(void)
33 {
34     derived *d = new derived();
35     base *b = d;
36     delete b;
37     getchar();
38     return 0;
39 }

　　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

 1 // A program without virtual destructor causing undefined behavior
 2 #include<iostream>
 3
 4 using namespace std;
 5
 6 class base
 7 {
 8 public:
 9     base()
10     {
11         cout<<"Constructing base \n";
12     }
13     virtual ~base()
14     {
15         cout<<"Destructing base \n";
16     }
17 };
18
19 class derived: public base
20 {
21 public:
22     derived()
23     {
24         cout<<"Constructing derived \n";
25     }
26     ~derived()
27     {
28         cout<<"Destructing derived \n";
29     }
30 };
31
32 int main(void)
33 {
34     derived *d = new derived();
35     base *b = d;
36     delete b;
37     getchar();
38     return 0;
39 }

　　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