Virtual Default Destructors in C++

Question

I\'ve got a large set of inherited classes (criteria) which inherit from a base class (criterion). Here\'s criterion\'s code

class criterion
{
p         


        

Answer 1

Yes - the base class needs a virtual destructor, even if it's empty. If that is not done, then when something delete's a derived object through a base pointer/reference, the derived object's member objects will not get a chance to destroy themselves properly.

Derived classes do not need to declare or define their own destructor unless they need something other than default destructor behavior.

Answer 2

The recommendation is to insert:

virtual ~criterion() {}

Starting from C++11, you can use = default; instead of an empty body {}.

This is to avoid problems with deleting from a base class' pointer. Otherwise you will leak memory as derived classes' destructors will not be called.

criterion *c = new fastFilter();
delete c; // leaks

Answer 3

One small change from what others have already answered:

Instead of

virtual void ~criterion() = 0;

the required version is:

    virtual ~criterion() {}  //Note: Removed void as destructors not allowed 
                             //  a return type

To know more about virtual destructor have a look at this link from FAQ When should my destructor be virtual?

Answer 4

You don't need to make the destructor abstract, just give it a empty implementation:

virtual ~criterion() { }

This way you are not forced to implement it in every child class, but still each of them will have a (inherited) virtual destructor.