Destructors for C++ Interface-like classes

Question

Starting to use PC-Lint on an existing code base (fear and trepidation).

One thing that it complains about is the following:

 class IBatch  {  public:     virtual void StartBatch() =0;     virtual int CommitBatch() =0;  }; 

Which when another class derives from this to use it like an interface

base class 'IBatch' has no destructor 

So, the question: when you create Interface classes like the above, do you always include a virtual destructor? Why? (is it a style or a coding error?)

EDIT: Should have said that I do not expect or want the user of IBatch to destruct, they are a consumer of a service only, through this interface to some external implementing class (if that would make a difference)

Answer 1

A base class destructor should be either public and virtual, or protected and nonvirtual.

(Herb Sutter, Guru of the Week #18: "Virtuality")

Answer 2

Coding error - The destructor for your derived class will never get called if called via pointer to base class.

When you implement IBatch and you refer to your derived class by a pointer to a base class (pointer to IBatch) and you call delete on that pointer to base class you might end up with memory leak because the destructor for your derived class will never get called.

The basic rule is when a class has at least one virtual method it needs to have virtual destructor.

class IBatch {     public:        virtual void f() = 0; };  class A : public IBatch {     public:        void f() {}        ~A() {} };  IBatch * a = new A(); a->f();    // calls A::f() delete a;  // calls IBatch::~IBatch() not A::~A()