If call to a pure virtual function from destructor is UB, why can we use pure virtual desrtuctors?

Question

It is known that we can use pure virtual destructors, like this:

struct A {
    virtual ~A() = 0;
};
A::~A() {}

struct B : A {};

Because the Standard says in 10.4 [class.abstract] p2

A pure virtual function need be defined only if called ... with (12.4 [class.dtor])

And later in 12.4 [class.dtor] p9

A destructor can be declared virtual (10.3) or pure virtual (10.4); if any objects of that class or any derived class are created in the program, the destructor shall be defined.

What means that the code above is perfectly valid - A::~A may be pure virtual, it is defined, B::~B implicitly calls A::~A.

So far, so good.
And then I read 10.4 [class.abstract] p6:

Member functions can be called from a constructor (or destructor) of an abstract class; the effect of making a virtual call (10.3) to a pure virtual function directly or indirectly for the object being created (or destroyed) from such a constructor (or destructor) is undefined.

But that's exactly what we do here - we call a pure virtual function A::~A from a destructor.

So, isn't there some kind of contradiction?

The destructor is a special case, since even if it's pure virtual, it must still have a body (per the standard). The undefined part applies to pure virtual member function (which may not have a definition). — StoryTeller - Unslander Monica

quantdev quantdev · Accepted Answer · 2014-06-15T11:37:14

There is no contraction.

You call the virtual destructor of A from the destructor of B.A destructor is not a member of B.

Standard § 9.3

Member Functions

Functions declared in the definition of a class, excluding those declared with a friend specifier ( 11.3 ), are called member functions of that class

The standard states that there is undefined behavior when there is a virtual call from the abstract class destructor/constructor (which in your case, would be class A) to one of it's own pure virtual member function.

[...] Member functions can be called from a constructor (or destructor) of an abstract class [...];

Your quote says for destructors:

You can call member functions from the destructor of an abstract class
If you call a pure virtual method (with a virtual call) from the destructor of this same abstract class, there is undefined behavior.

If call to a pure virtual function from destructor is UB, why can we use pure virtual desrtuctors?

2 Answers