Is it safe to call other virtual methods in virtual destructor?

Question

struct A
{
    virtual ~A() { this->f(); }

    virtual void f() {};
};

struct B : A
{
    int* p;

    B() : p(new int) {}

    ~B()
    {
        delete p;
        p = 0;
    }

    void f() override { *p = 0; }
};

int main()
{
    delete new B; // Is it safe?
}

Cubbi · Accepted Answer · 2013-08-27T03:21:42.483

It is safe if you know the rules, and the rules say that within a destructor, the dynamic type of the object is the class whose destructor is executing.

While B::~B() is executing, the type of the object is B (and if you call f() then, you'll get dispatched to B::f()

While A::~A() is executing, the type of the object is A, and if you call f() then, you get undefined behavior per §10.4[class.abstract]/6

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

Or, as clang++ reports it,

test.cc:5:20: warning: call to pure virtual member function 'f'; overrides of 'f' in subclasses are not available in the destructor of 'A'
    virtual ~A() { this->f(); }
                   ^
test.cc:7:5: note: 'f' declared here
    virtual void f() = 0;
    ^
1 warning generated.

EDIT: OP edited pure virtual out.. so, then, while A::~A() is executing, virtual call to f() gets dispatched to A::f()

It's not undefined if a body has been provided for the pure virtual. — Neil Kirk, Aug 27 '13 at 03:19
I asked a question to be sure.. http://stackoverflow.com/questions/18456450/is-it-safe-to-call-a-pure-virtual-function-in-an-abstract-constructor-destructor — Neil Kirk, Aug 27 '13 at 03:28

score 1 · Answer 2 · answered Aug 27 '13 at 03:18

Note: the OP edited the code to remove the pure virtual part.

With what you have posted, no it is not safe. When you call a virtual function in a constructor or destructor of a class in an inheritance hierarchy, it does not call the most derived function as usual, but calls the function defined within the current class.

In this case, ~A calls A::f() which has no function body, so this is an error. If you provided a body for A::f() (you can do this with pure virtuals, just outside the class definition), then it would be "safe", although whether it does what you want is another matter.

Is it safe to call other virtual methods in virtual destructor?

2 Answers2