Correct way to inherit from a virtual class with non-virtual parent

Question

I've written this test code that uses three types: struct One is a normal type with no virtual members, struct Two : One has a pure virtual function and a virtual destructor, and struct Three : Two implements Two's interface.

#include <iostream>

struct One
{
    ~One() {
        std::cout << "~One()\n";
    }
};

struct Two : One
{
    virtual ~Two() {
        std::cout << "~Two()\n";
    }

    virtual void test() = 0;
};

struct Three : Two
{
    virtual ~Three() {
        std::cout << "~Three()\n";
    }

    virtual void test() {
        std::cout << "Three::test()\n";
    }
};

int main()
{
    Two* two = new Three;
    two->test();

    One* one = two;
    delete one;
}

Unsurprisingly, the output was this:

Three::test()
~One()

Is there any way to fix this other than making every destructor virtual? Or should programmers just be careful not to run into this situation? I find it odd that there's no warning when compiling this.

An easy fix: Use private or protected inheritance from `One` and expose functionality through using declarations. — Xeo, Jan 07 '12 at 11:39

score 2 · Accepted Answer · answered Jan 07 '12 at 11:30

2

The only "fix" is not to delete the objects through a pointer to One.

If this is a frequent problem, or not, depends on how your classes are used. For example, the standard library contains structs like unary_function without a virtual destructor, but we hardly ever see it misused like this.

answered Jan 07 '12 at 11:30

Bo Persson

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and to enforce that, the destructor of `One` should be protected – Chris Bednarski Jan 07 '12 at 11:34

score 2 · Answer 2 · answered Jan 07 '12 at 11:37

delete one invokes undefined behaviour, because the dynamic type of the object does not match the static type, and the static type does not have a virtual destructor.

The usual way to avoid problems like this is to make destructors be either public and virtual, or protected and non-virtual (on classes that are expected to be used in this way).

score 1 · Answer 3 · answered Jan 07 '12 at 11:26

1

You must be careful and make One's destructor virtual. Some compilers do warn about this.

answered Jan 07 '12 at 11:26

Gary

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score 1 · Answer 4 · answered Jan 07 '12 at 11:32

1

If you want working destructors in derived classes then you must define them as virtual. It is the only way.

answered Jan 07 '12 at 11:32

gumik

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Correct way to inherit from a virtual class with non-virtual parent

4 Answers4

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