Is it safe to not specify a destructor in a subclass of a base class with a virtual destructor?

Question

Consider a simple interface/implementation design:

class Base
{
public:

    virtual ~Base()
    {
        // Virtual empty destructor
    }
};

class Derived : public Base
{
public:

    virtual ~Derived()
    {
        // Lots of clean up code here
        // ...
    }
};

With this design, I understand that the following code is safe and valid:

Base* base = new Derived();
delete base; // Both Base::~Base and Derived::~Derived get called

However, imagine if there was a new class:

class DerivedEx : public Derived
{
public:

    // No destructor here, relying on default destructor generated by compiler
};

Is DerivedEx "safe"?

For safety, I always assumed that I'd have to implement a virtual empty destructor in DerivedEx. But I'm wondering if this is redundant and unnecessary and whether or not there are any 'gotchas' that I'm not aware of.

score 8 · Accepted Answer · answered Jan 11 '13 at 15:19

8

Yes, it's still safe.

A destructor is automatically virtual if any base class destructor is virtual. This is true whether you type ~Foo(); or virtual ~Foo(); or neither and just allow the compiler to generate the implicitly defined destructor.

answered Jan 11 '13 at 15:19

aschepler

70,891
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So you mean the `virtual` for `Derived::~Derived` would be redundant as well? – Zeenobit Jan 11 '13 at 15:24
1

@Farbod T. It's redundant only to the compiler. To your future human maintainers it's a lifesaver to understand what the program is doing. – Mark B Jan 11 '13 at 15:28

score 4 · Answer 2 · edited May 23 '17 at 11:43

4

It is safe. You just need to ensure that the destructor in the base class is virtual.
As long as your Base class destructor is virtual the compiler will take care of calling all the destructors in the correct order.

Whether you need a destructor in most derived class depends on that class itself & that decision is usually made based on Rule of Three.

edited May 23 '17 at 11:43

Community

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answered Jan 11 '13 at 15:18

Alok Save

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I'd remove the starting "No" from your answer, as the question stands "Is DerivedEx safe?" – Angew is no longer proud of SO Jan 11 '13 at 15:20
@Angew: Thanks. I guess my *"No"* was a result of my mind shouting No! no it is not required, when I had just reached that point in the question :-) – Alok Save Jan 11 '13 at 15:22

score 3 · Answer 3 · answered Jan 11 '13 at 15:21

3

Yes, it is safe.

The implicitly generated destructor will be virtual, since there is a base class with a virtual destructor, and will correctly call the destructors of all non-trivial members and base classes.

answered Jan 11 '13 at 15:21

Mike Seymour

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Is it safe to not specify a destructor in a subclass of a base class with a virtual destructor?

3 Answers3