Use of public destructor when the constructor is private

Question

I have seen code where the constructor has been declared as private while the destructor is public. What is the use of such a declaration? Is the destructor required to be public so that during inheritance the calls can be possible or is it a bug in the code?

The question might seem to be a bit short on information, but what I really want to know is if having a public destructor when the constructor is required to be private abides by the C++ rules?

Answer 1

Short Answer

Creating a constructor as private but the destructor as public has many practical uses.

You can use this paradigm to:

• Enforcing reference counting (See Hitesh Vaghani's example).
• Implement the singleton pattern
• Implement the factory pattern.

Long Answer

Above I hinted that you can use private constructors and destructors to implement several design patterns. Well, here's how...

Reference Counting

Using private destructor within an object lends itself to a reference counting system. This lets the developer have stronger control of an objects lifetime.

class MyReferenceObject
{
public:
    static MyReferenceObject* Create()
    {
        return new MyReferenceObject();
    }

    void retain()
    {
        m_ref_count++;
    }

    void release()
    {
        m_ref_count--;
        if (m_ref_count <= 0)
        {
            // Perform any resource/sub object cleanup.
            // Delete myself.
            delete this; // Dangerous example but demonstrates the principle.
        }
    }
private:

    int m_ref_count;

    MyReferenceObject()
    {
        m_ref_count = 1;
    }

    ~MyReferenceObject() { }

}

int main()
{
    new MyReferenceObject(); // Illegal.
    MyReferenceObject object; // Illegal, cannot be made on stack as destructor is private.

    MyReferenceObject* object = MyReferenceObject::Create(); // Creates a new instance of 'MyReferenceObject' with reference count.
    object->retain(); // Reference count of 2.
    object->release(); // Reference count of 1.
    object->release(); // Reference count of 0, object deletes itself from the heap.
}

This demonstrates how an object can manage itself and prevent developers from corrupting the memory system. Note that this is a dangerous example as MyReferenceObject deletes itself, see here for a list of things to consider when doing this.

Singleton

A major advantage to private constructors and destructors within a singleton class is that enforces the user to use it in only the manner that the code was design. A rogue singleton object can't be created (because it's enforced at compile time) and the user can't delete the singleton instance (again, enforced at compile time).

For example:

class MySingleton
{
public:
     MySingleton* Instance()
     {
        static MySingleton* instance = NULL;
        if (!instance)
        {
            instance = new MySingleton();
        }

        return instance;
     }
private:
    MySingleton() { }
    ~MySingleton() { } 
}

int main()
{
     new MySingleton(); // Illegal
     delete MySingleton::Instance(); // Illegal.
}

See how it is almost impossible for the code to be misused. The proper use of the MySingleton is enforce at compile time, thus ensuring that developers must use MySingleton as intended.

Factory

Using private constructors within the factory design pattern is an important mechanism to enforce the use of only the factory to create objects.

For example:

class MyFactoryObject
{
public:

protected:
    friend class MyFactory; // Allows the object factory to create instances of MyFactoryObject

    MyFactoryObject() {} // Can only be created by itself or a friend class (MyFactory).
}

class MyFactory
{
public:
    static MyFactoryObject* MakeObject()
    {

        // You can perform any MyFactoryObject specific initialisation here and it will carry through to wherever the factory method is invoked.
        return new MyFactoryObject();
    }
}

int main()
{
    new MyFactoryObject(); // Illegal.
    MyFactory::MakeObject(); // Legal, enforces the developer to make MyFactoryObject only through MyFactory.
}

This is powerful as it hides the creation of MyFactoryObject from the developer. You can use the factory method to perform any initilisation for MyFactoryObject (eg: setting a GUID, registering into a DB) and anywhere the factory method is used, that initilisation code will also take place.

Summary

This is just a few examples of how you can use private constructors and destructors to enforce the correct use of your API. If you want to get tricky, you can combine all these design patterns as well ;)

Answer 2

First thing: the destructor can be private.

having a public destructor when the constructor is required to be private abides by the C++ rules?

It's totally working in C++. In fact, a great example of this scenario is the singleton pattern, where the constructor is private and the destructor is public.

Answer 3

In reverse order.

Is the destructor required to be public so that during inheritance the calls can be possible or is it a bug in the code?

Actually, for inheritance to work the destructor should be at least protected. If you inherit from a class with a private destructor, then no destructor can be generated for the derived class, which actually prevents instantiation (you can still use static methods and attributes).

What is the use of such declaration?

Note that even though the constructor is private, without further indication the class has a (default generated) public copy constructor and copy assignment operator. This pattern occurs frequently with:

• the named constructor idiom
• a factory

Example of named constructor idiom:

class Angle {
public:
    static Angle FromDegrees(double d);
    static Angle FromRadian(double d);

private:
    Angle(double x): _value(x) {}
    double _value;
};

Because it is ambiguous whether x should be precised in degrees or radians (or whatever), the constructor is made private and named method are provided. This way, usage makes units obvious:

Angle a = Angle::FromDegrees(360);

Answer 4

You make constructor private if you want to prevent creating more then one instance of your class. That way you control the creation of intances not their destruction. Thus, the destructor may be public.

Answer 5

One example in my head, let's say you want to limit the class instance number to be 0 or 1. For example, for some singleton class, you want the application can temprary destroy the object to rducue memory usage. to implement this constructor will be private, but destructor will be public. see following code snippet.

class SingletoneBigMemoryConsumer
{
private:
    SingletoneBigMemoryConsumer()
    {
        // Allocate a lot of resource here.
    }

public:
    static SingletoneBigMemoryConsumer* getInstance()
    { 
        if (instance != NULL) 
            return instance;
        else
            return new SingletoneBigMemoryConsumer();
    }
    ~SingletoneBigMemoryConsumer()
    {
        // release the allocated resource.
        instance = NULL;
    }
private:
    // data memeber.
    static SingletoneBigMemoryConsumer* instance;
}



//Usage.
SingletoneBigMemoryConsumer* obj = SingletoneBigMemoryConsumer::getInstance();
// You cannot create more SingletoneBigMemoryConsumer here.
// After 1 seconds usage, delete it to reduce memory usage.
delete obj;
// You can create an new one when needed later

Answer 6

The owner of an object needs access to the destructor to destroy it. If the constuctors are private, there must be some accessible function to create an object. If that function transferes the ownership of the constructed object to the caller ( for example return a pointer to a object on the free store ) the caller must have the right to access the destructor when he decides to delete the object.