Constructors and destructor must be virtual?

Question

Exists some differences between a constructor or destructor be virtual or not? In this case, what should be done

class A {
public:
    A();
    ~A();
}

or

class A {
public:
    virtual A();
    virtual ~A();
}

Have isocpp fot this case?

Thanks...

Constructor: cannot be done. Destructor: it depends. See [*when to use virtual destructors?*](http://stackoverflow.com/questions/461203/when-to-use-virtual-destructors) — juanchopanza, Oct 03 '15 at 13:49

score 1 · Accepted Answer · edited May 23 '17 at 12:14

1

You cannot have Virtual constructor in C++ why no virtual Constructor.

Virtual destructors are useful when you can delete an instance of a derived class through a pointer to base class. Refer to When to use Virtual Destructor.

edited May 23 '17 at 12:14

Community

1
1

answered Oct 03 '15 at 13:54

Lorenzo Belli

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score 1 · Answer 2 · answered Oct 03 '15 at 13:57

You can't have virtual constructors, but a virtual destructor makes it possible to destroy an object through a base class pointer while calling the derived destructors apropriately.

For example, dont actually run this code

class A {
public:
    A() {}
    ~A() {}
};

class B : public A {
public:
    B() {}
    virtual ~B() {}
};

class C : public B {
public:
    C() : _p( new int(0) ) {}
    ~C() { delete _p; std::cout << "Deleted p" << std::endl; }
private:
    int *_p;
};


int main() {
    C *c1 = new C();
    C *c2 = new C();

    B *bPointer = c1;
    A *aPointer = c2;

    std::cout << "Deleting through B*" << std::endl;
    delete bPointer; // "deleted p"
    std::cout << "------------" << std::endl;
    std::cout << "Deleting through A*" << std::endl;
    delete aPointer; // No output

    return 0;
}

Class A's destructor should've been marked as virtual. You only need to write virtual on the topmost class of the hierarchy.

I'd say he should run it for learning purposes :) But not use it in production code! — Christian Hackl, Oct 03 '15 at 14:00

Constructors and destructor must be virtual?

2 Answers2