When do we need to define destructors?

Question

I read that destructors need to be defined when we have pointer members and when we define a base class, but I am not sure if I completely understand. One of the things I am not sure about is whether or not defining a default constructor is useless or not, since we are always given a default constructor by default. Also, I am not sure if we need to define default constructor to implement the RAII principle (do we just need to put resource allocation in a constructor and not define any destructor?).

class A
{

public:
    ~Account()
    {
        delete [] brandname;
        delete b;

        //do we need to define it?

    };

    something(){} =0; //virtual function (reason #1: base class)

private:
    char *brandname; //c-style string, which is a pointer member (reason #2: has a pointer member)
    B* b; //instance of class B, which is a pointer member (reason #2)
    vector<B*> vec; //what about this?



}

class B: public A
{
    public something()
    {
    cout << "nothing" << endl;
    }

    //in all other cases we don't need to define the destructor, nor declare it?
}

Answer 1

The rule of Three and The Rule of Zero

The good ol' way of handling resources was with the Rule of Three (now Rule of Five due to move semantic), but recently another rule is taking over: the Rule of Zero.

The idea, but you should really read the article, is that resource management should be left to other specific classes.

On this regard the standard library provides a nice set of tools like: std::vector, std::string, std::unique_ptr and std::shared_ptr, effectively removing the need for custom destructors, move/copy constructors, move/copy assignment and default constructors.

How to apply it to your code

In your code you have a lot of different resources, and this makes for a great example.

The string

If you notice brandname is effectively a "dynamic string", the standard library not only saves you from C-style string, but automatically manages the memory of the string with std::string.

The dynamically allocated B

The second resource appears to be a dynamically allocated B. If you are dynamically allocating for other reasons other than "I want an optional member" you should definitely use std::unique_ptr that will take care of the resource (deallocating when appropriate) automatically. On the other hand, if you want it to be an optional member you can use std::optional instead.

The collection of Bs

The last resource is just an array of Bs. That is easily managed with an std::vector. The standard library allows you to choose from a variety of different containers for your different needs; Just to mention some of them: std::deque, std::list and std::array.

Conclusion

To add all the suggestions up, you would end up with:

class A {
private:
    std::string brandname;
    std::unique_ptr<B> b;
    std::vector<B> vec;
public:
    virtual void something(){} = 0;
};

Which is both safe and readable.

Answer 2

As @nonsensickle points out, the questions is too broad... so I'm gonna try to tackle it with everything I know...

The first reason to re define the destructor would be in The Rule of Three which is on part the item 6 in Scott Meyers Effective C++ but not entirely. The rule of three says that if you re defined the destructor, copy constructor, or copy assignment operations then that means you should rewrite all three of them. The reason is that if you had to rewrite your own version for one, then the compiler defaults will no longer be valid for the rest.

Another example would be the one pointed out by Scott Meyers in Effective C++

When you try to delete a derived class object through a base class pointer and the base class has a non virtual destructor, the results are undefined.

And then he continues

If a class does not contain any virtual functions, that is often an indication that it is not meant to be used as a base class. When a class is not intended to be used as a base class, making the destructor virtual is usually a bad idea.

His conclusion on destructors for virtual is

The bottom line is that gratuitously declaring all destructors virtual is just as wrong as never declaring them virtual. In fact, many people summarize the situation this way: declare a virtual destructor in a class if and only if that class contains at least one virtual function.

And if it is not a Rule Of three case, then maybe you have a pointer member inside your object, and maybe you allocated memory to it inside your object, then, you need to manage that memory in the destructor, this is item 6 on his book

Be sure to check out @Jefffrey's answer on the Rule of Zero

Answer 3

There are precisely two things that necessitate defining a destructor:

1. When your object gets destructed, you need to perform some action other than destructing all class members.

   The vast majority of these actions once was freeing memory, with the RAII principle, these actions have moved into the destructors of the RAII containers, which the compiler takes care of calling. But these actions can be anything, like closing a file, or writing some data to a log, or ... . If you strictly follow the RAII principle, you will write RAII containers for all these other actions, so that only RAII containers have destructors defined.

2. When you need to destruct objects through a base class pointer.

   When you need to do this, you must define the destructor to be virtual within the base class. Otherwise, your derived destructors won't get called, independent of whether they are defined or not, and whether they are virtual or not. Here is an example:

   #include <iostream>
   
   class Foo {
       public:
           ~Foo() {
               std::cerr << "Foo::~Foo()\n";
           };
   };
   
   class Bar : public Foo {
       public:
           ~Bar() {
               std::cerr << "Bar::~Bar()\n";
           };
   };
   
   int main() {
       Foo* bar = new Bar();
       delete bar;
   }
   

   This program only prints Foo::~Foo(), the destructor of Bar is not called. There is no warning or error message. Only partially destructed objects, with all the consequences. So make sure you spot this condition yourself when it arises (or make a point to add virtual ~Foo() = default; to each and every nonderived class you define.

If none of these two conditions are met, you don't need to define a destructor, the default constructor will suffice.

---

Now to your example code:
When your member is a pointer to something (either as a pointer or a reference), the compiler does not know ...

• ... whether there are other pointers to this object.

• ... whether the pointer points to one object, or to an array.

Hence, the compiler can't deduce whether, or how to destruct whatever the pointer points to. So the default destructor never destructs anything behind a pointer.

This applies both to brandname and to b. Consequently, you need a destructor, because you need to do the deallocation yourself. Alternatively, you can use RAII containers for them (std::string, and a smart pointer variant).

This reasoning does not apply to vec because this variable directly includes a std::vector<> within the objects. Consequently, the compiler knows that vec must be destructed, which in turn will destruct all its elements (it's a RAII container, after all).

Answer 4

If you dynamically allocate memory, and you want this memory to be deallocated only when the object itself is "terminated", then you need to have a destructor.

The object can be "terminated" in two ways:

1. If it was statically allocated, then it is "terminated" implicitly (by the compiler).
2. If it was dynamically allocated, then it is "terminated" explicitly (by calling delete).

When "terminated" explicitly using a pointer of a base-class type, the destructor has to be virtual.

Answer 5

We know that if a destructor is not provided, the compiler will generate one.

This means that anything beyond simple cleanup, such as primitive types, will require a destructor.

In many cases, dynamic allocation or resource acquisition during construction, has a clean up phase. For example, dynamically allocated memory may need to be deleted.

If the class represents a hardware element, the element may need to be turned off, or placed into a safe state.

Containers may need to delete all of their elements.

In summary, if the class acquires resources or requires specialized cleanup (let's say in a determined order), there should be destructor.