Why is a destructor function required in C plus plus classes?

A destructor is not a function per-se. Functions have returns values but destructors do not (not even void). However, other than the lack of a return value, a destructor's body is no different to a function body.

Contrary to belief, destructors are not required by every class of object. Destructors are only required when we need to explicitly release a resource when an object of the class falls from scope, but not all resources need to be released explicitly. Consider the following class:

struct point {

int x, y;

};

This class does not require a destructor because it is self-contained. That is, all the resources consumed by an object of the class are contained within the object itself, thus when the object falls from scope, the memory it consumed is automatically released. There is nothing to destroy.

Now consider the following class:

class my_vector {

std::vector<int> v;

};

Again, this class requires no destructor. When objects of this class fall from scope, the vector's destructor is invoked, releasing its resources. Thus the only resource consumed by this class is the vector itself, and that's self-contained by the class.

Now let's consider a class that does require a destructor:

template<typename T>

class smart_pointer {

T* ptr;

// ...

};

Here, we have a class that encapsulates a "naked" pointer variable of some type T. That pointer could feasibly refer to any object of type T in memory. However, being a "smart pointer", the assumption is that objects of this class will "own" the memory they refer to. As the owner, it makes sense to yield ownership whenever object's of the class fall from scope. If we didn't provide a destructor, ptr would fall from scope, but the memory it referred to would not. So to ensure the memory is released, we must explicitly release it via the class destructor:

template<typename T> class smart_pointer {

T* ptr;

// ...

public:

~smart_pointer () { delete ptr; }

};

To summarise: a destructor is only required when an object acquires a non-shared resource that is not released automatically when objects of the class fall from scope. A vector is a resource handle, so when it falls from scope, its resources are released automatically. Similarly with smart pointers. However a "naked" pointer is not a resource handle, so if our class "owns" a resource by holding a pointer to it, then we must release that pointer when objects of the class are destroyed. Resources cover many things besides raw memory. If we hold a reference to an open file, it would be prudent to close the file before the reference falls from scope. With a resource handle we get that assurance. Thus if we use resource handles rather than "raw" resources, then we need never write any destructors.

There can be other reasons to require a destructor besides resource management, such as to write an entry in a log file upon destruction, however resource management and other "housekeeping" task are not a requirement of every class thus a destructor is not required by every class.