in which case we need to disable default copy constructor and assign operator?

Question

If we put copy constructor and assign operator as private and provide no implementation, they will be disabled, like this:

class Test
{
   private:
       Test(const Test&);
       Test& operator=(const Test&);
};

but in which case we need to do this? I mean when should we do it like this?

Answer 1

When you want the objects of this class to be non-copyable.

There may be many reasons when objects can't be or shouldn't be copied to other objects. A few examples are:

1. Log files
2. Some mutices
3. In Singleton pattern
4. Object Factory
5. Some versions of smart pointers

For above examples, compiler provided version of default copy constructor and default assignment operators may lead to unexpected results.

c++11 onwarnds, you can use =delete syntax to delete the compiler provided default versions.

Another use is to force (restrict) copying of objects only via class utilities virtual Base* clone() for example.

Related: Rule of three or rule of five

Answer 2

Another common case is when the object is not shallow-copyable and the one who wrote the class doesn't want to implement deep copy for it, so he left those two disabled, with or without comments on the rationale.

For example, when constructor news an array and destructor deletes it, then the object not shallow-copyable. To implement deep copy, copy constructor needs to new an empty array to the same size and then copy each and every element, and copy-assignment has to delete the old one before doing the same thing.

It's a lot of boilerplate code to write. And it takes time to run, too.

So if this class is private to an implementation and copy-constructor and copy-assignment are neither mandatory nor helpful, why not just disable them?