What's the use of the private copy constructor in c++

Question

Why do people define a private copy constructor?

When is making the copy constructor and the assignment operator private a good design?

If there are no members in the class which are pointers or handles to a unique object (like file name), then wat other cases are there where private copy constructor is a good idea?

Same question apply for assignment operator. Given that majority of C++ revolves around copying of objects and passing by reference, are there any good designs which involve private copy constructor?

Answer 1

One use case is the singleton pattern where there can only be exactly one instance of a class. In this case, you need make your constructors and assignment operator= private so that there is no way of creating more than one object. The only way to create an object is via your GetInstance() function as shown below.

// An example of singleton pattern
class CMySingleton
{
public:
  static CMySingleton& GetInstance()
  {
    static CMySingleton singleton;
    return singleton;
  }

// Other non-static member functions
private:
  CMySingleton() {}                                  // Private constructor
  ~CMySingleton() {}
  CMySingleton(const CMySingleton&);                 // Prevent copy-construction
  CMySingleton& operator=(const CMySingleton&);      // Prevent assignment
};

int main(int argc, char* argv[])
{
  // create a single instance of the class
  CMySingleton &object = CMySingleton::GetInstance();

  // compile fail due to private constructor
  CMySingleton object1;
  // compile fail due to private copy constructor
  CMySingleton object2(object);
  // compile fail due to private assignment operator
  object1 = object;

  // ..
  return 0;
}

Answer 2

Some objects represent particular entities that can't or shouldn't be copied. For example, you may prevent copying of an object that represents the log file used by an application, corresponding to the expectation that a single log file will be used by all parts of the code. Use of an accidentally or inappropriately copied object could lead to out-of-order content appearing in the log, inaccurate records of current log size, multiple attempts (some failing) to "roll" to a new log filename or rename the existing one.

Another use is to enforce copying via a virtual function. As constructors can't be virtual, a common practice is to prevent direct access to the copy constructor and provide a virtual Base* clone() method that returns a copy of the actual run-time type to which a pointer points. This prevents the accidental slicing that Base b(derived) would exhibit.

Another example: a dead-simple smart pointer object that simply deletes the pointer it's given in the constructor: if it doesn't support reference counting or some other manner of handling multiple owners, and doesn't want to have risk awkward unintended std::auto_ptr style transfer of ownership, then simply hiding the copy constructor gives a great little smart pointer that's fast and efficient for the limited cases where it's usable. A compile time error about attempting to copy it would effectively ask the programmer "hey - if you really want to do that change me to a shared pointer, otherwise back off!".

Answer 3

A very bad example:

class Vehicle : { int wheels; Vehicle(int w) : wheels(w) {} }

class Car : public Vehicle { Engine * engine; public Car(Engine * e) : Vehicle(4), engine(e) }

...

Car c(new Engine());

Car c2(c); // Now both cars share the same engine!

Vehicle v;
v = c; // This doesn't even make any sense; all you have is a Vehicle with 4 wheels but no engine.

What does it mean to "copy" a car? (Is a car a car model, or an instance of a car? Does copying it preserve the vehicle registration?)

What does it mean to assign a vehicle to another one?

If the operations are meaningless (or merely unimplemented), the standard thing to do is to make the copy constructor and assignment operator private, causing a compile error if they're used instead of weird behaviour.

Answer 4

A common reason to make copy constructor and copy assignment private is to disable default implementation of these operations. However, in C++ 0x there are special syntax =delete for such purpose. So in C++ 0x making copy ctor private seems to be resrtricted to very exotic cases.

Copy ctors and assignments are rather syntactic sugar; so such a "private sugar" seems as symptom of greed :)

Answer 5

Even if the contents of the object aren't pointers or other references, preventing people from copying the object can still be useful. Perhaps the class contains a lot of data, and copying is too heavyweight of an operation.

Answer 6

The "virtual constructor idiom" is an important case where a private or protected copy constructor is needed. A problem arises in C++ where you are given the pointer to a base class, of an object that is actually inherited from this base class, and you want to make a copy of it. Calling the copy constructor would not call the copy constructor of the inheriting class, but actually call the copy constructor of the base class.

Observe:

class Base {

public:
   Base( const Base & ref ){ std::cout << "Base copy constructor" ; }
};

class Derived : public Base {

public:
   Derived( const Derived & ref ) : Base(ref) { std::cout << "Derived copy constructor"; }
}

Base * obj = new Derived;
Base * obj2 = new Derived(*obj);

The code above would produce the output:

"Base copy constructor"

This is clearly not the behaviour the programmer wanted! The programmer was attempting to copy an object of type "Derived" but instead got back an object of type "Base"!!

The issue is rectified by using the aforementioned idiom. Observe the example written above, re-written to use this idiom:

class Base {

public:
  virtual Base * clone () const = 0; //this will need to be implemented by derived class

protected:
   Base( const Base & ref ){ std::cout << "Base copy constructor" ; }
};

class Derived : public Base {

public:
  virtual Base * clone () const {

    //call private copy constructor of class "Derived"
    return static_cast<Base *>( new Derived(*this) );
  }

//private copy constructor:
private:
   Derived( const Derived & ref ) : Base(ref) { std::cout << "Derived copy constructor"; }
}

Base * obj = new Derived;
Base * obj2 = obj->clone();

The code above would produce the output:

"Base copy constructor"
"Derived copy constructor"

In other words, the object that was constructed in of desired type "Derived", and not of the type "Base"!

As you can see, in the Derived type, the copy constructor was intentionally made private, because it would be bad API design to give programmers to ability to accidentally try to call the copy constructor manually, rather than using the clever interface provided by clone(). Put another way, a directly callable public copy constructor available could cause programmers to make the mistake mentioned in part 1. In this case, best practise would have the copy constructor be hidden from view, and only indirectly accessible by using the method "clone()".

Answer 7

You might want to implement some of the methods of the class using a copy constructor, but not to expose it outside of the class. So then you make it private. Like any other method.