Preventing Object Copy in C++ (3 Different Ways)

Many times, user wants that an instance of a C++ class should not be copied at all. So, the question is how do we achieve this ?

There are three ways to achieve this :

1. Keeping the Copy Constructor and Copy assignment operator as private in the class.

   Below is the C++ implementation to illustrate how this can be done.

   #include <iostream>
   using namespace std;
   
   class Base {
       int x;
   public:
       Base()    {  }
       Base(int y): x(y)     {  }
   private:
   
       // Copy constructor
       Base(const Base& obj) : x(obj.x) {   }
   
       // copy assignment operator
       Base& operator=(const Base& tmp_obj)
       {
           x = tmp_obj.x;
           return *this;
       }
   };
   
   int main()
   {
       Base b1(10);
       Base b2(b1); // calls copy constructor
       b2 = b1; // calls copy assignment operator
       return 0;
   }

   NOTE: This code does not compile as we cannot copy the object of this class and hence it will show this error.

   prog.cpp: In function 'int main()':
   prog.cpp:18:2: error: 'Base::Base(const Base&)' is private
     Base(const Base &obj) : x(obj.x) //Copy constructor
     ^
   prog.cpp:33:12: error: within this context
     Base b2(b1); // Calls copy constructor
               ^
   prog.cpp:22:8: error: 'Base& Base::operator=(const Base&)' is private
     Base& operator = (const Base& tmp_obj) // copy assignment operator
           ^
   prog.cpp:35:5: error: within this context
     b2 = b1; // calls copy assignment operator
        ^
   

2. Inherit a Dummy class with a private copy constructor and a private copy assignment operator.

   Below is the C++ implementation to illustrate how this can be done.

   #include <iostream>
   using namespace std;
   
   class Dummy {
   public:
       Dummy() {  }
   private:
       Dummy(const Dummy& temp_obj)  {   }
       Dummy& operator=(const Dummy& temp_obj)   {   }
   };
   
   class Base : public Dummy {
       int x;
   public:
       Base()  {   }
       Base(int y) : x(y)  {  }
   };
   
   int main()
   {
       Base b1(10);
       Base b2(b1); // Calls copy constructor
       b2 = b1; // Calls copy assignment operator
       return 0;
   }

   prog.cpp: In function 'int main()':
   
   prog.cpp:12:5: error: 
   'Dummy::Dummy(const Dummy&)' is private
        Dummy(const Dummy &temp_obj)
        ^
   prog.cpp:22:7: error: within this context
    class Base: public Dummy
          ^
   prog.cpp:16:12: error: 
   'Dummy& Dummy::operator=(const Dummy&)' is private
        Dummy& operator = (const Dummy &temp_obj)
               ^
   prog.cpp:22:7: error: within this context
    class Base: public Dummy
   

   NOTE: This code does not compile as we cannot copy the object of this class and hence it will show this error.

3. Using Deleted copy constructor and copy assignment operator: Above two ways are quite complex, C++11 has come up with a simpler solution i.e. just delete the copy constructor and assignment operator.

   Below is the C++ implementation to illustrate :

   // CPP program to demonstrate use Delete copy
   // constructor and delete assignment operator
   #include <iostream>
   using namespace std;
   
   class Base {
       int x;
   public:
       Base()   {     }
       Base(int y) : x(y) {    }
       Base(const Base& temp_obj) = delete;
       Base& operator=(const Base& temp_obj) = delete;
   };
   
   int main()
   {
       Base b1(10);
       Base b2(b1); // Calls copy constructor
       b2 = b1; // Calls copy assignment operator
       return 0;
   }

   prog.cpp: In function 'int main()':
   prog.cpp:24:15: error: use of deleted function
    'Base::Base(const Base&)'
        Base b2(b1); // Calls copy constructor
                  ^
   prog.cpp:16:5: note: declared here
        Base(const Base &temp_obj) = delete;
        ^
   prog.cpp:26:8: error: use of deleted function 
   'Base& Base::operator=(const Base&)'
        b2 = b1;  // Calls copy assignment operator
           ^
   prog.cpp:17:11: note: declared here
        Base& operator = (const Base &temp_obj) = delete;
              ^
   

   NOTE: This code does not work as we cannot copy the object of this class and hence it will show this error.

Reference:
https://ariya.io/2015/01/c-class-and-preventing-object-copy