Get a copy of "this" (current instance) in C++

Question

Hi,

I want to have a copy of the currently running instance.

When i change a value in the copy, original object is also affected. The copy acts as an instance.

How to avoid this? I need to create an independent copy of the calling object.

 Set operator+(Set s){
             Set temp = *this;  

             for(int i=0; s.elements[i] != '\0'; i++){
                     temp(s.elements[i]);
             }
             temp.elements[0] = 'X'; // <- this affects calling object also :(

             return temp;

         }

Answer 1

The problem is that Set temp = *this; makes a shallow copy, not a deep copy. You will have to modify the copy constructor and assignment operators for the Set class so that they make copies of all the member/contained objects.

E.g:

class Set
{
public:
    Set()
    {
        elements = new SomeOtherObject[12];
        // Could make elements a std::vector<SomeOtherObject>, instead
    }

    Set(const Set& other)
    {
        AssignFrom(other);
    }

    Set& operator=(const Set& other)
    {
        AssignFrom(other);
        return *this;
    }

private:
    void AssignFrom(const Set& other)
    {
        // Make copies of entire array here, as deep as you need to.
        // You could simply do a top-level deep copy, if you control all the
        // other objects, and make them do top-level deep copies, as well
    }

    SomeOtherObject* elements;
};

Answer 2

This probably depends on how Set is implemented. If the assignment operator and the copy constructor haven't been overloaded to do a deep copy(including elements) then it won't work as expected.

Answer 3

Have you implemented a copy constructor for your class? Default copy constructor will copy any pointer in your class, but not the content you are pointing to. You need to create a copy constructor or overload the '=' operator.

Answer 4

Not that your function already makes two copies, since it takes its argument and returns its result per copy:

Set operator+(Set s);

So you wouldn't have to copy s, because it's already copied. I suppose this is involuntarily, so you might want to read about how to pass objects to functions and how to return objects from function in C++.

The problem you're reporting, though, hints at your copy constructor not working properly. Did you implement the copy constructor or are you using the compiler-supplied one?

Answer 5

Ok. I went through rule of three and did the following changes... Can you point out what's wrong with this?

#include<iostream>
#include<cstring>

using namespace std;

class Set{
  char *elements;

  public:
         Set() {
              elements = new char('\0');
              index = -1;
         }

         Set(const Set& cpy){
                  *this = cpy;
         }


         Set operator+(Set s){
             Set temp = *this;       // IMPORTANT! copy constructor of Set is called, "this" is passed as argument
                                     // * = current OBJECT, else returns ADDRESS of current object

             for(int i=0; s.elements[i] != '\0'; i++){
                     temp(s.elements[i]);
             }

             return temp;

         }
         Set& operator=(Set s){  
              delete [] elements;
             elements = new char[strlen(s.elements) + 1];
             strcpy(elements,  s.elements); //overrides element of "this"

             return *this;
         }

    };

Answer 6

I would avoid a char pointer completely and use std::string instead. This way you dont even need a copy constructor and an assigment operator because the compiler generated once will do just fine. (because 'elements' of the 'Set' class is copy-constructible and has an assignment operator) Here is my solution:

#include <iostream>
#include <string>

class Set{
  std::string elements;

  public:
         Set() {
             elements = "";
         }

         explicit Set(char* _elements) {
             if (_elements)
                elements = _elements;
         }

         Set operator+(const Set& s){
             Set temp(*this);    

             temp.elements += s.elements;
             return temp;
         }



};

Btw. I added a constructor from char* so that 'elements' can somehow be initialized from outside. Not sure if this is what you wanted.