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Shallow and Deep Copy in C++
What is Shallow and Deep Copy in C++ ?
When using copy constructors, shallow copy is when objects share same memory locations for variables. While deep copy is when objects have their own individual memory locations
Difference between Shallow and Deep Copy
We recommend going through what is copy constructor in C++ page before moving ahead with this post
Shallow Copy
Whenever we do not create our own user-defined copy constructor and we do copying, the compiler creates its own hidden copy constructor.
- Whenever this happens the member variables etc share the same memory locations.
- Any change in object1 is also reflected in object2
This default copy constructor does shallow copy. The code below reflects what happens in shallow copy.
Note
This only happens in heap memory variables example pointers
Deep Copy
Whenever we create our own user-defined copy constructor and we do the copying.
- Whenever this happens the member variables etc for any objects have their own memory locations.
- For example, any change in object1 is not reflected in object2
User-defined copy constructor do a deep copy. Code below reflects what happens in shallow copy
Note
This only happens in heap memory variables example pointers
Shallow Copy Example
#include<iostream>
using namespace std;
class PrepInsta
{
int *var;
public:
// Parmeterized constructor
PrepInsta(int x) {
var = new int;
*var = x;
}
// setter
void setValue(int val){
*var = val;
}
// getter
int getValue(){
return *var;
}
// the compiler would create its own hidden copy constructor
// as we have not created our own user defined copy constructor here
// shallow copy will happen i.e. objects variables etc will share memory
};
int main()
{
// calling the normal constructor
PrepInsta obj1(10);
// calling default copy constructor of compiler (Shallow Copy)
PrepInsta obj2 = obj1;
cout << "Before value change - " << endl;
// printing the values, assigned by constructors
cout << "obj1.var = " << obj1.getValue() << endl;
cout << "obj2.var = " << obj2.getValue() << endl;
obj1.setValue(20);
cout << "\nAfter value change - " << endl;
// printing the values, assigned by constructors
cout << "obj1.var = " << obj1.getValue() << endl;
cout << "obj2.var = " << obj2.getValue() << endl;
// since we did shallow copy as we didnt make any
// user defined copy constructor
// change in obj1 is reflected in obj2
return 0;
}Output
Before value change - obj1.var = 10 obj2.var = 10 After value change - obj1.var = 20 obj2.var = 20
Deep Copy Example
#include<iostream>
using namespace std;
class PrepInsta
{
public:
int var;
PrepInsta(int x) {
var = x;
}
// Overriding default copy constructor
// with user defind copy contructor
PrepInsta(const PrepInsta &obj) {
var = obj.var;
}
};
int main()
{
// calling the normal constructor
PrepInsta obj1(10);
// calling user defined copy constructor (Deep Copy)
PrepInsta obj2 = obj1;
cout << "Before value change - " << endl;
// printing the values, assigned by constructors
cout << "obj1.var = " << obj1.var << endl;
cout << "obj2.var = " << obj2.var << endl;
obj1.var = 20;
cout << "\nAfter value change - " << endl;
// printing the values, assigned by constructors
cout << "obj1.var = " << obj1.var << endl;
cout << "obj2.var = " << obj2.var << endl;
// since we did deep copy using user defined copy constructor
// change in obj1 is not reflected in obj2
return 0;
}Output
Before value change - obj1.var = 10 obj2.var = 10 After value change - obj1.var = 20 obj2.var = 10
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