How to allocate memory with Constructor in deep copy? - c++

Question

I'm trying to understand Deep Copy. But I'm just confused about allocating dynamically memory by calling Constructor.
Here is my successful program for Deep copy:
#include<iostream>
using namespace std;
class A{
public:
int *p;
A(){
p=new int;
}
void set(int b){
*p=b;
}
int get(){
return *p;
}
void operator=(const A &k){
p = new int;
*p=*(k.p);
}
~A(){
delete p;
}
};
int main()
{
A obj;
obj.set(3);
A obj1;
obj1=obj;
obj1.set(5);
cout << obj.get() << endl;
cout << obj1.get() << endl;
}
Now I just want to ask that I created two objects and constructor will call for two times and in constructor there dynamically memory is allocating.
Then My Question is that pointer should point two different dynamically memory location(2 object and 2 pointer) or is pointer same as Static data members(then no deep copy should require)? Mean one pointer for all objects of class.

Answer 1

If you create two instances of that class, they will have two different p members each containing a different address of different regions in memory, yes.
You can probably tell if you run your program: it will display 3 and 5, if the pointer was the same, it would have displayed 5 twice.
Edit: as asked, some additional explanation (and a summary of what have been said in the comments)
First of all, your operator= is leaking memory, instead you should remember to release the memory already allocated in p before reassigning it:
void operator=(const A &k){
delete p;
// as JoshuaGreen states in the comments, you can set p to nullptr
// here, that way, if new fails and throws, p will be set to nullptr
// and you'll know it doesn't contain anything (you'll have to test
// it in other methods to benefit from this modification though,
// but it will be safer)
p = nullptr;
p = new int;
*p=*(k.p);
}
Although in this specific case, you could just avoid reallocating:
void operator=(const A &k){
// p = new int; // not needed
*p=*(k.p);
}
Now, that was indeed important to overload the assignment operator= (and you actually forgot to overload the copy constructor), let's see what would have happened if this assignment operator= wasn't defined.
Your class would instead look like this:
#include<iostream>
using namespace std;
class A{
public:
int *p;
A(){
p=new int;
}
void set(int b){
*p=b;
}
int get(){
return *p;
}
~A(){
delete p;
}
};
But actually, the compiler will generate an implicitly defined default copy constructor and default assignment operator= for you. Of course you don't see their implementation, but they would behave exactly the same as if the class was defined like this:
#include<iostream>
using namespace std;
class A{
public:
int *p;
A(){
p=new int;
}
A(const A& other) :
p(other.p) {} // ! we're copying the pointer instead of reallocating memory
void set(int b){
*p=b;
}
int get(){
return *p;
}
A& operator=(const A& other){
p = other.p; // same here!
}
~A(){
delete p;
}
};
When you're class deals with dynamically allocated memory, that's bad. Let's see what would happen in your main:
int main()
{
// allocate a new pointer to int, let's call it p
A obj;
// set the content of p to 3
obj.set(3);
// allocate a new pointer to int, let's call it p1
A obj1;
// /!\
// instead of copying the content of p to the content of p1, we're
// actually doing p1 = p here! we're leaking memory AND the two
// objects point on the same memory!
obj1=obj;
// set the content of p1 to 5, but p1 is now equal to p because of
// the bad assignment, so we're also setting p's content to 5
obj1.set(5);
// print the content of p (5)
cout << obj.get() << endl;
// print the content of p1 (5)
cout << obj1.get() << endl;
}
// delete the contents of p and p1 /!\ we're actually deleting the same
// allocated memory twice! that's bad
That's why you have to redefine the "big three" (copy constructor, copy assignment operator, and destructor)