Thread: Copy Construcor issue

Hi!

This is my code

Code:

//Main

#include "Class_header.h"
#include <iostream>
using namespace std;




int main ()
{


vectorOfint some_array;


some_array.print();


vectorOfint copy_of=some_array;         //Copying class


copy_of.print();


}

// defining methods

#include "Class_header.h"
#include <iostream>
using namespace std;




vectorOfint::vectorOfint()    //Constructor
{
for (int i=0;i<32;i++)
{
p_array[i]=i;
}


}


vectorOfint::vectorOfint(const vectorOfint& other) //Copy constructor


{
int *p_array=new int[32];
x=4;


for (int i=0;i<32;i++)
{
p_array[i]=other.p_array[i];
}
}






void vectorOfint::print()
{
for (int i=0;i<32;i++)
{
cout<<p_array[i]<<" ";
}
cout<<"\nx is "<<x<<'\n'<<'\n';
}

//Header

class vectorOfint
{


public:


vectorOfint();                          //Constructor
vectorOfint(const vectorOfint& other);  //Copy constructor
void print ();


private:


int *p_array=new int[32];
int x=0;
};

Problem is that if I call some_array.print(), it prints out numbers 0-31 and "x is 0" as it should. But if I call copy_of.print(); it prints out 32 random numbers and "x=4". It changes x but can´t copy members of array. Why?

Why?

Why are you trying to reallocate memory for your array inside the constructor? You've already allocated memory for the array in the class definition.


Why are you even using dynamic memory instead of a "normal" statically allocated array since you're always seem to be trying to create an array of 32 elements?

Jim

He's not just reallocating the space, but assigning the new pointer to a local variable.

With this code I just wanted to see how copy constructor works (I´m total beginner). As I understand, there is meaning to use copy constructor only if you use pointers and book that I read and videos I watched showed that you should allocate new memory for pointers. My English is not very good so I can't say completely what I mean but I hope you understand enough.

As I understand, there is meaning to use copy constructor only if you use pointers and book that I read and videos I watched showed that you should allocate new memory for pointers.

That may be true, but you're already allocating memory every time you create an instance of the class.

Code:

class vectorOfint
{
 
...
   private:
      int *p_array=new int[32]; // This allocates memory every time you create an instance of the class.

And because you manually allocate memory for that pointer every time you create an instance of the class you need to write a destructor, copy constructor and assignment operator, and possibly the move copy consturctor and move assignment operator.

Because of the above assignment you don't need to try to do the allocation in your constructors.

Jim

Originally Posted by jimblumberg

That may be true, but you're already allocating memory every time you create an instance of the class.

And because you manually allocate memory for that pointer every time you create an instance of the class you need to write a destructor, copy constructor and assignment operator, and possibly the move copy consturctor and move assignment operator.

Because of the above assignment you don't need to try to do the allocation in your constructors.

Jim

I did not write destructor and assigment operator this time because I tried to keep things simple for myself. I´m just not familiar with these things jet. What I did was simplified attempt to create something like in this video

Rule of Three Implementation in C++ - YouTube

At 9:23 guy in this video creates class with pointer data member and later he creates copy constructor and in that (at 13:20) he creates new pointer to avoid making what he calls "shallow copy". Instead he creates "deep copy".

Is it not right?

I did not write destructor and assigment operator this time because I tried to keep things simple for myself. I´m just not familiar with these things jet.

Then perhaps you're not ready to be using pointers in your classes, or perhaps not ready for pointers period.

At 9:23 guy in this video creates class with pointer data member and later he creates copy constructor and in that (at 13:20) he creates new pointer to avoid making what he calls "shallow copy". Instead he creates "deep copy".

I'm not interested in watching some video of unknown quality, however that video probably didn't allocate memory for the pointer in the class definition as you did. I really suggest you find and use a good book on C++ programming instead of relying on YouTube.

Code:

struct something
{
    char *some_uninitialized_pointer;
    char *some_initialized_pointer = new char[32];

With the above code the two pointers would have to be handled differently in your constructor, one would need to have the memory allocated but the other wouldn't. However the destructor should delete the memory for both of those pointers.

Jim

You all were right. Next time I try to be more careful and check things up before posting. Thank you!