How do i make copy constructor for 2 pointer variable in c++?

Question

I have two pointer variables as data members. How do I make copies of these two data members? How do I make a deep copy constructor for this? I tried many times, but it's not working.

class abc {
    float *number1;
    float *number2;

public:
    abc(float l,float w)
    {
       number1 = new float;
       number2 = new float;
       number1 = &l;
       number2 = &w;
    }
    
    float get_1()
    {
        return *number1;
    }
    
    float get_2()
    {
        return *number2;    
    }
    
    abc(const abc& ob)
    {
        number1 = ob.number1;
        number2 = ob.number2;
    }
    
    void display()
    {
        cout << "Number 1 : " << *number1 << endl;
        cout << "Number 2 : " << *number2 << endl;
    }
};

John, let's start with code alignment first. Then read this: https://www.learncpp.com/cpp-tutorial/shallow-vs-deep-copying/ — unalignedmemoryaccess, Apr 19 '21 at 14:29
Why dynamically allocate two `float`s instead of: `float number1; float number2;` and not have to bother? — Ted Lyngmo, Apr 19 '21 at 14:29
@TedLyngmo ... and overwrite the pointers by meanlgless local pointers, causing memory leak? — MikeCAT, Apr 19 '21 at 14:30
If you change the `float*`'s into just regular `float`'s, you don't need to do anything. — NathanOliver, Apr 19 '21 at 14:30
Maybe one day it will be array of floats, so he wants to learn...Slow down people.. — unalignedmemoryaccess, Apr 19 '21 at 14:31
@MikeCAT Pretty sure they mean change the class members to `float` from `float*` — NathanOliver, Apr 19 '21 at 14:31
But I make deep constructor and i have no idea how to deal with — john, Apr 19 '21 at 14:31
If change float pointer to float variable i can easily make shallow copy but my task to deep copy — john, Apr 19 '21 at 14:32
@john Again: Why are you using `new` there then at all, if you don't want to share pointers?!? _"If change float pointer to float variable i can easily make shallow copy but my task to deep copy"_ Huh, what please? — πάντα ῥεῖ, Apr 19 '21 at 14:32
I am using new for allocating new memory when i make new object — john, Apr 19 '21 at 14:33
@john Have you been programming in another language where using `new` is the proper way to do this? — Ted Lyngmo, Apr 19 '21 at 14:35
"I try many times but its not working" <- not a problem description. Explain what you have tried, what output you want, what output you get instead, and how that is wrong. — underscore_d, Apr 19 '21 at 14:37
"but my task to deep copy" Is your instructor deliberately teaching you bad C++, or do they just not know better? — Caleth, Apr 19 '21 at 14:43
I want that 2 data member copy into another object when i call copy constructor — john, Apr 19 '21 at 14:46
Of course one shouldn't store floats as pointers in a case like this, but one also wouldn't name their class "abc" and members "number1" and "number2". Man's just giving a simple example to learn a new concept, people should chill about it. — Antti_M, Apr 19 '21 at 14:48
@tilz0R one day they will learn about `std::vector`, there is no reason to use pointers here — 463035818_is_not_an_ai, Apr 19 '21 at 15:39
@largest_prime_is_463035818 do you think vector is faster than new on microcontroller? What is value is big? Who handles realloc for vector. — unalignedmemoryaccess, Apr 19 '21 at 15:40

mch · Answer 1 · 2021-04-19T14:47:06.053

Also your constructor is wrong:

abc(float l,float w)
{
   number1= new float;
   number2=new float;
   number1 =&l; //this will overwrite the pointer returned by new with the address of a local variable
   number2=&w; //the same
}

you should do this:

abc(float l,float w)
{
   number1= new float(l); // allocate a float and set the value to l
   number2=new float(w); // allocate a float and set the value to w
}

And the copy constructor is quite similar:

abc(const abc& ob)
{
    number1 = new float(*ob.number1); // allocate a float and set the value to number1 from the other object
    number2 = new float(*ob.number2); // allocate a float and set the value to number2 from the other object
}

This example can be improved further by using the initializer list of the constructors:

abc(float l,float w) :
        number1 { new float(l)},
        number2 { new float(w)}
{
}

abc(const abc& ob) :
        number1 { new float(*ob.number1)},
        number2 { new float(*ob.number2)}
{
}

You mean `new float(*ob.number1)` instead of `new float(ob.number1)`? (also for `number2`) — MikeCAT, Apr 19 '21 at 14:41
@john If you are going to give feedback on answers, you'd better be more precise. "not working" doesn't provide any insight into what it is that's not working. — Ted Lyngmo, Apr 19 '21 at 14:45

Vlad from Moscow · Answer 2 · 2021-04-19T14:53:23.993

For starters this constructor

abc(float l,float w)
{
   number1 = new float;
   number2 = new float;
   number1 = &l;
   number2 = &w;
}

produces memory leaks. At first a memory was allocated in these statements

   number1 = new float;
   number2 = new float;

and its addresses were assigned to pointers. But then at once the pointers were reassigned

   number1 = &l;
   number2 = &w;

So the addresses to the allocated memory were lost.

The constructor can look at least the following way

abc( float l, float w ) : number1( new float( l ) ), number2( new float( w ) )
{
}

These functions

float get_1()
{
    return *number1;
}

float get_2()
{
    return *number2;    
}

should be declared like

float get_1() const
{
    return *number1;
}

float get_2() const
{
    return *number2;    
}

In general it is better to define these functions like

const float & get_1() const
{
    return *number1;
}

const float & get_2() const
{
    return *number2;    
}

float & get_1()
{
    return *number1;
}

float & get_2()
{
    return *number2;    
}

The copy constructor can look like

abc( const abc &ob) 
    : number1( new float( *ob.number1 ) ), number2( new float( *ob.number2 ) )
{
}

The function display should look at least like

void display() const 
{
    cout << "Number 1 : " << *number1 << endl;
    cout << "Number 2 : " << *number2 << endl;
}

Instead of this function you could define a friend operator <<. For example

class abc
{
    //...
    friend std::ostream & operator <<( std::ostream &os, const abc &ob )
    {
        os << "Number 1 : " << *ob.number1 << std::endl;
        os << "Number 2 : " << *ob.number2 << std::endl;

        return os;
    }
}

Also you should define explicitly the copy assignment operator and the destructor.

ravenspoint · Answer 3 · 2021-04-19T14:32:09.520

0

void display()
{
    cout<<"Number 1 : "<<number1<<endl;
    cout<<"Number 2 :"<<number2<endl;
}

This will print the location in memory of the numbers. Is that what you want? To display the value of the numbers

void display()
{
    cout<<"Number 1 : "<< *number1 << endl;
    cout<<"Number 2 : "<< *number2 << endl;
}

edited Apr 19 '21 at 14:32

answered Apr 19 '21 at 14:31

ravenspoint

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This is not really an answer. He wants to do copy of the data in the right way, so new alloc and copy then. – unalignedmemoryaccess Apr 19 '21 at 14:31
1

I didn't give you negative feedback, but I want to point that asking him what he wants is not SO answer :) I rather give positive feedback when possible, and nothing when not. – unalignedmemoryaccess Apr 19 '21 at 14:35

How do i make copy constructor for 2 pointer variable in c++?

3 Answers3