Copy a C++ object at a pointer to a new pointer

Question

I have a pointer like this.

MyClass *p;  //MyClass is a complex class

I need to create a new pointer that has a copy of data that is inside the object pointed by p. After copying, I need to change my new object without changing the object pointed by *p.

When I do MyClass *q = p it copies the pointer. What is the correct way to do this?

I want to do something like:

MyClass *q = &p;

and when I now change an attribute of the object pointed by q, I should not affect the object at *p.

Please help me with this.

The instructions (documentation)? Or you can just try it and see. — Galik
– Galik, Commented Feb 4, 2018 at 0:42

Galik · Accepted Answer · 2018-02-04 00:46:36Z

3

If MyClass has a copy constructor then you can simply construct a new one from the old one like this:

MyClass* p = some_function();

MyClass* q = new MyClass(*p); // use copy constructor

Of course you should be using smart pointers for this kind of thing:

std::unique_ptr<MyClass> p = some_function();

std::unique_ptr<MyClass> q = std::make_unique<MyClass>(*p); // copy constructor

This won't work if MyClass is polymorphic, in which case a virtual clone() function would be required.

answered Feb 4, 2018 at 0:46

Galik

49k5 gold badges85 silver badges126 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

harish Over a year ago

Thank you, I will check this. Can you please suggest a good C++ book to start with? There are many on the internet if I search, but please recommend one.

Galik Over a year ago

@harsh This is a recommended book for beginners: rads.stackoverflow.com/amzn/click/0321992784

harish Over a year ago

Thanks a lot @Galik

Kamyar Seifi · Accepted Answer · 2018-02-04 04:32:11Z

2

First, your second pointer needs to be pointing to some allocated memory which has the same type as your first object:

Myclass *d = something();  //pointing to some object in memory 

Myclass *q = new Myclass(); //allocating space in memory for the same type

then you can use the copy operator to copy the data like this:

(*q) = (*d)

answered Feb 4, 2018 at 4:32

Kamyar Seifi

873 bronze badges

1 Comment

harish Over a year ago

Thank you I got another answer, so I did not check this.

Collectives™ on Stack Overflow

Copy a C++ object at a pointer to a new pointer

2 Answers 2

3 Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

3 Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related