Copying Classes with Dynamic Memory

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Deep copy vs Shallow Copy
What is the difference between overloading operator= and overloading the copy constructor?

I see two ways of copying a class:

Copy Constructor
operator=

My question is, which one should make a new copy of dynamically allocated memory (2 classes with the same data and 2 instances of dynamic memory) and which should simply move the class to a new memory location (one class with the same dynamically allocated memory but the class is in a different place)?

there are two different operations and i need to distinguish which one is desired — Lauer, Oct 24 '12 at 13:15
@user1204406 the only sane way to distinguish between a copy and a move is to use copy and move constructors and copy and move assignment operators, with rvalue references from C++11. — R. Martinho Fernandes, Oct 24 '12 at 13:17
The only way you can copy a class is by using copy/paste in your editor. Maybe you mean "copy an object"? — Kerrek SB, Oct 24 '12 at 13:18
Credit goes to R. Martinho Fernandes for answering the question out of the eight responses — Lauer, Oct 24 '12 at 14:58
This question and its closure are being discussed here: http://meta.stackexchange.com/questions/152792/ — Bart, Oct 27 '12 at 18:58
@andrey, other close voters: can you review the meta question Bart linked to and see if you still think this is a dupe? — Jaydles, Oct 27 '12 at 19:15

score 1 · Answer 1 · answered Oct 24 '12 at 13:34

A copy constructor creates a new object and initializes its state based on an existing object:

A x(y); // x is now in the same state as y

An assignment operator takes an existing object and changes its state to match another existing object:

A x; // x is in the default state
x = y; // x is now in the same state as y

Whatever decisions you make about the state should apply equally to both.

score 0 · Answer 2 · edited May 23 '17 at 11:55

0

You must implement both because you can declare variable through copy constructor. You can assign to variable. For example:

Class a; //some heap allocations inside
Class b(a); //must be valid
Class c = a; //must be valid too

And dont forget about destructor. Simple way is achieve copy and assign operator is to use copy when assign then swap(copy&swap). Copy & swap and Rule of three

edited May 23 '17 at 11:55

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answered Oct 24 '12 at 13:18

Denis Ermolin

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Copying Classes with Dynamic Memory

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