What is the difference between memberwise copy, bitwise copy, shallow copy and deep copy?

Question

I have googled these terms but I am still confused.

Some people said that memberwise copy is deep copy and the bitwise copy is shallow copy but someone said it is not.

Can anyone explain to me that which type of copy of the default copy constructor and the user-defined copy constructor use?

Answer 1

Member-wise Copy

Is when you visit each member and explicitly copy it, invoking its copy constructor. It is usually tantamount to deep-copy. It is the right and proper way of copying things. The opposite is bit-wise copy, which is a hack, see below.

Bit-wise Copy

Is a specific form of shallow copy. It is when you simply copy the bits of the source class to the target class, using memcpy() or something similar. Constructors are not invoked, so you tend to get a class which appears to be all right but things start breaking in horrible ways as soon as you start using it. This is the opposite of member-wise copy, and is a quick and dirty hack that can sometimes be used when we know that there are no constructors to be invoked and no internal structures to be duplicated. For a discussion of what may go wrong with this, see this Q&A: C++ bitwise vs memberwise copying?

Shallow Copy

Refers to copying just the immediate members of an object, without duplicating whatever structures are pointed by them. It is what you get when you do a bit-wise copy.

^{(Note that there is no such thing as "shadow copy". I mean, there is such a thing, in file systems, but that's probably not what you had in mind.)}

Deep Copy

Refers to not only copying the immediate members of an object, but also duplicating whatever structures are pointed by them. It is what you normally get when you do member-wise copy.

To summarize:

There are two categories:

• Shallow Copy
• Deep Copy

Then, there are two widely used techniques:

• Bit-wise Copy (a form of Shallow Copy)
• Member-wise Copy (a form of Deep Copy)

As for the hear-say about someone who said something and someone who said something else: bit-wise copy is definitely always shallow copy. Member-wise copy is usually deep copy, but you may of course foul it up, so you may be thinking that you are making a deep copy while in fact you are not. Proper member-wise copy relies on having proper copy constructors.

Finally:

The default copy constructor will do a bit-wise copy if the object is known to be trivially copyable, or a member-wise copy if not. However, the compiler does not always have enough information to perform a proper copy of each member. For example, a pointer is copied by making a copy of the pointer, not by making a copy of the pointed object. That's why you should generally not rely on the compiler providing you with a default copy constructor when your object is not trivially copyable.

A user-supplied constructor may do whatever type of copy the user likes. Hopefully, the user will choose wisely and do a member-wise copy.