Custom copy constructor and adding a field

Question

Duplicate of this.

In C++ you sometimes have to implement the copy constructor yourself (when you have pointer as a member usually). Over compiler generated copy constructor this has the disadvantage that when you add a member field and forget to add the copying line in the copy constructor, you have a problem, which is often hard to track down. I like to program defensively and this worries me a bit.

One solution would be to use memcpy and then just handle the pointers properly, but this is discouraged as I understand it.

Answer 1

No, do not use memcpy. You run the risk of overwriting data that you don't even know exists (vtables, for example.) Also, if the field that you've added is itself a pointer, then you haven't solved anything.

The best practices for programming defensively for this situation are:

• Be in the habit of examining your class constructors, destructors, etc. whenever you add fields

• Have automated tests

• Use memory debugging tools such as Purify (open-source tools are also available that may help.)

Answer 2

Don't do it - it won't work either way if someone adds a pointer member variable and forgets about the copy constructor - I'd suggest you add a compile time assert link text in the copy constuctor for the sizeof of your class - if the sizeof will change - the compile assert will fail and the code will not compile untill someone changes the condition - the possibility of someone changing the condition that sits next to a comment about making sure to copy all members correctly and not doing that is quite low ;)

Answer 3

Don't use naked pointers - use a smart pointer that will do the copy for you, if needed. Yo then no longer need to write a copy constructor - I have only written a single one in the past five years.