c++ - Why is the argument of the copy constructor a reference rather than a pointer?

Question

Why is the argument of the copy constructor a reference rather than a pointer?

Why can't we use the pointer instead?

Answer 1

There are many reasons:

1. References cannot be NULL. OK, it's possible to create a NULL reference, but it's also possible to cast a std::vector<int>* into a std::vector<SomeType>*. That doesn't mean such a cast has defined behavior. And neither does creating a NULL reference. Pointers have defined behavior when set to NULL; references do not. References are therefore always expected to refer to actual objects.

2. Variables and temporaries cannot be implicitly converted into pointers to their types. For obvious reasons. We don't want pointers to temporaries running around, which is why the standard expressly forbids doing it (at least when the compiler can tell you are doing it). But we are allowed to have references to them; these are implicitly created.

3. Because of point number 2, using pointers rather than references would require every copy operation to use the address-of operator (&). Oh wait, the C++ committee foolishly allowed that to be overloaded. So any copy operation would need to actually use std::addressof, a C++11 feature, to get the address. So every copy would need to look like Type t{std::addressof(v)}; Or you could just use references.