what is auto_ptr_ref, what it achieves and how it achieves it

Question

auto_ptr_ref documentation here says this

This is an instrumental class to allow certain conversions that allow auto_ptr objects to be passed to and returned from functions.

Can somebody explain how auto_ptr_ref helps in achieving this. I just want to understand the auto_ptr class and its internals

Answer 1

It is rather confusing. Basically, auto_ptr_ref exists because the auto_ptr copy constructor isn't really a copy constructor in the standard sense of the word.

Copy constructors typically have a signature that looks like this:

X(const X &b);

The auto_ptr copy constructor has a signature that looks like this:

X(X &b)

This is because auto_ptr needs to modify the object being copied from in order to set its pointer to 0 to facilitate the ownership semantics of auto_ptr.

Sometimes, temporaries cannot match a copy constructor that doesn't declare its argument const. This is where auto_ptr_ref comes in. The compiler won't be able to call the non-const version of the copy constructor, but it can call the conversion operator. The conversion operator creates an auto_ptr_ref object that's just sort of a temporary holder for the pointer. The auto_ptr constructor or operator = is called with the auto_ptr_ref argument.

If you notice, the conversion operator in auto_ptr that automatically converts to an auto_ptr_ref does a release on the source auto_ptr, just like the copy constructor does.

It's kind of a weird little dance that happens behind the scenes because auto_ptr modifies the thing being copied from.

Random related tanget about C++0x and unique_ptr

In C++0x, auto_ptr is deprecated in favor of unique_ptr. unique_ptr doesn't even have a copy constructor and uses the new 'move constructor' which is explicit about the fact that it will modify the object being moved from and leave it in a useless (but still valid) state. Temporaries (aka rvalues) are explicitly always allowed to be arguments to a move constructor.

The move constructor in C++0x has a number of other big benefits. It enables the standard STL containers to store unique_ptrs and do the right thing, as opposed to how auto_ptrs cannot be. It also mostly eliminates the need for the 'swap' function as the whole purpose of the swap function is usually to be a move constructor or move assignment operator that never throws.

Which is the other expectation. The move constructor and move assignment operator (much like a destructor) are never supposed to throw.

Answer 2

I just found a very good link and a name for this technique "Move Constructors" or "Colvin-Gibbons trick"

http://en.wikibooks.org/wiki/More_C%2B%2B_Idioms/Move_Constructor

Answer 3

As a result, it is possible to store smart pointers in containters (for example, QMap) with help of auto_ptr_ref. The only need is to initialize auto_ptr_ref from auto_ptr when inserting, and back auto_ptr from auto_ptr_ref to operate.