# auto_ptr

ISO C++
auto_ptr

## Limitations

Explaining all of the fun and delicious things that can happen with misuse of the `auto_ptr` class template (called AP here) would take some time. Suffice it to say that the use of AP safely in the presence of copying has some subtleties.

The AP class is a really nifty idea for a smart pointer, but it is one of the dumbest of all the smart pointers -- and that's fine.

AP is not meant to be a supersmart solution to all resource leaks everywhere. Neither is it meant to be an effective form of garbage collection (although it can help, a little bit). And it can notbe used for arrays!

AP is meant to prevent nasty leaks in the presence of exceptions. That's all. This code is AP-friendly:

        // Not a recommend naming scheme, but good for web-based FAQs.
        typedef std::auto_ptr<MyClass>  APMC;

        extern function_taking_MyClass_pointer (MyClass*);
        extern some_throwable_function ();

        void func (int data)
        {
    	APMC  ap (new MyClass(data));

    	some_throwable_function();   // this will throw an exception

    	function_taking_MyClass_pointer (ap.get());
        }

When an exception gets thrown, the instance of MyClass that's been created on the heap will be `delete`'d as the stack is unwound past `func()`.

Changing that code as follows is not AP-friendly:

    	APMC  ap (new MyClass[22]);

You will get the same problems as you would without the use of AP:

    	char*  array = new char[10];       // array new...
    	...
    	delete array;                      // ...but single-object delete

AP cannot tell whether the pointer you've passed at creation points to one or many things. If it points to many things, you are about to die. AP is trivial to write, however, so you could write your own `auto_array_ptr` for that situation (in fact, this has been done many times; check the mailing lists, Usenet, Boost, etc).

## Use in Containers

All of the containers described in the standard library require their contained types to have, among other things, a copy constructor like this:

        struct My_Type
        {
    	My_Type (My_Type const&);
        };

Note the const keyword; the object being copied shouldn't change. The template class `auto_ptr` (called AP here) does not meet this requirement. Creating a new AP by copying an existing one transfers ownership of the pointed-to object, which means that the AP being copied must change, which in turn means that the copy ctors of AP do not take const objects.

The resulting rule is simple: Never ever use a container of auto_ptr objects. The standard says that undefined behavior is the result, but it is guaranteed to be messy.

To prevent you from doing this to yourself, the concept checks built in to this implementation will issue an error if you try to compile code like this:

        #include <vector>
        #include <memory>

        void f()
        {
    	std::vector< std::auto_ptr<int> >   vec_ap_int;
        }

Should you try this with the checks enabled, you will see an error.