Complete scoped_ptr.adoc

doc/smart_ptr/scoped_ptr.adoc

@@ -8,8 +8,204 @@ http://www.boost.org/LICENSE_1_0.txt
 ////
 
 [#scoped_ptr]
-# scoped_ptr
+# scoped_ptr: Scoped Object Ownership
 :toc:
 :toc-title:
-:idprefix:
+:idprefix: scoped_ptr_
 
+## Description
+
+The `scoped_ptr` class template stores a pointer to a dynamically allocated object.
+(Dynamically allocated objects are allocated with the {cpp} `new` expression.) The
+object pointed to is guaranteed to be deleted, either on destruction of the `scoped_ptr`,
+or via an explicit `reset`. See the <<scoped_ptr_example,example>>.
+
+`scoped_ptr` is a simple solution for simple needs. It supplies a basic "resource acquisition
+is initialization" facility, without shared-ownership or transfer-of-ownership semantics.
+Both its name and enforcement of semantics (by being  noncopyable) signal its intent to retain
+ownership solely within the current scope. Because it is noncopyable, it is safer than `shared_ptr`
+for pointers which should not be copied.
+
+Because `scoped_ptr` is simple, in its usual implementation every operation is as fast as for a
+built-in pointer and it has no more space overhead that a built-in pointer.
+
+`scoped_ptr` cannot be used in {cpp} Standard Library containers. Use `shared_ptr` or `std::unique_ptr`
+if you need a smart pointer that can.
+
+`scoped_ptr` cannot correctly hold a pointer to a dynamically allocated array. See `scoped_array` for that usage.
+
+The class template is parameterized on `T`, the type of the object pointed to. Destroying `T` must not thow exceptions,
+and `T` must be complete at the point `scoped_ptr<T>::~scoped_ptr` is instantiated.
+
+## Synopsis
+
+`scoped_ptr` is defined in `<boost/smart_ptr/scoped_ptr.hpp>`.
+
+```
+namespace boost
+{
+
+template<class T> class scoped_ptr
+{
+private:
+
+    scoped_ptr(scoped_ptr const&);
+    scoped_ptr& operator=(scoped_ptr const&);
+
+    void operator==(scoped_ptr const&) const;
+    void operator!=(scoped_ptr const&) const;
+
+public:
+
+    typedef T element_type;
+
+    explicit scoped_ptr(T * p = 0) noexcept;
+    ~scoped_ptr() noexcept;
+
+    void reset(T * p = 0) noexcept;
+
+    T & operator*() const noexcept;
+    T * operator->() const noexcept;
+    T * get() const noexcept;
+
+    explicit operator bool() const noexcept;
+
+    void swap(scoped_ptr & b) noexcept;
+};
+
+template<class T> void swap(scoped_ptr<T> & a, scoped_ptr<T> & b) noexcept;
+
+} // namespace boost
+```
+
+## Members
+
+### element_type
+
+    typedef T element_type;
+
+Provides the type of the stored pointer.
+
+### constructor
+
+    explicit scoped_ptr(T * p = 0) noexcept;
+
+Constructs a `scoped_ptr`, storing a copy of `p`, which must have been allocated via a
+{cpp} `new` expression or be 0. `T` is not required be a complete type.
+
+### destructor
+
+    ~scoped_ptr() noexcept;
+
+Destroys the object pointed to by the stored pointer, if any, as if by using
+`delete this\->get()`. `T` must be a complete type.
+
+### reset
+
+    void reset(T * p = 0) noexcept;
+
+Deletes the object pointed to by the stored pointer and then stores a copy of
+`p`, which must have been allocated via a {cpp} `new` expression or be 0.
+
+Since the previous object needs to be deleted, `T` must be a complete type.
+
+### indirection
+
+    T & operator*() const noexcept;
+
+Returns a reference to the object pointed to by the stored pointer. Behavior is undefined if the stored pointer is 0.
+
+    T * operator->() const noexcept;
+
+Returns the stored pointer. Behavior is undefined if the stored pointer is 0.
+
+### get
+
+    T * get() const noexcept;
+
+Returns the stored pointer. `T` need not be a complete type.
+
+### conversions
+
+    explicit operator bool () const noexcept; // never throws
+
+Returns `get() != 0`.
+
+### swap
+
+    void swap(scoped_ptr & b) noexcept;
+
+Exchanges the contents of the two smart pointers. `T` need not be a complete type.
+
+## Free Functions
+
+### swap
+
+    template<class T> void swap(scoped_ptr<T> & a, scoped_ptr<T> & b) noexcept;
+
+Equivalent to `a.swap(b)`.
+
+## Example
+
+Here's an example that uses `scoped_ptr`.
+
+```
+#include <boost/scoped_ptr.hpp>
+#include <iostream>
+
+struct Shoe { ~Shoe() { std::cout << "Buckle my shoe\n"; } };
+
+class MyClass {
+    boost::scoped_ptr<int> ptr;
+  public:
+    MyClass() : ptr(new int) { *ptr = 0; }
+    int add_one() { return ++*ptr; }
+};
+
+int main()
+{
+    boost::scoped_ptr<Shoe> x(new Shoe);
+    MyClass my_instance;
+    std::cout << my_instance.add_one() << '\n';
+    std::cout << my_instance.add_one() << '\n';
+}
+```
+
+The example program produces the beginning of a child's nursery rhyme:
+
+```
+1
+2
+Buckle my shoe
+```
+
+## Rationale
+
+The primary reason to use `scoped_ptr` rather than `std::auto_ptr` or `std::unique_ptr` is to let readers of your code
+know that you intend "resource acquisition is initialization" to be applied only for the current scope, and have no intent to transfer ownership.
+
+A secondary reason to use `scoped_ptr` is to prevent a later maintenance programmer from adding a function that transfers
+ownership by returning the `auto_ptr`, because the maintenance programmer saw `auto_ptr`, and assumed ownership could safely be transferred.
+
+Think of `bool` vs `int`. We all know that under the covers `bool` is usually just an `int`. Indeed, some argued against including bool in the {cpp}
+standard because of that. But by coding `bool` rather than `int`, you tell your readers what your intent is. Same with `scoped_ptr`; by using it you are signaling intent.
+
+It has been suggested that `scoped_ptr<T>` is equivalent to `std::auto_ptr<T> const`. Ed Brey pointed out, however, that `reset` will not work on a `std::auto_ptr<T> const`.
+
+## Handle/Body Idiom
+
+One common usage of `scoped_ptr` is to implement a handle/body (also called pimpl) idiom which avoids exposing the body (implementation) in the header file.
+
+The `link:../../example/scoped_ptr_example_test.cpp[scoped_ptr_example_test.cpp]` sample program includes a header file,
+`link:../../example/scoped_ptr_example.hpp[scoped_ptr_example.hpp]`, which uses a `scoped_ptr<>` to an incomplete type to hide the
+implementation. The instantiation of member functions which require a complete type occurs in the `link:../../example/scoped_ptr_example.cpp[scoped_ptr_example.cpp]`
+implementation file.
+
+## Frequently Asked Questions
+
+[qanda]
+Why doesn't `scoped_ptr` have a `release()` member?::
+
+  When reading source code, it is valuable to be able to draw conclusions about program behavior based on the types being used. If `scoped_ptr` had a `release()` member,
+  it would become possible to transfer ownership of the held pointer, weakening its role as a way of limiting resource lifetime to a given context. Use `std::auto_ptr` where
+  transfer of ownership is required. (supplied by Dave Abrahams)