doc/smart_ptr/scoped_ptr.adoc

////
Copyright 2017 Peter Dimov

Distributed under the Boost Software License, Version 1.0.

See accompanying file LICENSE_1_0.txt or copy at
http://www.boost.org/LICENSE_1_0.txt
////

[#scoped_ptr]
# scoped_ptr: Scoped Object Ownership
:toc:
:toc-title:
: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;

  template<class T>
    bool operator==( scoped_ptr<T> const & p, std::nullptr_t ) noexcept;
  template<class T>
    bool operator==( std::nullptr_t, scoped_ptr<T> const & p ) noexcept;

  template<class T>
    bool operator!=( scoped_ptr<T> const & p, std::nullptr_t ) noexcept;
  template<class T>
    bool operator!=( std::nullptr_t, scoped_ptr<T> const & p ) noexcept;
}
```

## 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`.

NOTE: On C++03 compilers, the return value is of an unspecified type.

### 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)`.

### comparisons

    template<class T> bool operator==( scoped_ptr<T> const & p, std::nullptr_t ) noexcept;

    template<class T> bool operator==( std::nullptr_t, scoped_ptr<T> const & p ) noexcept;

Returns `p.get() == nullptr`.

    template<class T> bool operator!=( scoped_ptr<T> const & p, std::nullptr_t ) noexcept;

    template<class T> bool operator!=( std::nullptr_t, scoped_ptr<T> const & p ) noexcept;

Returns `p.get() != nullptr`.

## 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)