doc/smart_ptr/intrusive_ptr.adoc

////
Copyright 2003-2005, 2013, 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
////

[#intrusive_ptr]
# intrusive_ptr: Managing Objects with Embedded Counts
:toc:
:toc-title:
:idprefix: intrusive_ptr_

## Description

The `intrusive_ptr` class template stores a pointer to an object with an embedded reference count.
Every new `intrusive_ptr` instance increments the reference count by using an unqualified call to the
function `intrusive_ptr_add_ref`, passing it the pointer as an argument. Similarly, when an `intrusive_ptr`
is destroyed, it calls `intrusive_ptr_release`; this function is responsible for destroying the object when
its reference count drops to zero. The user is expected to provide suitable definitions of these two functions.
On compilers that support argument-dependent lookup, `intrusive_ptr_add_ref` and `intrusive_ptr_release` should
be defined in the namespace that corresponds to their parameter; otherwise, the definitions need to go in namespace
`boost`. The library provides a helper base class template `<<intrusive_ref_counter,intrusive_ref_counter>>` which
may help adding support for `intrusive_ptr` to user types.

The class template is parameterized on `T`, the type of the object pointed to. `intrusive_ptr<T>` can be implicitly
converted to `intrusive_ptr<U>` whenever `T*` can be implicitly converted to `U*`.

The main reasons to use `intrusive_ptr` are:

* Some existing frameworks or OSes provide objects with embedded reference counts;
* The memory footprint of `intrusive_ptr` is the same as the corresponding raw pointer;
* `intrusive_ptr<T>` can be constructed from an arbitrary raw pointer of type `T*`.

As a general rule, if it isn't obvious whether `intrusive_ptr` better fits your needs than `shared_ptr`, try a `shared_ptr`-based design first.

## Synopsis

`intrusive_ptr` is defined in `<boost/smart_ptr/intrusive_ptr.hpp>`.

```
namespace boost {

  template<class T> class intrusive_ptr {
  public:

    typedef T element_type;

    intrusive_ptr() noexcept;
    intrusive_ptr(T * p, bool add_ref = true);

    intrusive_ptr(intrusive_ptr const & r);
    template<class Y> intrusive_ptr(intrusive_ptr<Y> const & r);

    ~intrusive_ptr();

    intrusive_ptr & operator=(intrusive_ptr const & r);
    template<class Y> intrusive_ptr & operator=(intrusive_ptr<Y> const & r);
    intrusive_ptr & operator=(T * r);

    void reset();
    void reset(T * r);
    void reset(T * r, bool add_ref);

    T & operator*() const noexcept;
    T * operator->() const noexcept;
    T * get() const noexcept;
    T * detach() noexcept;

    explicit operator bool () const noexcept;

    void swap(intrusive_ptr & b) noexept;
  };

  template<class T, class U>
    bool operator==(intrusive_ptr<T> const & a, intrusive_ptr<U> const & b) noexcept;

  template<class T, class U>
    bool operator!=(intrusive_ptr<T> const & a, intrusive_ptr<U> const & b) noexcept;

  template<class T, class U>
    bool operator==(intrusive_ptr<T> const & a, U * b) noexcept;

  template<class T, class U>
    bool operator!=(intrusive_ptr<T> const & a, U * b) noexcept;

  template<class T, class U>
    bool operator==(T * a, intrusive_ptr<U> const & b) noexcept;

  template<class T, class U>
    bool operator!=(T * a, intrusive_ptr<U> const & b) noexcept;

  template<class T, class U>
    bool operator<(intrusive_ptr<T> const & a, intrusive_ptr<U> const & b) noexcept;

  template<class T> void swap(intrusive_ptr<T> & a, intrusive_ptr<T> & b) noexcept;

  template<class T> T * get_pointer(intrusive_ptr<T> const & p) noexcept;

  template<class T, class U>
    intrusive_ptr<T> static_pointer_cast(intrusive_ptr<U> const & r) noexcept;

  template<class T, class U>
    intrusive_ptr<T> const_pointer_cast(intrusive_ptr<U> const & r) noexcept;

  template<class T, class U>
    intrusive_ptr<T> dynamic_pointer_cast(intrusive_ptr<U> const & r) noexcept;

  template<class E, class T, class Y>
    std::basic_ostream<E, T> & operator<< (std::basic_ostream<E, T> & os,
      intrusive_ptr<Y> const & p);
}
```

## Members

### element_type

```
typedef T element_type;
```

Provides the type of the template parameter T.

### constructors

```
intrusive_ptr() noexcept;
```

[none]
* {blank}
+
Postconditions:: `get() == 0`.

```
intrusive_ptr(T * p, bool add_ref = true);
```

[none]
* {blank}
+
Effects:: `if(p != 0 && add_ref) intrusive_ptr_add_ref(p);`.
Postconditions:: `get() == p`.

```
intrusive_ptr(intrusive_ptr const & r);
```
```
template<class Y> intrusive_ptr(intrusive_ptr<Y> const & r);
```

[none]
* {blank}
+
Effects:: `T * p = r.get(); if(p != 0) intrusive_ptr_add_ref(p);`.
Postconditions:: `get() == r.get()`.

### destructor

```
~intrusive_ptr();
```

[none]
* {blank}
+
Effects:: `if(get() != 0) intrusive_ptr_release(get());`.

### assignment

```
intrusive_ptr & operator=(intrusive_ptr const & r);
```
```
template<class Y> intrusive_ptr & operator=(intrusive_ptr<Y> const & r);
```
```
intrusive_ptr & operator=(T * r);
```

[none]
* {blank}
+
Effects:: Equivalent to `intrusive_ptr(r).swap(*this)`.
Returns:: `*this`.

### reset

```
void reset();
```

[none]
* {blank}
+
Effects:: Equivalent to `intrusive_ptr().swap(*this)`.

```
void reset(T * r);
```

[none]
* {blank}
+
Effects:: Equivalent to `intrusive_ptr(r).swap(*this)`.

```
void reset(T * r, bool add_ref);
```

[none]
* {blank}
+
Effects:: Equivalent to `intrusive_ptr(r, add_ref).swap(*this)`.

### indirection

```
T & operator*() const noexcept;
```

[none]
* {blank}
+
Requirements:: `get() != 0`.
Returns:: `*get()`.

```
T * operator->() const noexcept;
```

[none]
* {blank}
+
Requirements:: `get() != 0`.
Returns:: `get()`.

### get

```
T * get() const noexcept;
```

[none]
* {blank}
+
Returns:: the stored pointer.

### detach

```
T * detach() noexcept;
```

[none]
* {blank}
+
Returns:: the stored pointer.
Postconditions:: `get() == 0`.

NOTE: The returned pointer has an elevated reference count. This allows conversion of an `intrusive_ptr`
back to a raw pointer, without the performance overhead of acquiring and dropping an extra reference.
It can be viewed as the complement of the non-reference-incrementing constructor.

CAUTION: Using `detach` escapes the safety of automatic reference counting provided by `intrusive_ptr`.
It should by used only where strictly necessary (such as when interfacing to an existing API), and when
the implications are thoroughly understood.

### conversions

```
explicit operator bool () const noexcept;
```

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+
Returns:: `get() != 0`.

NOTE: This conversion operator allows `intrusive_ptr` objects to be used in boolean contexts,
like `if (p && p\->valid()) {}`.

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

### swap

```
void swap(intrusive_ptr & b) noexcept;
```

[none]
* {blank}
+
Effects:: Exchanges the contents of the two smart pointers.

## Free Functions

### comparison

```
template<class T, class U>
  bool operator==(intrusive_ptr<T> const & a, intrusive_ptr<U> const & b) noexcept;
```

[none]
* {blank}
+
Returns:: `a.get() == b.get()`.

```
template<class T, class U>
  bool operator!=(intrusive_ptr<T> const & a, intrusive_ptr<U> const & b) noexcept;
```

[none]
* {blank}
+
Returns:: `a.get() != b.get()`.

```
template<class T, class U>
  bool operator==(intrusive_ptr<T> const & a, U * b) noexcept;
```

[none]
* {blank}
+
Returns:: `a.get() == b`.

```
template<class T, class U>
  bool operator!=(intrusive_ptr<T> const & a, U * b) noexcept;
```

[none]
* {blank}
+
Returns:: `a.get() != b`.

```
template<class T, class U>
  bool operator==(T * a, intrusive_ptr<U> const & b) noexcept;
```

[none]
* {blank}
+
Returns:: `a == b.get()`.

```
template<class T, class U>
  bool operator!=(T * a, intrusive_ptr<U> const & b) noexcept;
```

[none]
* {blank}
+
Returns:: `a != b.get()`.

```
template<class T, class U>
  bool operator<(intrusive_ptr<T> const & a, intrusive_ptr<U> const & b) noexcept;
```

[none]
* {blank}
+
Returns:: `std::less<T *>()(a.get(), b.get())`.

NOTE: Allows `intrusive_ptr` objects to be used as keys in associative containers.

### swap

```
template<class T> void swap(intrusive_ptr<T> & a, intrusive_ptr<T> & b) noexcept;
```

[none]
* {blank}
+
Effects:: Equivalent to `a.swap(b)`.

### get_pointer

```
template<class T> T * get_pointer(intrusive_ptr<T> const & p) noexcept;
```

[none]
* {blank}
+
Returns:: `p.get()`.

NOTE: Provided as an aid to generic programming. Used by `mem_fn`.

### static_pointer_cast

```
template<class T, class U>
  intrusive_ptr<T> static_pointer_cast(intrusive_ptr<U> const & r) noexcept;
```

[none]
* {blank}
+
Returns:: `intrusive_ptr<T>(static_cast<T*>(r.get()))`.

### const_pointer_cast

```
template<class T, class U>
  intrusive_ptr<T> const_pointer_cast(intrusive_ptr<U> const & r) noexcept;
```

[none]
* {blank}
+
Returns:: `intrusive_ptr<T>(const_cast<T*>(r.get()))`.

### dynamic_pointer_cast

```
template<class T, class U>
  intrusive_ptr<T> dynamic_pointer_cast(intrusive_ptr<U> const & r) noexcept;
```

[none]
* {blank}
+
Returns:: `intrusive_ptr<T>(dynamic_cast<T*>(r.get()))`.

### operator<<

```
template<class E, class T, class Y>
  std::basic_ostream<E, T> & operator<< (std::basic_ostream<E, T> & os,
    intrusive_ptr<Y> const & p);
```

[none]
* {blank}
+
Effects:: `os << p.get();`.
Returns:: `os`.