doc/smart_ptr/local_shared_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
////

[#local_shared_ptr]
# local_shared_ptr: Shared Ownership within a Single Thread
:toc:
:toc-title:
:idprefix: local_shared_ptr_

## Description

`local_shared_ptr` is nearly identical to `shared_ptr`, with the only difference of note being that its reference count is
updated with non-atomic operations. As such, a `local_shared_ptr` and all its copies must reside in (be local to) a single
thread (hence the name.)

`local_shared_ptr` can be converted to `shared_ptr` and vice versa. Creating a `local_shared_ptr` from a `shared_ptr` creates
a new local reference count; this means that two `local_shared_ptr` instances, both created from the same `shared_ptr`, refer
to the same object but don't share the same count, and as such, can safely be used by two different threads.

.Two local_shared_ptr instances created from a shared_ptr
```
shared_ptr<X> p1( new X );

local_shared_ptr<X> p2( p1 ); // p2.local_use_count() == 1
local_shared_ptr<X> p3( p1 ); // p3.local_use_count() also 1
```

Creating the second `local_shared_ptr` from the first one, however, does lead to the two sharing the same count:

.A local_shared_ptr created from another local_shared_ptr
```
shared_ptr<X> p1( new X );

local_shared_ptr<X> p2( p1 ); // p2.local_use_count() == 1
local_shared_ptr<X> p3( p2 ); // p3.local_use_count() == 2
```

Two `shared_ptr` instances created from the same `local_shared_ptr` do share ownership:

.Two shared_ptr instances created from a local_shared_ptr
```
local_shared_ptr<X> p1( new X );

shared_ptr<X> p2( p1 ); // p2.use_count() == 2
shared_ptr<X> p3( p1 ); // p3.use_count() == 3
```

Here `p2.use_count()` is 2, because `p1` holds a reference, too.

One can think of `local_shared_ptr<T>` as `shared_ptr<shared_ptr<T>>`, with the outer `shared_ptr` using non-atomic operations for
its count. Converting from `local_shared_ptr` to `shared_ptr` gives you a copy of the inner `shared_ptr`; converting from `shared_ptr`
wraps it into an outer `shared_ptr` with a non-atomic use count (conceptually speaking) and returns the result.

## Synopsis

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

```
namespace boost {

  template<class T> class local_shared_ptr {
  public:

    typedef /*see below*/ element_type;

    // constructors

    constexpr local_shared_ptr() noexcept;
    constexpr local_shared_ptr(std::nullptr_t) noexcept;

    template<class Y> explicit local_shared_ptr(Y * p);

    template<class Y, class D> local_shared_ptr(Y * p, D d);
    template<class D> local_shared_ptr(std::nullptr_t p, D d);

    template<class Y, class D, class A> local_shared_ptr(Y * p, D d, A a);
    template<class D, class A> local_shared_ptr(std::nullptr_t p, D d, A a);

    local_shared_ptr(local_shared_ptr const & r) noexcept;
    template<class Y> local_shared_ptr(local_shared_ptr<Y> const & r) noexcept;

    local_shared_ptr(local_shared_ptr && r) noexcept;
    template<class Y> local_shared_ptr(local_shared_ptr<Y> && r) noexcept;

    template<class Y> local_shared_ptr( shared_ptr<Y> const & r );
    template<class Y> local_shared_ptr( shared_ptr<Y> && r );

    template<class Y> local_shared_ptr(local_shared_ptr<Y> const & r, element_type * p) noexcept;
    template<class Y> local_shared_ptr(local_shared_ptr<Y> && r, element_type * p) noexcept;

    template<class Y, class D> local_shared_ptr(std::unique_ptr<Y, D> && r);
    
    // destructor

    ~local_shared_ptr() noexcept;

    // assignment

    local_shared_ptr & operator=(local_shared_ptr const & r) noexcept;
    template<class Y> local_shared_ptr & operator=(local_shared_ptr<Y> const & r) noexcept;

    local_shared_ptr & operator=(local_shared_ptr const && r) noexcept;
    template<class Y> local_shared_ptr & operator=(local_shared_ptr<Y> const && r) noexcept;

    template<class Y, class D> local_shared_ptr & operator=(std::unique_ptr<Y, D> && r);

    local_shared_ptr & operator=(std::nullptr_t) noexcept;

    // reset
	
    void reset() noexcept;

    template<class Y> void reset(Y * p);
    template<class Y, class D> void reset(Y * p, D d);
    template<class Y, class D, class A> void reset(Y * p, D d, A a);

    template<class Y> void reset(local_shared_ptr<Y> const & r, element_type * p) noexcept;
    template<class Y> void reset(local_shared_ptr<Y> && r, element_type * p) noexcept;

    // accessors
    
    T & operator*() const noexcept; // only valid when T is not an array type
    T * operator->() const noexcept; // only valid when T is not an array type

    // only valid when T is an array type
    element_type & operator[](std::ptrdiff_t i) const noexcept;

    element_type * get() const noexcept;

    long local_use_count() const noexcept;

    // conversions

    explicit operator bool() const noexcept;

    template<class Y> operator shared_ptr<Y>() const noexcept;
    template<class Y> operator weak_ptr<Y>() const noexcept;

    // swap
    
    void swap(local_shared_ptr & b) noexcept;

    // owner_before
    
    template<class Y> bool owner_before(local_shared_ptr<Y> const & rhs) const noexcept;
  };

  // comparisons
  
  template<class T, class U>
    bool operator==(local_shared_ptr<T> const & a, local_shared_ptr<U> const & b) noexcept;
  template<class T, class U>
    bool operator==(local_shared_ptr<T> const & a, shared_ptr<U> const & b) noexcept;
  template<class T, class U>
    bool operator==(shared_ptr<T> const & a, local_shared_ptr<U> const & b) noexcept;

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

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

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

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

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

  // get_pointer
  
  template<class T>
    typename local_shared_ptr<T>::element_type *
      get_pointer(local_shared_ptr<T> const & p) noexcept;

  // casts
  
  template<class T, class U>
    local_shared_ptr<T> static_pointer_cast(local_shared_ptr<U> const & r) noexcept;

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

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

  template<class T, class U>
    local_shared_ptr<T> reinterpret_pointer_cast(local_shared_ptr<U> const & r) noexcept;

  // stream I/O

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

  // get_deleter
  
  template<class D, class T> D * get_deleter(local_shared_ptr<T> const & p) noexcept;
}
```

## Members

### element_type
```
typedef ... element_type;
```
`element_type` is `T` when `T` is not an array type, and `U` when `T` is `U[]` or `U[N]`.

### default constructor
```
constexpr local_shared_ptr() noexcept;
```
```
constexpr local_shared_ptr(std::nullptr_t) noexcept;
```
[none]
* {blank}
+
Effects:: Constructs an empty `local_shared_ptr`.
Postconditions:: `local_use_count() == 0 && get() == 0`.

### pointer constructor
```
template<class Y> explicit local_shared_ptr(Y * p);
```
[none]
* {blank}
+
Effects:: Constructs a `local_shared_ptr` that owns `shared_ptr<T>( p )`.

Postconditions:: `local_use_count() == 1 && get() == p`.

Throws:: `std::bad_alloc`, or an implementation-defined exception when a resource other than memory could not be obtained.

### constructors taking a deleter
```
template<class Y, class D> local_shared_ptr(Y * p, D d);
```
```
template<class D> local_shared_ptr(std::nullptr_t p, D d);
```
[none]
* {blank}
+
Effects:: Constructs a `local_shared_ptr` that owns `shared_ptr<T>( p, d )`.

Postconditions:: `local_use_count() == 1 && get() == p`.

Throws:: `std::bad_alloc`, or an implementation-defined exception when a resource other than memory could not be obtained.

```
template<class Y, class D, class A> local_shared_ptr(Y * p, D d, A a);
```
```
template<class D, class A> local_shared_ptr(std::nullptr_t p, D d, A a);
```
[none]
* {blank}
+
Effects:: Constructs a `local_shared_ptr` that owns `shared_ptr<T>( p, d, a )`.

Postconditions:: `local_use_count() == 1 && get() == p`.

Throws:: `std::bad_alloc`, or an implementation-defined exception when a resource other than memory could not be obtained.

### copy and converting constructors
```
local_shared_ptr(local_shared_ptr const & r) noexcept;
```
```
template<class Y> local_shared_ptr(local_shared_ptr<Y> const & r) noexcept;
```
[none]
* {blank}
+
Requires:: `Y*` should be convertible to `T*`.

Effects:: If `r` is empty, constructs an empty `local_shared_ptr`; otherwise, constructs a `local_shared_ptr` that shares ownership with `r`.

Postconditions:: `get() == r.get() && local_use_count() == r.local_use_count()`.

### move constructors
```
local_shared_ptr(local_shared_ptr && r) noexcept;
```
```
template<class Y> local_shared_ptr(local_shared_ptr<Y> && r) noexcept;
```
[none]
* {blank}
+
Requires:: `Y*` should be convertible to `T*`.

Effects:: Move-constructs a `local_shared_ptr` from `r`.

Postconditions:: `*this` contains the old value of `r`. `r` is empty and `r.get() == 0`.

### shared_ptr constructor
```
template<class Y> local_shared_ptr( shared_ptr<Y> const & r );
```
```
template<class Y> local_shared_ptr( shared_ptr<Y> && r );
```
[none]
* {blank}
+
Effects:: Constructs a `local_shared_ptr` that owns `r`.

Postconditions:: `local_use_count() == 1`. `get()` returns the old value of `r.get()`.

Throws:: `std::bad_alloc`, or an implementation-defined exception when a resource other than memory could not be obtained.

### aliasing constructor
```
template<class Y> local_shared_ptr(local_shared_ptr<Y> const & r, element_type * p) noexcept;
```
[none]
* {blank}
+
Effects:: constructs a `local_shared_ptr` that shares ownership with `r` and stores `p`.

Postconditions:: `get() == p && local_use_count() == r.local_use_count()`.

### aliasing move constructor
```
template<class Y> local_shared_ptr(local_shared_ptr<Y> && r, element_type * p) noexcept;
```
[none]
* {blank}
+
Effects:: Move-constructs a `local_shared_ptr` from `r`, while storing `p` instead.

Postconditions:: `get() == p` and `local_use_count()` equals the old count of `r`. `r` is empty and `r.get() == 0`.

### unique_ptr constructor
```
template<class Y, class D> local_shared_ptr(std::unique_ptr<Y, D> && r);
```
[none]
* {blank}
+
Requires:: `Y*` should be convertible to `T*`.

Effects::
- When `r.get() == 0`, equivalent to `local_shared_ptr()`;
- Otherwise, constructs a `local_shared_ptr` that owns `shared_ptr<T>( std::move(r) )`.

Throws:: `std::bad_alloc`, or an implementation-defined exception when a resource other than memory could not be obtained.

Exception safety:: If an exception is thrown, the constructor has no effect.

### destructor
```
~local_shared_ptr() noexcept;
```
[none]
* {blank}
+
Effects::
- If `*this` is empty, or shares ownership with another `local_shared_ptr` instance (`local_use_count() > 1`), there are no side effects.
- Otherwise, destroys the owned `shared_ptr`.

### assignment
```
local_shared_ptr & operator=(local_shared_ptr const & r) noexcept;
```
```
template<class Y> local_shared_ptr & operator=(local_shared_ptr<Y> const & r) noexcept;
```
[none]
* {blank}
+
Effects:: Equivalent to `local_shared_ptr(r).swap(*this)`.
Returns:: `*this`.

```
local_shared_ptr & operator=(local_shared_ptr && r) noexcept;
```
```
template<class Y> local_shared_ptr & operator=(local_shared_ptr<Y> && r) noexcept;
```
```
template<class Y, class D> local_shared_ptr & operator=(std::unique_ptr<Y, D> && r);
```
[none]
* {blank}
+
Effects:: Equivalent to `local_shared_ptr(std::move(r)).swap(*this)`.
Returns:: `*this`.

```
local_shared_ptr & operator=(std::nullptr_t) noexcept;
```
[none]
* {blank}
+
Effects:: Equivalent to `local_shared_ptr().swap(*this)`.
Returns:: `*this`.

### reset
```
void reset() noexcept;
```
[none]
* {blank}
+
Effects:: Equivalent to `local_shared_ptr().swap(*this)`.

```
template<class Y> void reset(Y * p);
```
[none]
* {blank}
+
Effects:: Equivalent to `local_shared_ptr(p).swap(*this)`.

```
template<class Y, class D> void reset(Y * p, D d);
```
[none]
* {blank}
+
Effects:: Equivalent to `local_shared_ptr(p, d).swap(*this)`.

```
template<class Y, class D, class A> void reset(Y * p, D d, A a);
```
[none]
* {blank}
+
Effects:: Equivalent to `local_shared_ptr(p, d, a).swap(*this)`.

```
template<class Y> void reset(local_shared_ptr<Y> const & r, element_type * p) noexcept;
```
[none]
* {blank}
+
Effects:: Equivalent to `local_shared_ptr(r, p).swap(*this)`.

```
template<class Y> void reset(local_shared_ptr<Y> && r, element_type * p) noexcept;
```
[none]
* {blank}
+
Effects:: Equivalent to `local_shared_ptr(std::move(r), p).swap(*this)`.

### indirection
```
T & operator*() const noexcept;
```
[none]
* {blank}
+
Requires:: `T` should not be an array type.
Returns:: `*get()`.

```
T * operator->() const noexcept;
```
[none]
* {blank}
+
Requires:: `T` should not be an array type.
Returns:: `get()`.

```
element_type & operator[](std::ptrdiff_t i) const noexcept;
```
[none]
* {blank}
+
Requires:: `T` should be an array type. The stored pointer must not be 0. `i >= 0`. If `T` is `U[N]`, `i < N`.
Returns:: `get()[i]`.

### get

```
element_type * get() const noexcept;
```
[none]
* {blank}
+
Returns:: The stored pointer.

### local_use_count
```
long local_use_count() const noexcept;
```
[none]
* {blank}
+
Returns:: The number of `local_shared_ptr` objects, `*this` included, that share ownership with `*this`, or 0 when `*this` is empty.

### conversions
```
explicit operator bool() const noexcept;
```
[none]
* {blank}
+
Returns:: `get() != 0`.

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

```
template<class Y> operator shared_ptr<Y>() const noexcept;
```
```
template<class Y> operator weak_ptr<Y>() const noexcept;
```
[none]
* {blank}
+
Requires:: `T*` should be convertible to `Y*`.
Returns:: a copy of the owned `shared_ptr`.

### swap
```
void swap(local_shared_ptr & b) noexcept;
```
[none]
* {blank}
+
Effects:: Exchanges the contents of the two smart pointers.

### owner_before
```
template<class Y> bool owner_before(local_shared_ptr<Y> const & rhs) const noexcept;
```
[none]
* {blank}
+
Effects:: See the description of `operator<`.

## Free Functions

### comparison
```
template<class T, class U>
  bool operator==(local_shared_ptr<T> const & a, local_shared_ptr<U> const & b) noexcept;
```
```
template<class T, class U>
  bool operator==(local_shared_ptr<T> const & a, shared_ptr<U> const & b) noexcept;
```
```
template<class T, class U>
  bool operator==(shared_ptr<T> const & a, local_shared_ptr<U> const & b) noexcept;
```
[none]
* {blank}
+
Returns:: `a.get() == b.get()`.

```
template<class T, class U>
  bool operator!=(local_shared_ptr<T> const & a, local_shared_ptr<U> const & b) noexcept;
```
```
template<class T, class U>
  bool operator!=(local_shared_ptr<T> const & a, shared_ptr<U> const & b) noexcept;
```
```
template<class T, class U>
  bool operator!=(shared_ptr<T> const & a, local_shared_ptr<U> const & b) noexcept;
```
[none]
* {blank}
+
Returns:: `a.get() != b.get()`.

```
template<class T> bool operator==(local_shared_ptr<T> const & p, std::nullptr_t) noexcept;
```
```
template<class T> bool operator==(std::nullptr_t, local_shared_ptr<T> const & p) noexcept;
```
[none]
* {blank}
+
Returns:: `p.get() == 0`.

```
template<class T> bool operator!=(local_shared_ptr<T> const & p, std::nullptr_t) noexcept;
```
```
template<class T> bool operator!=(std::nullptr_t, local_shared_ptr<T> const & p) noexcept;
```
[none]
* {blank}
+
Returns:: `p.get() != 0`.

```
template<class T, class U>
  bool operator<(local_shared_ptr<T> const & a, local_shared_ptr<U> const & b) noexcept;
```
[none]
* {blank}
+
Returns:: An unspecified value such that
  - `operator<` is a strict weak ordering as described in section [lib.alg.sorting] of the {cpp} standard;
  - under the equivalence relation defined by `operator<`, `!(a < b) && !(b < a)`, two `local_shared_ptr` instances
    are equivalent if and only if they share ownership or are both empty.

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

NOTE: The rest of the comparison operators are omitted by design.

### swap
```
template<class T> void swap(local_shared_ptr<T> & a, local_shared_ptr<T> & b) noexcept;
```
[none]
* {blank}
+
Effects:: Equivalent to `a.swap(b)`.

### get_pointer
```
template<class T>
  typename local_shared_ptr<T>::element_type *
    get_pointer(local_shared_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>
  local_shared_ptr<T> static_pointer_cast(local_shared_ptr<U> const & r) noexcept;
```
[none]
* {blank}
+
Requires:: The expression `static_cast<T*>( (U*)0 )` must be well-formed.
Returns:: `local_shared_ptr<T>( r, static_cast<typename local_shared_ptr<T>::element_type*>(r.get()) )`.

CAUTION: The seemingly equivalent expression `local_shared_ptr<T>(static_cast<T*>(r.get()))` will eventually
result in undefined behavior, attempting to delete the same object twice.

### const_pointer_cast
```
template<class T, class U>
  local_shared_ptr<T> const_pointer_cast(local_shared_ptr<U> const & r) noexcept;
```
[none]
* {blank}
+
Requires:: The expression `const_cast<T*>( (U*)0 )` must be well-formed.
Returns:: `local_shared_ptr<T>( r, const_cast<typename local_shared_ptr<T>::element_type*>(r.get()) )`.

### dynamic_pointer_cast
```
template<class T, class U>
    local_shared_ptr<T> dynamic_pointer_cast(local_shared_ptr<U> const & r) noexcept;
```
[none]
* {blank}
+
Requires:: The expression `dynamic_cast<T*>( (U*)0 )` must be well-formed.
Returns::
  - When `dynamic_cast<typename local_shared_ptr<T>::element_type*>(r.get())` returns a nonzero value `p`, `local_shared_ptr<T>(r, p)`;
  - Otherwise, `local_shared_ptr<T>()`.

### reinterpret_pointer_cast
```
template<class T, class U>
  local_shared_ptr<T> reinterpret_pointer_cast(local_shared_ptr<U> const & r) noexcept;
```
[none]
* {blank}
+
Requires:: The expression `reinterpret_cast<T*>( (U*)0 )` must be well-formed.
Returns:: `local_shared_ptr<T>( r, reinterpret_cast<typename local_shared_ptr<T>::element_type*>(r.get()) )`.

### operator<<
```
template<class E, class T, class Y>
  std::basic_ostream<E, T> &
    operator<< (std::basic_ostream<E, T> & os, local_shared_ptr<Y> const & p);
```
[none]
* {blank}
+
Effects:: `os << p.get();`.
Returns:: `os`.

### get_deleter
```
template<class D, class T>
  D * get_deleter(local_shared_ptr<T> const & p) noexcept;
```
[none]
* {blank}
+
Returns:: If `*this` owns a `shared_ptr` instance `p`, `get_deleter<D>( p )`, otherwise 0.