////
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
////

[#atomic_shared_ptr]
# atomic_shared_ptr
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:toc-title:
:idprefix: atomic_shared_ptr_

## Description

The class template `atomic_shared_ptr<T>` implements the interface of `std::atomic`
for a contained value of type `shared_ptr<T>`. Concurrent access to `atomic_shared_ptr`
is not a data race.

## Synopsis

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

```
namespace boost {

  template<class T> class atomic_shared_ptr {
  private:

    shared_ptr<T> p_; // exposition only

    atomic_shared_ptr(const atomic_shared_ptr&) = delete;
    atomic_shared_ptr& operator=(const atomic_shared_ptr&) = delete;

  public:

    constexpr atomic_shared_ptr() noexcept;
    atomic_shared_ptr( shared_ptr<T> p ) noexcept;

    atomic_shared_ptr& operator=( shared_ptr<T> r ) noexcept;

    bool is_lock_free() const noexcept;

    shared_ptr<T> load( int = 0 ) const noexcept;
    operator shared_ptr<T>() const noexcept;

    void store( shared_ptr<T> r, int = 0 ) noexcept;

    shared_ptr<T> exchange( shared_ptr<T> r, int = 0 ) noexcept;

    bool compare_exchange_weak( shared_ptr<T>& v, const shared_ptr<T>& w, int, int ) noexcept;
    bool compare_exchange_weak( shared_ptr<T>& v, const shared_ptr<T>& w, int = 0 ) noexcept;
    bool compare_exchange_strong( shared_ptr<T>& v, const shared_ptr<T>& w, int, int ) noexcept;
    bool compare_exchange_strong( shared_ptr<T>& v, const shared_ptr<T>& w, int = 0 ) noexcept;

    bool compare_exchange_weak( shared_ptr<T>& v, shared_ptr<T>&& w, int, int ) noexcept;
    bool compare_exchange_weak( shared_ptr<T>& v, shared_ptr<T>&& w, int = 0 ) noexcept;
    bool compare_exchange_strong( shared_ptr<T>& v, shared_ptr<T>&& w, int, int ) noexcept;
    bool compare_exchange_strong( shared_ptr<T>& v, shared_ptr<T>&& w, int = 0 ) noexcept;
  };
}
```

## Members

```
constexpr atomic_shared_ptr() noexcept;
```
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+
Effects:: Default-initializes `p_`.

```
atomic_shared_ptr( shared_ptr<T> p ) noexcept;
```
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+
Effects:: Initializes `p_` to `p`.

```
atomic_shared_ptr& operator=( shared_ptr<T> r ) noexcept;
```
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+
Effects:: `p_.swap(r)`.
Returns:: `*this`.

```
bool is_lock_free() const noexcept;
```
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+
Returns:: `false`.

NOTE: This implementation is not lock-free.

```
shared_ptr<T> load( int = 0 ) const noexcept;
```
```
operator shared_ptr<T>() const noexcept;
```
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+
Returns:: `p_`.

NOTE: The `int` argument is intended to be of type `memory_order`, but is ignored.
  This implementation is lock-based and therefore always sequentially consistent.

```
void store( shared_ptr<T> r, int = 0 ) noexcept;
```
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+
Effects:: `p_.swap(r)`.

```
shared_ptr<T> exchange( shared_ptr<T> r, int = 0 ) noexcept;
```
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+
Effects:: `p_.swap(r)`.
Returns:: The old value of `p_`.

```
bool compare_exchange_weak( shared_ptr<T>& v, const shared_ptr<T>& w, int, int ) noexcept;
```
```
bool compare_exchange_weak( shared_ptr<T>& v, const shared_ptr<T>& w, int = 0 ) noexcept;
```
```
bool compare_exchange_strong( shared_ptr<T>& v, const shared_ptr<T>& w, int, int ) noexcept;
```
```
bool compare_exchange_strong( shared_ptr<T>& v, const shared_ptr<T>& w, int = 0 ) noexcept;
```
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* {blank}
+
Effects:: If `p_` is equivalent to `v`, assigns `w` to `p_`, otherwise assigns `p_` to `v`.
Returns:: `true` if `p_` was equivalent to `v`, `false` otherwise.
Remarks:: Two `shared_ptr` instances are equivalent if they store the same pointer value and _share ownership_.

```
bool compare_exchange_weak( shared_ptr<T>& v, shared_ptr<T>&& w, int, int ) noexcept;
```
```
bool compare_exchange_weak( shared_ptr<T>& v, shared_ptr<T>&& w, int = 0 ) noexcept;
```
```
bool compare_exchange_strong( shared_ptr<T>& v, shared_ptr<T>&& w, int, int ) noexcept;
```
```
bool compare_exchange_strong( shared_ptr<T>& v, shared_ptr<T>&& w, int = 0 ) noexcept;
```
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+
Effects:: If `p_` is equivalent to `v`, assigns `std::move(w)` to `p_`, otherwise assigns `p_` to `v`.
Returns:: `true` if `p_` was equivalent to `v`, `false` otherwise.
Remarks:: The old value of `w` is not preserved in either case.