[saferecl.hp.holder]

# 32 Concurrency support library [[thread]](./#thread)

## 32.11 Safe reclamation [[saferecl]](saferecl#hp.holder)

### 32.11.3 Hazard pointers [[saferecl.hp]](saferecl.hp#holder)

#### 32.11.3.4 Class hazard_pointer [saferecl.hp.holder]

#### [32.11.3.4.1](#general) General [[saferecl.hp.holder.general]](saferecl.hp.holder.general)

namespace std {class hazard_pointer {public: hazard_pointer() noexcept;
 hazard_pointer(hazard_pointer&&) noexcept;
 hazard_pointer& operator=(hazard_pointer&&) noexcept; ~hazard_pointer(); bool empty() const noexcept; template<class T> T* protect(const atomic<T*>& src) noexcept; template<class T> bool try_protect(T*& ptr, const atomic<T*>& src) noexcept; template<class T> void reset_protection(const T* ptr) noexcept; void reset_protection(nullptr_t = nullptr) noexcept; void swap(hazard_pointer&) noexcept; };}

[1](#general-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L13488)

An object of type hazard_pointer is either empty or[*owns*](#def:owning,hazard_pointer "32.11.3.4.1 General [saferecl.hp.holder.general]") a hazard pointer[.](#general-1.sentence-1)

Each hazard pointer is owned by
exactly one object of type hazard_pointer[.](#general-1.sentence-2)

[*Note [1](#general-note-1)*:

An empty hazard_pointer object is different from
a hazard_pointer object
that owns an unassociated hazard pointer[.](#general-1.sentence-3)

An empty hazard_pointer object does not own any hazard pointers[.](#general-1.sentence-4)

— *end note*]

#### [32.11.3.4.2](#ctor) Constructors, destructor, and assignment [[saferecl.hp.holder.ctor]](saferecl.hp.holder.ctor)

[🔗](#lib:hazard_pointer,constructor)

`hazard_pointer() noexcept;
`

[1](#ctor-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L13508)

*Postconditions*: *this is empty[.](#ctor-1.sentence-1)

[🔗](#lib:hazard_pointer,constructor_)

`hazard_pointer(hazard_pointer&& other) noexcept;
`

[2](#ctor-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L13519)

*Postconditions*: If other is empty, *this is empty[.](#ctor-2.sentence-1)

Otherwise,*this owns the hazard pointer originally owned by other;other is empty[.](#ctor-2.sentence-2)

[🔗](#lib:hazard_pointer,destructor)

`~hazard_pointer();
`

[3](#ctor-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L13533)

*Effects*: If *this is not empty,
destroys the hazard pointer owned by *this,
thereby ending its current protection epoch[.](#ctor-3.sentence-1)

[🔗](#lib:operator=,hazard_pointer)

`hazard_pointer& operator=(hazard_pointer&& other) noexcept;
`

[4](#ctor-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L13546)

*Effects*: If this == &other is true, no effect[.](#ctor-4.sentence-1)

Otherwise, if *this is not empty,
destroys the hazard pointer owned by *this,
thereby ending its current protection epoch[.](#ctor-4.sentence-2)

[5](#ctor-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L13553)

*Postconditions*: If other was empty, *this is empty[.](#ctor-5.sentence-1)

Otherwise, *this owns the hazard pointer originally
owned by other[.](#ctor-5.sentence-2)

If this != &other is true, other is empty[.](#ctor-5.sentence-3)

[6](#ctor-6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L13560)

*Returns*: *this[.](#ctor-6.sentence-1)

#### [32.11.3.4.3](#mem) Member functions [[saferecl.hp.holder.mem]](saferecl.hp.holder.mem)

[🔗](#lib:empty,hazard_pointer)

`bool empty() const noexcept;
`

[1](#mem-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L13573)

*Returns*: true if and only if *this is empty[.](#mem-1.sentence-1)

[🔗](#lib:protect,hazard_pointer)

`template<class T> T* protect(const atomic<T*>& src) noexcept;
`

[2](#mem-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L13584)

*Effects*: Equivalent to:T* ptr = src.load(memory_order::relaxed);while (!try_protect(ptr, src)) {}return ptr;

[🔗](#lib:try_protect,hazard_pointer)

`template<class T> bool try_protect(T*& ptr, const atomic<T*>& src) noexcept;
`

[3](#mem-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L13600)

*Mandates*: T is a hazard-protectable type[.](#mem-3.sentence-1)

[4](#mem-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L13604)

*Preconditions*: *this is not empty[.](#mem-4.sentence-1)

[5](#mem-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L13608)

*Effects*: Performs the following steps in order:

- [(5.1)](#mem-5.1)

  Initializes a variable old of type T* with the value of ptr[.](#mem-5.1.sentence-1)

- [(5.2)](#mem-5.2)

  Evaluates reset_protection(old)[.](#mem-5.2.sentence-1)

- [(5.3)](#mem-5.3)

  Assigns the value of src.load(memory_order​::​acquire) to ptr[.](#mem-5.3.sentence-1)

- [(5.4)](#mem-5.4)

  If old == ptr is false,
evaluates reset_protection()[.](#mem-5.4.sentence-1)

[6](#mem-6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L13619)

*Returns*: old == ptr[.](#mem-6.sentence-1)

[🔗](#lib:reset_protection,hazard_pointer)

`template<class T> void reset_protection(const T* ptr) noexcept;
`

[7](#mem-7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L13630)

*Mandates*: T is a hazard-protectable type[.](#mem-7.sentence-1)

[8](#mem-8)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L13634)

*Preconditions*: *this is not empty[.](#mem-8.sentence-1)

[9](#mem-9)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L13638)

*Effects*: If ptr is a null pointer value, invokes reset_protection()[.](#mem-9.sentence-1)

Otherwise,
associates the hazard pointer owned by *this with *ptr,
thereby ending the current protection epoch[.](#mem-9.sentence-2)

[10](#mem-10)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L13645)

*Complexity*: Constant[.](#mem-10.sentence-1)

[🔗](#lib:reset_protection,hazard_pointer_)

`void reset_protection(nullptr_t = nullptr) noexcept;
`

[11](#mem-11)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L13656)

*Preconditions*: *this is not empty[.](#mem-11.sentence-1)

[12](#mem-12)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L13660)

*Postconditions*: The hazard pointer owned by *this is unassociated[.](#mem-12.sentence-1)

[13](#mem-13)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L13664)

*Complexity*: Constant[.](#mem-13.sentence-1)

[🔗](#lib:swap,hazard_pointer)

`void swap(hazard_pointer& other) noexcept;
`

[14](#mem-14)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L13675)

*Effects*: Swaps the hazard pointer ownership of this object with that of other[.](#mem-14.sentence-1)

[*Note [1](#mem-note-1)*:

The owned hazard pointers, if any, remain unchanged during the swap and
continue to be associated with the respective objects
that they were protecting before the swap, if any[.](#mem-14.sentence-2)

No protection epochs are ended or initiated[.](#mem-14.sentence-3)

— *end note*]

[15](#mem-15)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L13685)

*Complexity*: Constant[.](#mem-15.sentence-1)

#### [32.11.3.4.4](#nonmem) Non-member functions [[saferecl.hp.holder.nonmem]](saferecl.hp.holder.nonmem)

[🔗](#lib:make_hazard_pointer)

`hazard_pointer make_hazard_pointer();
`

[1](#nonmem-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L13698)

*Effects*: Constructs a hazard pointer[.](#nonmem-1.sentence-1)

[2](#nonmem-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L13702)

*Returns*: A hazard_pointer object that owns the newly-constructed hazard pointer[.](#nonmem-2.sentence-1)

[3](#nonmem-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L13706)

*Throws*: May throw bad_alloc if memory for the hazard pointer could not be allocated[.](#nonmem-3.sentence-1)

[🔗](#lib:swap,hazard_pointer_)

`void swap(hazard_pointer& a, hazard_pointer& b) noexcept;
`

[4](#nonmem-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L13718)

*Effects*: Equivalent to a.swap(b)[.](#nonmem-4.sentence-1)