cds  2.2.0
cds::intrusive::SkipListSet< GC, T, Traits > Class Template Reference

Lock-free skip-list set. More...

#include <cds/intrusive/impl/skip_list.h>

Inheritance diagram for cds::intrusive::SkipListSet< GC, T, Traits >:
cds::container::SkipListSet< GC, T, Traits >

Public Types

typedef GC gc
 Garbage collector.
 
typedef T value_type
 type of value stored in the skip-list
 
typedef Traits traits
 Traits template parameter.
 
typedef traits::hook hook
 hook type
 
typedef hook::node_type node_type
 node type
 
typedef implementation_defined key_comparator
 key comparison functor based on opt::compare and opt::less option setter.
 
typedef traits::disposer disposer
 item disposer
 
typedef get_node_traits< value_type, node_type, hook >::type node_traits
 node traits
 
typedef traits::item_counter item_counter
 Item counting policy.
 
typedef traits::memory_model memory_model
 Memory ordering, see cds::opt::memory_model option.
 
typedef traits::random_level_generator random_level_generator
 random level generator
 
typedef traits::allocator allocator_type
 allocator for maintaining array of next pointers of the node
 
typedef traits::back_off back_off
 Back-off strategy.
 
typedef traits::stat stat
 internal statistics type
 
typedef gc::template guarded_ptr< value_typeguarded_ptr
 Guarded pointer.
 

Public Member Functions

 SkipListSet ()
 Default constructor. More...
 
 ~SkipListSet ()
 Clears and destructs the skip-list.
 
bool insert (value_type &val)
 Inserts new node. More...
 
template<typename Func >
bool insert (value_type &val, Func f)
 Inserts new node. More...
 
template<typename Func >
std::pair< bool, bool > update (value_type &val, Func func, bool bInsert=true)
 Updates the node. More...
 
bool unlink (value_type &val)
 Unlinks the item val from the set. More...
 
template<typename Q >
guarded_ptr extract (Q const &key)
 Extracts the item from the set with specified key. More...
 
template<typename Q , typename Less >
guarded_ptr extract_with (Q const &key, Less pred)
 Extracts the item from the set with comparing functor pred. More...
 
guarded_ptr extract_min ()
 Extracts an item with minimal key from the list. More...
 
guarded_ptr extract_max ()
 Extracts an item with maximal key from the list. More...
 
template<typename Q >
bool erase (Q const &key)
 Deletes the item from the set. More...
 
template<typename Q , typename Less >
bool erase_with (Q const &key, Less pred)
 Deletes the item from the set with comparing functor pred. More...
 
template<typename Q , typename Func >
bool erase (Q const &key, Func f)
 Deletes the item from the set. More...
 
template<typename Q , typename Less , typename Func >
bool erase_with (Q const &key, Less pred, Func f)
 Deletes the item from the set with comparing functor pred. More...
 
template<typename Q , typename Func >
bool find (Q &key, Func f)
 Finds key. More...
 
template<typename Q , typename Less , typename Func >
bool find_with (Q &key, Less pred, Func f)
 Finds the key key with pred predicate for comparing. More...
 
template<typename Q >
bool contains (Q const &key)
 Checks whether the set contains key. More...
 
template<typename Q , typename Less >
bool contains (Q const &key, Less pred)
 Checks whether the set contains key using pred predicate for searching. More...
 
template<typename Q >
guarded_ptr get (Q const &key)
 Finds key and return the item found. More...
 
template<typename Q , typename Less >
guarded_ptr get_with (Q const &key, Less pred)
 Finds key and return the item found. More...
 
size_t size () const
 Returns item count in the set. More...
 
bool empty () const
 Checks if the set is empty.
 
void clear ()
 Clears the set (not atomic) More...
 
stat const & statistics () const
 Returns const reference to internal statistics.
 

Static Public Member Functions

static constexpr unsigned int max_height () noexcept()
 Returns maximum height of skip-list. The max height is a constant for each object and does not exceed 32.
 

Static Public Attributes

static unsigned int const c_nMaxHeight
 Max node height. The actual node height should be in range [0 .. c_nMaxHeight) More...
 
static size_t const c_nHazardPtrCount = c_nMaxHeight * 2 + 3
 Count of hazard pointer required for the skip-list.
 

Protected Types

typedef node_type::atomic_marked_ptr atomic_node_ptr
 Atomic marked node pointer.
 
typedef node_type::marked_ptr marked_node_ptr
 Node marked pointer.
 

Forward iterators (only for debugging purpose)

typedef skip_list::details::iterator< gc, node_traits, back_off, false > iterator
 Iterator type. More...
 
typedef skip_list::details::iterator< gc, node_traits, back_off, true > const_iterator
 Const iterator type.
 
iterator begin ()
 Returns a forward iterator addressing the first element in a set.
 
const_iterator begin () const
 Returns a forward const iterator addressing the first element in a set.
 
const_iterator cbegin () const
 Returns a forward const iterator addressing the first element in a set.
 
iterator end ()
 Returns a forward iterator that addresses the location succeeding the last element in a set.
 
const_iterator end () const
 Returns a forward const iterator that addresses the location succeeding the last element in a set.
 
const_iterator cend () const
 Returns a forward const iterator that addresses the location succeeding the last element in a set.
 

Detailed Description

template<class GC, typename T, typename Traits = skip_list::traits>
class cds::intrusive::SkipListSet< GC, T, Traits >

Lock-free skip-list set.

The implementation of well-known probabilistic data structure called skip-list invented by W.Pugh in his papers:

  • [1989] W.Pugh Skip Lists: A Probabilistic Alternative to Balanced Trees
  • [1990] W.Pugh A Skip List Cookbook

A skip-list is a probabilistic data structure that provides expected logarithmic time search without the need of rebalance. The skip-list is a collection of sorted linked list. Nodes are ordered by key. Each node is linked into a subset of the lists. Each list has a level, ranging from 0 to 32. The bottom-level list contains all the nodes, and each higher-level list is a sublist of the lower-level lists. Each node is created with a random top level (with a random height), and belongs to all lists up to that level. The probability that a node has the height 1 is 1/2. The probability that a node has the height N is 1/2 ** N (more precisely, the distribution depends on an random generator provided, but our generators have this property).

The lock-free variant of skip-list is implemented according to book

  • [2008] M.Herlihy, N.Shavit "The Art of Multiprocessor Programming", chapter 14.4 "A Lock-Free Concurrent Skiplist".

Template arguments:

Warning
The skip-list requires up to 67 hazard pointers that may be critical for some GCs for which the guard count is limited (like as gc::HP). Those GCs should be explicitly initialized with hazard pointer enough:
cds::gc::HP myhp( 67 )
. Otherwise an run-time exception may be raised when you try to create skip-list object.

There are several specializations of SkipListSet for each GC. You should include:

  • <cds/intrusive/skip_list_hp.h> for gc::HP garbage collector
  • <cds/intrusive/skip_list_dhp.h> for gc::DHP garbage collector
  • <cds/intrusive/skip_list_nogc.h> for cds_intrusive_SkipListSet_nogc for append-only set
  • <cds/intrusive/skip_list_rcu.h> for RCU type

Iterators

The class supports a forward iterator (iterator and const_iterator). The iteration is ordered. The iterator object is thread-safe: the element pointed by the iterator object is guarded, so, the element cannot be reclaimed while the iterator object is alive. However, passing an iterator object between threads is dangerous.

Warning
Due to concurrent nature of skip-list set it is not guarantee that you can iterate all elements in the set: any concurrent deletion can exclude the element pointed by the iterator from the set, and your iteration can be terminated before end of the set. Therefore, such iteration is more suitable for debugging purpose only

Remember, each iterator object requires 2 additional hazard pointers, that may be a limited resource for GC like as gc::HP (for gc::DHP the count of guards is unlimited).

The iterator class supports the following minimalistic interface:

struct iterator {
// Default ctor
// Copy ctor
iterator( iterator const& s);
value_type * operator ->() const;
value_type& operator *() const;
// Pre-increment
iterator& operator ++();
// Copy assignment
iterator& operator = (const iterator& src);
bool operator ==(iterator const& i ) const;
bool operator !=(iterator const& i ) const;
};

Note, the iterator object returned by end(), cend() member functions points to nullptr and should not be dereferenced.

How to use

You should incorporate skip_list::node into your struct T and provide appropriate skip_list::traits::hook in your Traits template parameters. Usually, for Traits you define a struct based on skip_list::traits.

Example for gc::HP and base hook:

// Include GC-related skip-list specialization
#include <cds/intrusive/skip_list_hp.h>
// Data stored in skip list
struct my_data: public cds::intrusive::skip_list::node< cds::gc::HP >
{
// key field
std::string strKey;
// other data
// ...
};
// my_data compare functor
struct my_data_cmp {
int operator()( const my_data& d1, const my_data& d2 )
{
return d1.strKey.compare( d2.strKey );
}
int operator()( const my_data& d, const std::string& s )
{
return d.strKey.compare(s);
}
int operator()( const std::string& s, const my_data& d )
{
return s.compare( d.strKey );
}
};
// Declare your traits
struct my_traits: public cds::intrusive::skip_list::traits
{
typedef my_data_cmp compare;
};
// Declare skip-list set type

Equivalent option-based code:

// GC-related specialization
#include <cds/intrusive/skip_list_hp.h>
struct my_data {
// see above
};
struct compare {
// see above
};
// Declare option-based skip-list set
typedef cds::intrusive::SkipListSet< cds::gc::HP
,my_data
>::type
> option_based_set;

Member Typedef Documentation

§ iterator

template<class GC, typename T, typename Traits = skip_list::traits>
typedef skip_list::details::iterator< gc, node_traits, back_off, false > cds::intrusive::SkipListSet< GC, T, Traits >::iterator

Iterator type.

The forward iterator has some features:

  • it has no post-increment operator
  • to protect the value, the iterator contains a GC-specific guard + another guard is required locally for increment operator. For some GC (like as gc::HP), a guard is a limited resource per thread, so an exception (or assertion) "no free guard" may be thrown if the limit of guard count per thread is exceeded.
  • The iterator cannot be moved across thread boundary because it contains thread-private GC's guard.
  • Iterator ensures thread-safety even if you delete the item the iterator points to. However, in case of concurrent deleting operations there is no guarantee that you iterate all item in the list. Moreover, a crash is possible when you try to iterate the next element that has been deleted by concurrent thread.
Warning
Use this iterator on the concurrent container for debugging purpose only.

The iterator interface:

class iterator {
public:
// Default constructor
// Copy construtor
iterator( iterator const& src );
// Dereference operator
value_type * operator ->() const;
// Dereference operator
value_type& operator *() const;
// Preincrement operator
iterator& operator ++();
// Assignment operator
iterator& operator = (iterator const& src);
// Equality operators
bool operator ==(iterator const& i ) const;
bool operator !=(iterator const& i ) const;
};

Constructor & Destructor Documentation

§ SkipListSet()

template<class GC, typename T, typename Traits = skip_list::traits>
cds::intrusive::SkipListSet< GC, T, Traits >::SkipListSet ( )
inline

Default constructor.

The constructor checks whether the count of guards is enough for skip-list and may raise an exception if not.

Member Function Documentation

§ clear()

template<class GC, typename T, typename Traits = skip_list::traits>
void cds::intrusive::SkipListSet< GC, T, Traits >::clear ( )
inline

Clears the set (not atomic)

The function unlink all items from the set. The function is not atomic, i.e., in multi-threaded environment with parallel insertions this sequence

set.clear();
assert( set.empty());

the assertion could be raised.

For each item the disposer will be called after unlinking.

§ contains() [1/2]

template<class GC, typename T, typename Traits = skip_list::traits>
template<typename Q >
bool cds::intrusive::SkipListSet< GC, T, Traits >::contains ( Q const &  key)
inline

Checks whether the set contains key.

The function searches the item with key equal to key and returns true if it is found, and false otherwise.

§ contains() [2/2]

template<class GC, typename T, typename Traits = skip_list::traits>
template<typename Q , typename Less >
bool cds::intrusive::SkipListSet< GC, T, Traits >::contains ( Q const &  key,
Less  pred 
)
inline

Checks whether the set contains key using pred predicate for searching.

The function is similar to contains( key ) but pred is used for key comparing. Less functor has the interface like std::less. Less must imply the same element order as the comparator used for building the set.

§ erase() [1/2]

template<class GC, typename T, typename Traits = skip_list::traits>
template<typename Q >
bool cds::intrusive::SkipListSet< GC, T, Traits >::erase ( Q const &  key)
inline

Deletes the item from the set.

The function searches an item with key equal to key in the set, unlinks it from the set, and returns true. If the item with key equal to key is not found the function return false.

Note the compare functor should accept a parameter of type Q that can be not the same as value_type.

§ erase() [2/2]

template<class GC, typename T, typename Traits = skip_list::traits>
template<typename Q , typename Func >
bool cds::intrusive::SkipListSet< GC, T, Traits >::erase ( Q const &  key,
Func  f 
)
inline

Deletes the item from the set.

The function searches an item with key equal to key in the set, call f functor with item found, unlinks it from the set, and returns true. The disposer specified in Traits class template parameter is called by garbage collector GC asynchronously.

The Func interface is

struct functor {
void operator()( value_type const& item );
};

If the item with key equal to key is not found the function return false.

Note the compare functor should accept a parameter of type Q that can be not the same as value_type.

§ erase_with() [1/2]

template<class GC, typename T, typename Traits = skip_list::traits>
template<typename Q , typename Less >
bool cds::intrusive::SkipListSet< GC, T, Traits >::erase_with ( Q const &  key,
Less  pred 
)
inline

Deletes the item from the set with comparing functor pred.

The function is an analog of erase(Q const&) but pred predicate is used for key comparing.

Less functor has the semantics like std::less but should take arguments of type value_type and Q in any order. pred must imply the same element order as the comparator used for building the set.

§ erase_with() [2/2]

template<class GC, typename T, typename Traits = skip_list::traits>
template<typename Q , typename Less , typename Func >
bool cds::intrusive::SkipListSet< GC, T, Traits >::erase_with ( Q const &  key,
Less  pred,
Func  f 
)
inline

Deletes the item from the set with comparing functor pred.

The function is an analog of erase(Q const&, Func) but pred predicate is used for key comparing.

Less functor has the semantics like std::less but should take arguments of type value_type and Q in any order. pred must imply the same element order as the comparator used for building the set.

§ extract()

template<class GC, typename T, typename Traits = skip_list::traits>
template<typename Q >
guarded_ptr cds::intrusive::SkipListSet< GC, T, Traits >::extract ( Q const &  key)
inline

Extracts the item from the set with specified key.

The function searches an item with key equal to key in the set, unlinks it from the set, and returns it as guarded_ptr object. If key is not found the function returns an empty guarded pointer.

Note the compare functor should accept a parameter of type Q that can be not the same as value_type.

The disposer specified in Traits class template parameter is called automatically by garbage collector GC specified in class' template parameters when returned guarded_ptr object will be destroyed or released.

Note
Each guarded_ptr object uses the GC's guard that can be limited resource.

Usage:

skip_list theList;
// ...
{
skip_list::guarded_ptr gp(theList.extract( 5 ));
if ( gp ) {
// Deal with gp
// ...
}
// Destructor of gp releases internal HP guard
}

§ extract_max()

template<class GC, typename T, typename Traits = skip_list::traits>
guarded_ptr cds::intrusive::SkipListSet< GC, T, Traits >::extract_max ( )
inline

Extracts an item with maximal key from the list.

The function searches an item with maximal key, unlinks it, and returns the pointer to item as guarded_ptr object. If the skip-list is empty the function returns an empty guarded_ptr.

Note
Due the concurrent nature of the list, the function extracts nearly maximal key. It means that the function gets rightmost item and tries to unlink it. During unlinking, a concurrent thread may insert an item with key greater than rightmost item's key. So, the function returns the item with maximum key at the moment of list traversing.

The disposer specified in Traits class template parameter is called by garbage collector GC asynchronously when returned guarded_ptr object will be destroyed or released.

Note
Each guarded_ptr object uses the GC's guard that can be limited resource.

Usage:

skip_list theList;
// ...
{
skip_list::guarded_ptr gp( theList.extract_max( gp ));
if ( gp ) {
// Deal with gp
//...
}
// Destructor of gp releases internal HP guard
}

§ extract_min()

template<class GC, typename T, typename Traits = skip_list::traits>
guarded_ptr cds::intrusive::SkipListSet< GC, T, Traits >::extract_min ( )
inline

Extracts an item with minimal key from the list.

The function searches an item with minimal key, unlinks it, and returns it as guarded_ptr object. If the skip-list is empty the function returns an empty guarded pointer.

Note
Due the concurrent nature of the list, the function extracts nearly minimum key. It means that the function gets leftmost item and tries to unlink it. During unlinking, a concurrent thread may insert an item with key less than leftmost item's key. So, the function returns the item with minimum key at the moment of list traversing.

The disposer specified in Traits class template parameter is called by garbage collector GC automatically when returned guarded_ptr object will be destroyed or released.

Note
Each guarded_ptr object uses the GC's guard that can be limited resource.

Usage:

skip_list theList;
// ...
{
skip_list::guarded_ptr gp(theList.extract_min());
if ( gp ) {
// Deal with gp
//...
}
// Destructor of gp releases internal HP guard
}

§ extract_with()

template<class GC, typename T, typename Traits = skip_list::traits>
template<typename Q , typename Less >
guarded_ptr cds::intrusive::SkipListSet< GC, T, Traits >::extract_with ( Q const &  key,
Less  pred 
)
inline

Extracts the item from the set with comparing functor pred.

The function is an analog of extract(Q const&) but pred predicate is used for key comparing.

Less functor has the semantics like std::less but should take arguments of type value_type and Q in any order. pred must imply the same element order as the comparator used for building the set.

§ find()

template<class GC, typename T, typename Traits = skip_list::traits>
template<typename Q , typename Func >
bool cds::intrusive::SkipListSet< GC, T, Traits >::find ( Q &  key,
Func  f 
)
inline

Finds key.

The function searches the item with key equal to key and calls the functor f for item found. The interface of Func functor is:

struct functor {
void operator()( value_type& item, Q& key );
};

where item is the item found, key is the find function argument.

The functor can change non-key fields of item. Note that the functor is only guarantee that item cannot be disposed during functor is executing. The functor does not serialize simultaneous access to the set item. If such access is possible you must provide your own synchronization on item level to exclude unsafe item modifications.

Note the compare functor specified for class Traits template parameter should accept a parameter of type Q that can be not the same as value_type.

The function returns true if key is found, false otherwise.

§ find_with()

template<class GC, typename T, typename Traits = skip_list::traits>
template<typename Q , typename Less , typename Func >
bool cds::intrusive::SkipListSet< GC, T, Traits >::find_with ( Q &  key,
Less  pred,
Func  f 
)
inline

Finds the key key with pred predicate for comparing.

The function is an analog of find(Q&, Func) but pred is used for key compare.

Less functor has the semantics like std::less but should take arguments of type value_type and Q in any order. pred must imply the same element order as the comparator used for building the set.

§ get()

template<class GC, typename T, typename Traits = skip_list::traits>
template<typename Q >
guarded_ptr cds::intrusive::SkipListSet< GC, T, Traits >::get ( Q const &  key)
inline

Finds key and return the item found.

The function searches the item with key equal to key and returns the pointer to the item found as guarded_ptr. If key is not found the function returns an empt guarded pointer.

The disposer specified in Traits class template parameter is called by garbage collector GC asynchronously when returned guarded_ptr object will be destroyed or released.

Note
Each guarded_ptr object uses one GC's guard which can be limited resource.

Usage:

skip_list theList;
// ...
{
skip_list::guarded_ptr gp(theList.get( 5 ));
if ( gp ) {
// Deal with gp
//...
}
// Destructor of guarded_ptr releases internal HP guard
}

Note the compare functor specified for class Traits template parameter should accept a parameter of type Q that can be not the same as value_type.

§ get_with()

template<class GC, typename T, typename Traits = skip_list::traits>
template<typename Q , typename Less >
guarded_ptr cds::intrusive::SkipListSet< GC, T, Traits >::get_with ( Q const &  key,
Less  pred 
)
inline

Finds key and return the item found.

The function is an analog of get( Q const&) but pred is used for comparing the keys.

Less functor has the semantics like std::less but should take arguments of type value_type and Q in any order. pred must imply the same element order as the comparator used for building the set.

§ insert() [1/2]

template<class GC, typename T, typename Traits = skip_list::traits>
bool cds::intrusive::SkipListSet< GC, T, Traits >::insert ( value_type val)
inline

Inserts new node.

The function inserts val in the set if it does not contain an item with key equal to val.

Returns true if val is placed into the set, false otherwise.

§ insert() [2/2]

template<class GC, typename T, typename Traits = skip_list::traits>
template<typename Func >
bool cds::intrusive::SkipListSet< GC, T, Traits >::insert ( value_type val,
Func  f 
)
inline

Inserts new node.

This function is intended for derived non-intrusive containers.

The function allows to split creating of new item into two part:

  • create item with key only
  • insert new item into the set
  • if inserting is success, calls f functor to initialize value-field of val.

The functor signature is:

void func( value_type& val );

where val is the item inserted. User-defined functor f should guarantee that during changing val no any other changes could be made on this set's item by concurrent threads. The user-defined functor is called only if the inserting is success.

§ size()

template<class GC, typename T, typename Traits = skip_list::traits>
size_t cds::intrusive::SkipListSet< GC, T, Traits >::size ( ) const
inline

Returns item count in the set.

The value returned depends on item counter type provided by Traits template parameter. If it is atomicity::empty_item_counter this function always returns 0. Therefore, the function is not suitable for checking the set emptiness, use empty() for this purpose.

§ unlink()

template<class GC, typename T, typename Traits = skip_list::traits>
bool cds::intrusive::SkipListSet< GC, T, Traits >::unlink ( value_type val)
inline

Unlinks the item val from the set.

The function searches the item val in the set and unlink it from the set if it is found and is equal to val.

Difference between erase() and unlink() functions: erase() finds a key and deletes the item found. unlink() finds an item by key and deletes it only if val is an item of that set, i.e. the pointer to item found is equal to &val .

The disposer specified in Traits class template parameter is called by garbage collector GC asynchronously.

The function returns true if success and false otherwise.

§ update()

template<class GC, typename T, typename Traits = skip_list::traits>
template<typename Func >
std::pair<bool, bool> cds::intrusive::SkipListSet< GC, T, Traits >::update ( value_type val,
Func  func,
bool  bInsert = true 
)
inline

Updates the node.

The operation performs inserting or changing data with lock-free manner.

If the item val is not found in the set, then val is inserted into the set iff bInsert is true. Otherwise, the functor func is called with item found. The functor func signature is:

void func( bool bNew, value_type& item, value_type& val );

with arguments:

  • bNew - true if the item has been inserted, false otherwise
  • item - item of the set
  • val - argument val passed into the update() function If new item has been inserted (i.e. bNew is true) then item and val arguments refer to the same thing.

Returns std::pair<bool, bool> where first is true if operation is successful, i.e. the node has been inserted or updated, second is true if new item has been added or false if the item with key already exists.

Warning
See insert item troubleshooting

Field Documentation

§ c_nMaxHeight

template<class GC, typename T, typename Traits = skip_list::traits>
unsigned int const cds::intrusive::SkipListSet< GC, T, Traits >::c_nMaxHeight
static
Initial value:
= std::conditional<
(random_level_generator::c_nUpperBound <= skip_list::c_nHeightLimit),
std::integral_constant< unsigned int, random_level_generator::c_nUpperBound >,
std::integral_constant< unsigned int, skip_list::c_nHeightLimit >
>::type::value

Max node height. The actual node height should be in range [0 .. c_nMaxHeight)

The max height is specified by random level generator constant m_nUpperBound but it should be no more than 32 (skip_list::c_nHeightLimit).


The documentation for this class was generated from the following files:

cds 2.2.0 Developed by Maxim Khizhinsky aka khizmax 2007 - 2017
Autogenerated Wed Jan 4 2017 08:49:52 by Doxygen 1.8.12