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## Class template stack

boost::lockfree::stack

## Synopsis

    // In header: <boost/lockfree/stack.hpp>

    template<typename T, ... Options>
    class stack {
    public:
      // types
      typedef T                                 value_type;
      typedef implementation_defined::allocator allocator;
      typedef implementation_defined::size_type size_type;

      // construct/copy/destruct
      stack(void);
      template<typename U>
        explicit stack(typename node_allocator::template rebind< U >::other const &);
      explicit stack(allocator const &);
      explicit stack(size_type);
      template<typename U>
        stack(size_type,
              typename node_allocator::template rebind< U >::other const &);
      ~stack(void);

      // public member functions
      bool is_lock_free(void) const;
      void reserve(size_type);
      void reserve_unsafe(size_type);
      bool push(T const &);
      bool bounded_push(T const &);
      template<typename ConstIterator>
        ConstIterator push(ConstIterator, ConstIterator);
      template<typename ConstIterator>
        ConstIterator bounded_push(ConstIterator, ConstIterator);
      bool unsynchronized_push(T const &);
      template<typename ConstIterator>
        ConstIterator unsynchronized_push(ConstIterator, ConstIterator);
      bool pop(T &);
      template<typename U> bool pop(U &);
      bool unsynchronized_pop(T &);
      template<typename U> bool unsynchronized_pop(U &);
      bool empty(void) const;
    };

## Description

The stack class provides a multi-writer/multi-reader stack, pushing and popping is lock-free, construction/destruction has to be synchronized. It uses a freelist for memory management, freed nodes are pushed to the freelist and not returned to the OS before the stack is destroyed.

Policies:

- `boost::lockfree::fixed_sized<>`, defaults to `boost::lockfree::fixed_sized<false>`
  Can be used to completely disable dynamic memory allocations during push in order to ensure lockfree behavior.
  If the data structure is configured as fixed-sized, the internal nodes are stored inside an array and they are addressed by array indexing. This limits the possible size of the stack to the number of elements that can be addressed by the index type (usually 2**16-2), but on platforms that lack double-width compare-and-exchange instructions, this is the best way to achieve lock-freedom.

- `boost::lockfree::capacity<>`, optional
  If this template argument is passed to the options, the size of the stack is set at compile-time.
  It this option implies `fixed_sized<true>`

- `boost::lockfree::allocator<>`, defaults to `boost::lockfree::allocator<std::allocator<void>>`
  Specifies the allocator that is used for the internal freelist

Requirements:

- T must have a copy constructor

### `stack` public construct/copy/destruct

1.     stack(void);

   Construct stack.

2.     template<typename U>
         explicit stack(typename node_allocator::template rebind< U >::other const & alloc);

3.     explicit stack(allocator const & alloc);

4.     explicit stack(size_type n);

   Construct stack, allocate n nodes for the freelist.

5.     template<typename U>
         stack(size_type n,
               typename node_allocator::template rebind< U >::other const & alloc);

6.     ~stack(void);

   Destroys stack, free all nodes from freelist.

   | Note
   not thread-safe

### `stack` public member functions

1.     bool is_lock_free(void) const;

   | Warning
   It only checks, if the top stack node and the freelist can be modified in a lock-free manner. On most platforms, the whole implementation is lock-free, if this is true. Using c++0x-style atomics, there is no possibility to provide a completely accurate implementation, because one would need to test every internal node, which is impossible if further nodes will be allocated from the operating system.

   Returns: | true, if implementation is lock-free.

2.     void reserve(size_type n);

   Allocate n nodes for freelist

   | Note
   thread-safe, may block if memory allocator blocks

   Requires: | only valid if no capacity<> argument given

3.     void reserve_unsafe(size_type n);

   Allocate n nodes for freelist

   | Note
   not thread-safe, may block if memory allocator blocks

   Requires: | only valid if no capacity<> argument given

4.     bool push(T const & v);

   Pushes object t to the stack.

   | Note
   Thread-safe. If internal memory pool is exhausted and the memory pool is not fixed-sized, a new node will be allocated from the OS. This may not be lock-free.

   Postconditions: | object will be pushed to the stack, if internal node can be allocated
   Returns: | true, if the push operation is successful.
   Throws: | if memory allocator throws

5.     bool bounded_push(T const & v);

   Pushes object t to the stack.

   | Note
   Thread-safe and non-blocking. If internal memory pool is exhausted, the push operation will fail

   Postconditions: | object will be pushed to the stack, if internal node can be allocated
   Returns: | true, if the push operation is successful.

6.     template<typename ConstIterator>
         ConstIterator push(ConstIterator begin, ConstIterator end);

   Pushes as many objects from the range [begin, end) as freelist node can be allocated.

   | Note
   Operation is applied atomically Thread-safe. If internal memory pool is exhausted and the memory pool is not fixed-sized, a new node will be allocated from the OS. This may not be lock-free.

   Returns: | iterator to the first element, which has not been pushed
   Throws: | if memory allocator throws

7.     template<typename ConstIterator>
         ConstIterator bounded_push(ConstIterator begin, ConstIterator end);

   Pushes as many objects from the range [begin, end) as freelist node can be allocated.

   | Note
   Operation is applied atomically Thread-safe and non-blocking. If internal memory pool is exhausted, the push operation will fail

   Returns: | iterator to the first element, which has not been pushed
   Throws: | if memory allocator throws

8.     bool unsynchronized_push(T const & v);

   Pushes object t to the stack.

   | Note
   Not thread-safe. If internal memory pool is exhausted and the memory pool is not fixed-sized, a new node will be allocated from the OS. This may not be lock-free.

   Postconditions: | object will be pushed to the stack, if internal node can be allocated
   Returns: | true, if the push operation is successful.
   Throws: | if memory allocator throws

9.     template<typename ConstIterator>
         ConstIterator unsynchronized_push(ConstIterator begin, ConstIterator end);

   Pushes as many objects from the range [begin, end) as freelist node can be allocated.

   | Note
   Not thread-safe. If internal memory pool is exhausted and the memory pool is not fixed-sized, a new node will be allocated from the OS. This may not be lock-free.

   Returns: | iterator to the first element, which has not been pushed
   Throws: | if memory allocator throws

10.     bool pop(T & ret);

    Pops object from stack.

    | Note
    Thread-safe and non-blocking

    Postconditions: | if pop operation is successful, object will be copied to ret.
    Returns: | true, if the pop operation is successful, false if stack was empty.

11.     template<typename U> bool pop(U & ret);

    Pops object from stack.

    | Note
    Thread-safe and non-blocking

    Requires: | type T must be convertible to U
    Postconditions: | if pop operation is successful, object will be copied to ret.
    Returns: | true, if the pop operation is successful, false if stack was empty.

12.     bool unsynchronized_pop(T & ret);

    Pops object from stack.

    | Note
    Not thread-safe, but non-blocking

    Postconditions: | if pop operation is successful, object will be copied to ret.
    Returns: | true, if the pop operation is successful, false if stack was empty.

13.     template<typename U> bool unsynchronized_pop(U & ret);

    Pops object from stack.

    | Note
    Not thread-safe, but non-blocking

    Requires: | type T must be convertible to U
    Postconditions: | if pop operation is successful, object will be copied to ret.
    Returns: | true, if the pop operation is successful, false if stack was empty.

14.     bool empty(void) const;

    | Note
    It only guarantees that at some point during the execution of the function the stack has been empty. It is rarely practical to use this value in program logic, because the stack can be modified by other threads.

    Returns: | true, if stack is empty.