boost/intrusive/pointer_plus_bits.hpp

   1 /////////////////////////////////////////////////////////////////////////////
   2 //
   3 // (C) Copyright Ion Gaztanaga  2007-2008
   4 //
   5 // Distributed under the Boost Software License, Version 1.0.
   6 //    (See accompanying file LICENSE_1_0.txt or copy at
   7 //          http://www.boost.org/LICENSE_1_0.txt)
   8 //
   9 // See http://www.boost.org/libs/intrusive for documentation.
  10 //
  11 /////////////////////////////////////////////////////////////////////////////
  12
  13 #ifndef BOOST_INTRUSIVE_POINTER_PLUS_BITS_HPP
  14 #define BOOST_INTRUSIVE_POINTER_PLUS_BITS_HPP
  15
  16 #include <boost/intrusive/detail/mpl.hpp> //ls_zeros
  17 #include <boost/intrusive/detail/assert.hpp> //BOOST_INTRUSIVE_INVARIANT_ASSERT
  18
  19 namespace boost {
  20 namespace intrusive {
  21
  22 //!This trait class is used to know if a pointer
  23 //!can embed extra bits of information if
  24 //!it's going to be used to point to objects
  25 //!with an alignment of "Alignment" bytes.
  26 template<class VoidPointer, std::size_t Alignment>
  27 struct max_pointer_plus_bits
  28 {
  29    static const std::size_t value = 0;
  30 };
  31
  32 //!This is a specialization for raw pointers.
  33 //!Raw pointers can embed extra bits in the lower bits
  34 //!if the alignment is multiple of 2pow(NumBits).
  35 template<std::size_t Alignment>
  36 struct max_pointer_plus_bits<void*, Alignment>
  37 {
  38    static const std::size_t value = detail::ls_zeros<Alignment>::value;
  39 };
  40
  41 //!This is class that is supposed to have static methods
  42 //!to embed extra bits of information in a pointer.
  43 //!This is a declaration and there is no default implementation,
  44 //!because operations to embed the bits change with every pointer type.
  45 //!
  46 //!An implementation that detects that a pointer type whose
  47 //!has_pointer_plus_bits<>::value is non-zero can make use of these
  48 //!operations to embed the bits in the pointer.
  49 template<class Pointer, std::size_t NumBits>
  50 struct pointer_plus_bits;
  51
  52 //!This is the specialization to embed extra bits of information
  53 //!in a raw pointer. The extra bits are stored in the lower bit of the pointer.
  54 template<class T, std::size_t NumBits>
  55 struct pointer_plus_bits<T*, NumBits>
  56 {
  57    static const std::size_t Mask = ((std::size_t(1u) << NumBits) - 1);
  58    typedef T*        pointer;
  59
  60    static pointer get_pointer(pointer n)
  61    {  return pointer(std::size_t(n) & ~Mask);  }
  62
  63    static void set_pointer(pointer &n, pointer p)
  64    {
  65       BOOST_INTRUSIVE_INVARIANT_ASSERT(0 == (std::size_t(p) & Mask));
  66       n = pointer(std::size_t(p) | (std::size_t(n) & Mask));
  67    }
  68
  69    static std::size_t get_bits(pointer n)
  70    {  return (std::size_t(n) & Mask);  }
  71
  72    static void set_bits(pointer &n, std::size_t c)
  73    {
  74       BOOST_INTRUSIVE_INVARIANT_ASSERT(c <= Mask);
  75       n = pointer(std::size_t(get_pointer(n)) | c);
  76    }
  77 };
  78
  79 } //namespace intrusive
  80 } //namespace boost
  81
  82 #endif //BOOST_INTRUSIVE_POINTER_PLUS_BITS_HPP