libphobos/src/std/experimental/allocator/building_blocks/null_allocator.d

   1 ///
   2 module std.experimental.allocator.building_blocks.null_allocator;
   3
   4 /**
   5 $(D NullAllocator) is an emphatically empty implementation of the allocator
   6 interface. Although it has no direct use, it is useful as a "terminator" in
   7 composite allocators.
   8 */
   9 struct NullAllocator
  10 {
  11     import std.typecons : Ternary;
  12     /**
  13     $(D NullAllocator) advertises a relatively large _alignment equal to 64 KB.
  14     This is because $(D NullAllocator) never actually needs to honor this
  15     alignment and because composite allocators using $(D NullAllocator)
  16     shouldn't be unnecessarily constrained.
  17     */
  18     enum uint alignment = 64 * 1024;
  19     // /// Returns $(D n).
  20     //size_t goodAllocSize(size_t n) shared const
  21     //{ return .goodAllocSize(this, n); }
  22     /// Always returns $(D null).
  23     void[] allocate(size_t) shared { return null; }
  24     /// Always returns $(D null).
  25     void[] alignedAllocate(size_t, uint) shared { return null; }
  26     /// Always returns $(D null).
  27     void[] allocateAll() shared { return null; }
  28     /**
  29     These methods return $(D false).
  30     Precondition: $(D b is null). This is because there is no other possible
  31     legitimate input.
  32     */
  33     bool expand(ref void[] b, size_t s) shared
  34     { assert(b is null); return s == 0; }
  35     /// Ditto
  36     bool reallocate(ref void[] b, size_t) shared
  37     { assert(b is null); return false; }
  38     /// Ditto
  39     bool alignedReallocate(ref void[] b, size_t, uint) shared
  40     { assert(b is null); return false; }
  41     /// Returns $(D Ternary.no).
  42     Ternary owns(void[]) shared const { return Ternary.no; }
  43     /**
  44     Returns $(D Ternary.no).
  45     */
  46     Ternary resolveInternalPointer(const void*, ref void[]) shared const
  47     { return Ternary.no; }
  48     /**
  49     No-op.
  50     Precondition: $(D b is null)
  51     */
  52     bool deallocate(void[] b) shared { assert(b is null); return true; }
  53     /**
  54     No-op.
  55     */
  56     bool deallocateAll() shared { return true; }
  57     /**
  58     Returns $(D Ternary.yes).
  59     */
  60     Ternary empty() shared const { return Ternary.yes; }
  61     /**
  62     Returns the $(D shared) global instance of the $(D NullAllocator).
  63     */
  64     static shared NullAllocator instance;
  65 }
  66
  67 @system unittest
  68 {
  69     assert(NullAllocator.instance.alignedAllocate(100, 0) is null);
  70     assert(NullAllocator.instance.allocateAll() is null);
  71     auto b = NullAllocator.instance.allocate(100);
  72     assert(b is null);
  73     assert(NullAllocator.instance.expand(b, 0));
  74     assert(!NullAllocator.instance.expand(b, 42));
  75     assert(!NullAllocator.instance.reallocate(b, 42));
  76     assert(!NullAllocator.instance.alignedReallocate(b, 42, 0));
  77     NullAllocator.instance.deallocate(b);
  78     NullAllocator.instance.deallocateAll();
  79
  80     import std.typecons : Ternary;
  81     assert(NullAllocator.instance.empty() == Ternary.yes);
  82     assert(NullAllocator.instance.owns(null) == Ternary.no);
  83     void[] p;
  84     assert(NullAllocator.instance.resolveInternalPointer(null, p) == Ternary.no);
  85 }