1// Written in the D programming language.
2/**
3Source: $(PHOBOSSRC std/experimental/allocator/building_blocks/null_allocator.d)
4*/
5module std.experimental.allocator.building_blocks.null_allocator;
6
7/**
8`NullAllocator` is an emphatically empty implementation of the allocator
9interface. Although it has no direct use, it is useful as a "terminator" in
10composite allocators.
11*/
12struct NullAllocator
13{
14    import std.typecons : Ternary;
15
16    nothrow @nogc pure @safe:
17    /**
18    `NullAllocator` advertises a relatively large _alignment equal to 64 KB.
19    This is because `NullAllocator` never actually needs to honor this
20    alignment and because composite allocators using `NullAllocator`
21    shouldn't be unnecessarily constrained.
22    */
23    enum uint alignment = 64 * 1024;
24    // /// Returns `n`.
25    //size_t goodAllocSize(size_t n) shared const
26    //{ return .goodAllocSize(this, n); }
27    /// Always returns `null`.
28    void[] allocate(size_t) shared { return null; }
29    /// Always returns `null`.
30    void[] alignedAllocate(size_t, uint) shared { return null; }
31    /// Always returns `null`.
32    void[] allocateAll() shared { return null; }
33    /**
34    These methods return `false`.
35    Precondition: $(D b is null). This is because there is no other possible
36    legitimate input.
37    */
38    bool expand(ref void[] b, size_t s) shared
39    { assert(b is null); return s == 0; }
40    /// Ditto
41    bool reallocate(ref void[] b, size_t) shared
42    { assert(b is null); return false; }
43    /// Ditto
44    bool alignedReallocate(ref void[] b, size_t, uint) shared
45    { assert(b is null); return false; }
46    /// Returns `Ternary.no`.
47    Ternary owns(const void[]) shared const { return Ternary.no; }
48    /**
49    Returns `Ternary.no`.
50    */
51    Ternary resolveInternalPointer(const void*, ref void[]) shared const
52    { return Ternary.no; }
53    /**
54    No-op.
55    Precondition: $(D b is null)
56    */
57    bool deallocate(void[] b) shared { assert(b is null); return true; }
58    /**
59    No-op.
60    */
61    bool deallocateAll() shared { return true; }
62    /**
63    Returns `Ternary.yes`.
64    */
65    Ternary empty() shared const { return Ternary.yes; }
66    /**
67    Returns the `shared` global instance of the `NullAllocator`.
68    */
69    static shared NullAllocator instance;
70}
71
72nothrow @nogc pure @safe unittest
73{
74    alias a = NullAllocator.instance;
75
76    assert(a.alignedAllocate(100, 0) is null);
77    assert(a.allocateAll() is null);
78    auto b = a.allocate(100);
79    assert(b is null);
80    assert(a.expand(b, 0));
81    assert(!a.expand(b, 42));
82    assert(!a.reallocate(b, 42));
83    assert(!a.alignedReallocate(b, 42, 0));
84    assert(a.deallocate(b));
85    assert(a.deallocateAll());
86
87    import std.typecons : Ternary;
88    assert(a.empty == Ternary.yes);
89    assert(a.owns(null) == Ternary.no);
90
91    void[] p;
92    assert(a.resolveInternalPointer(null, p) == Ternary.no);
93}
94