1------------------------------------------------------------------------------
2--                                                                          --
3--                         GNAT LIBRARY COMPONENTS                          --
4--                                                                          --
5--       A D A . C O N T A I N E R S . B O U N D E D _ H O L D E R S        --
6--                                                                          --
7--                                 S p e c                                  --
8--                                                                          --
9--            Copyright (C) 2015-2019, Free Software Foundation, Inc.       --
10--                                                                          --
11-- This specification is derived from the Ada Reference Manual for use with --
12-- GNAT. The copyright notice above, and the license provisions that follow --
13-- apply solely to the  contents of the part following the private keyword. --
14--                                                                          --
15-- GNAT is free software;  you can  redistribute it  and/or modify it under --
16-- terms of the  GNU General Public License as published  by the Free Soft- --
17-- ware  Foundation;  either version 3,  or (at your option) any later ver- --
18-- sion.  GNAT is distributed in the hope that it will be useful, but WITH- --
19-- OUT ANY WARRANTY;  without even the  implied warranty of MERCHANTABILITY --
20-- or FITNESS FOR A PARTICULAR PURPOSE.                                     --
21--                                                                          --
22-- As a special exception under Section 7 of GPL version 3, you are granted --
23-- additional permissions described in the GCC Runtime Library Exception,   --
24-- version 3.1, as published by the Free Software Foundation.               --
25--                                                                          --
26-- You should have received a copy of the GNU General Public License and    --
27-- a copy of the GCC Runtime Library Exception along with this program;     --
28-- see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see    --
29-- <http://www.gnu.org/licenses/>.                                          --
30------------------------------------------------------------------------------
31
32private with System;
33
34generic
35   type Element_Type (<>) is private;
36   Max_Size_In_Storage_Elements : Natural :=
37     Element_Type'Max_Size_In_Storage_Elements;
38   with function "=" (Left, Right : Element_Type) return Boolean is <>;
39
40package Ada.Containers.Bounded_Holders is
41   pragma Annotate (CodePeer, Skip_Analysis);
42
43   --  This package is patterned after Ada.Containers.Indefinite_Holders. It is
44   --  used to treat indefinite subtypes as definite, but without using heap
45   --  allocation. For example, you might like to say:
46   --
47   --     type A is array (...) of T'Class; -- illegal
48   --
49   --  Instead, you can instantiate this package with Element_Type => T'Class,
50   --  and say:
51   --
52   --     type A is array (...) of Holder;
53   --
54   --  Each object of type Holder is allocated Max_Size_In_Storage_Elements
55   --  bytes. If you try to create a holder from an object of type Element_Type
56   --  that is too big, an exception is raised (assuming assertions are
57   --  enabled). This applies to To_Holder and Set. If you pass an Element_Type
58   --  object that is smaller than Max_Size_In_Storage_Elements, it works fine,
59   --  but some space is wasted.
60   --
61   --  NOTE: If assertions are disabled, and you try to use an Element that is
62   --  too big, execution is erroneous, and anything can happen, such as
63   --  overwriting arbitrary memory locations.
64   --
65   --  Element_Type must not be an unconstrained array type. It can be a
66   --  class-wide type or a type with non-defaulted discriminants.
67   --
68   --  The 'Size of each Element_Type object must be a multiple of
69   --  System.Storage_Unit; e.g. creating Holders from 5-bit objects won't
70   --  work.
71
72   type Holder is private;
73
74   function "=" (Left, Right : Holder) return Boolean;
75
76   function To_Holder (New_Item : Element_Type) return Holder;
77   function "+" (New_Item : Element_Type) return Holder renames To_Holder;
78
79   function Get (Container : Holder) return Element_Type;
80
81   procedure Set (Container : in out Holder; New_Item  : Element_Type);
82
83private
84
85   --  The implementation uses low-level tricks (Address clauses and unchecked
86   --  conversions of access types) to treat the elements as storage arrays.
87
88   pragma Assert (Element_Type'Alignment <= Standard'Maximum_Alignment);
89   --  This prevents elements with a user-specified Alignment that is too big
90
91   type Storage_Element is mod 2 ** System.Storage_Unit;
92   type Storage_Array is array (Positive range <>) of Storage_Element;
93   type Holder is record
94      Data : Storage_Array (1 .. Max_Size_In_Storage_Elements);
95   end record
96     with Alignment => Standard'Maximum_Alignment;
97   --  We would like to say "Alignment => Element_Type'Alignment", but that
98   --  is illegal because it's not static, so we use the maximum possible
99   --  (default) alignment instead.
100
101   type Element_Access is access all Element_Type;
102   pragma Assert (Element_Access'Size = Standard'Address_Size,
103                  "cannot instantiate with an array type");
104   --  If Element_Access is a fat pointer, Element_Type must be an
105   --  unconstrained array, which is not allowed. Arrays won't work, because
106   --  the 'Address of an array points to the first element, thus losing the
107   --  bounds.
108
109   pragma No_Strict_Aliasing (Element_Access);
110   --  Needed because we are unchecked-converting from Address to
111   --  Element_Access (see package body), which is a violation of the
112   --  normal aliasing rules enforced by gcc.
113
114end Ada.Containers.Bounded_Holders;
115