1 // -*- C++ -*-
2
3 // Copyright (C) 2005-2021 Free Software Foundation, Inc.
4 //
5 // This file is part of the GNU ISO C++ Library. This library is free
6 // software; you can redistribute it and/or modify it under the terms
7 // of the GNU General Public License as published by the Free Software
8 // Foundation; either version 3, or (at your option) any later
9 // version.
10
11 // This library is distributed in the hope that it will be useful, but
12 // WITHOUT ANY WARRANTY; without even the implied warranty of
13 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
14 // General Public License for more details.
15
16 // You should have received a copy of the GNU General Public License
17 // along with this library; see the file COPYING3. If not see
18 // <http://www.gnu.org/licenses/>.
19
20
21 // Copyright (C) 2004 Ami Tavory and Vladimir Dreizin, IBM-HRL.
22
23 // Permission to use, copy, modify, sell, and distribute this software
24 // is hereby granted without fee, provided that the above copyright
25 // notice appears in all copies, and that both that copyright notice
26 // and this permission notice appear in supporting documentation. None
27 // of the above authors, nor IBM Haifa Research Laboratories, make any
28 // representation about the suitability of this software for any
29 // purpose. It is provided "as is" without express or implied
30 // warranty.
31
32 /**
33 * @file hash_initial_size_example.cpp
34 * An example of setting an initial size for a container object.
35 */
36
37 /**
38 * This example shows how to set the initial size of a hash-based
39 * container object through its resize-policy object.
40 */
41
42 #include <cassert>
43 #include <ext/pb_ds/assoc_container.hpp>
44 #include <ext/pb_ds/hash_policy.hpp>
45
46 using namespace std;
47 using namespace __gnu_pbds;
48
49 // A simple hash functor.
50 // hash could serve instead of this functor, but it is not yet
51 // standard everywhere.
52 struct int_hash
53 {
54 size_t
operator ()int_hash55 operator()(const int& r_i) const
56 { return r_i; }
57 };
58
main()59 int main()
60 {
61 // Resize policy type.
62 typedef
63 hash_standard_resize_policy<
64 // Size-policy type.
65 hash_exponential_size_policy<>,
66 // Trigger-policy type.
67 hash_load_check_resize_trigger<>,
68 /* Allow external access to size.
69 * This is just used in this example for using the
70 * get_actual_size method (which won't be accessible without
71 * this flag.
72 */
73 true>
74 resize_policy_t;
75
76 // A collision-probing hash table mapping ints to chars.
77 typedef
78 gp_hash_table<
79 int,
80 char,
81 int_hash,
82 equal_to<
83 int>,
84 // Combining function.
85 direct_mask_range_hashing<>,
86 // Probe function.
87 linear_probe_fn<>,
88 // Resize policy.
89 resize_policy_t>
90 map_t;
91
92 // A resize-policy object with suggested initial size 256.
93 resize_policy_t res(hash_exponential_size_policy<>(256));
94
95 map_t g(int_hash(),
96 equal_to<int>(),
97 direct_mask_range_hashing<>(),
98 linear_probe_fn<>(),
99 res);
100
101 // Check the actual size of the container object. In this case, this
102 // should be the initial size given by the size policy object.
103 assert(g.get_actual_size() == 256);
104
105 // The logical size of g, though is 0 (it does not contain any elements).
106 assert(g.size() == 0);
107
108 return 0;
109 }
110
111