1 
2 /**
3  *    Copyright (C) 2018-present MongoDB, Inc.
4  *
5  *    This program is free software: you can redistribute it and/or modify
6  *    it under the terms of the Server Side Public License, version 1,
7  *    as published by MongoDB, Inc.
8  *
9  *    This program is distributed in the hope that it will be useful,
10  *    but WITHOUT ANY WARRANTY; without even the implied warranty of
11  *    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
12  *    Server Side Public License for more details.
13  *
14  *    You should have received a copy of the Server Side Public License
15  *    along with this program. If not, see
16  *    <http://www.mongodb.com/licensing/server-side-public-license>.
17  *
18  *    As a special exception, the copyright holders give permission to link the
19  *    code of portions of this program with the OpenSSL library under certain
20  *    conditions as described in each individual source file and distribute
21  *    linked combinations including the program with the OpenSSL library. You
22  *    must comply with the Server Side Public License in all respects for
23  *    all of the code used other than as permitted herein. If you modify file(s)
24  *    with this exception, you may extend this exception to your version of the
25  *    file(s), but you are not obligated to do so. If you do not wish to do so,
26  *    delete this exception statement from your version. If you delete this
27  *    exception statement from all source files in the program, then also delete
28  *    it in the license file.
29  */
30 
31 #pragma once
32 
33 #include <cstring>
34 
35 #include "mongo/base/data_view.h"
36 
37 namespace mongo {
38 
39 struct ZeroInitTag_t {
ZeroInitTag_tZeroInitTag_t40     ZeroInitTag_t(){};
41 };
42 
43 const ZeroInitTag_t kZeroInitTag;
44 
45 template <typename Layout, typename ConstView, typename View>
46 class EncodedValueStorage {
47 protected:
EncodedValueStorage()48     EncodedValueStorage() {}
49 
50     // This explicit constructor is provided to allow for easy zeroing
51     // during creation of a value.  You might prefer this over an
52     // uninitialised value if the zeroed version provides a useful base
53     // state.  Such cases might include a set of counters that begin at
54     // zero, flags that start off false or a larger structure where some
55     // significant portion of storage falls into those kind of use cases.
56     // Use this where you might have used calloc(1, sizeof(type)) in C.
57     //
58     // The added value of providing it as a constructor lies in the ability
59     // of subclasses to easily inherit a zeroed base state during
60     // initialization.
EncodedValueStorage(ZeroInitTag_t)61     explicit EncodedValueStorage(ZeroInitTag_t) {
62         std::memset(_data, 0, sizeof(_data));
63     }
64 
65 public:
view()66     View view() {
67         return _data;
68     }
69 
constView()70     ConstView constView() const {
71         return _data;
72     }
73 
View()74     operator View() {
75         return view();
76     }
77 
ConstView()78     operator ConstView() const {
79         return constView();
80     }
81 
82 private:
83     char _data[sizeof(Layout)];
84 };
85 
86 }  // namespace mongo
87