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LSST Data Management Base Package
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refwrap_utils.h
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1// -*- LSST-C++ -*-
2/*
3 * This file is part of afw.
4 *
5 * Developed for the LSST Data Management System.
6 * This product includes software developed by the LSST Project
7 * (https://www.lsst.org).
8 * See the COPYRIGHT file at the top-level directory of this distribution
9 * for details of code ownership.
10 *
11 * This program is free software: you can redistribute it and/or modify
12 * it under the terms of the GNU General Public License as published by
13 * the Free Software Foundation, either version 3 of the License, or
14 * (at your option) any later version.
15 *
16 * This program is distributed in the hope that it will be useful,
17 * but WITHOUT ANY WARRANTY; without even the implied warranty of
18 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
19 * GNU General Public License for more details.
20 *
21 * You should have received a copy of the GNU General Public License
22 * along with this program. If not, see <https://www.gnu.org/licenses/>.
23 */
24
25#ifndef LSST_AFW_TYPEHANDLING_DETAIL_REFWRAP_UTILS_H
26#define LSST_AFW_TYPEHANDLING_DETAIL_REFWRAP_UTILS_H
27
28#include <functional>
29#include <variant>
30
31namespace lsst {
32namespace afw {
33namespace typehandling {
34namespace detail {
35
36// An equality comparison function object for std::variant visitors that
37// contain only std::reference_wrapper types. Because reference_wrapper doesn't
38// define operator==, std::variant::operator== can't do this for us.
40
41 template <typename T>
43 return a.get() == b.get();
44 }
45
46 template <typename T, typename U>
48 static_assert(!std::is_same_v<T, U>);
49 return false;
50 }
51
52 template <typename ...E>
54 std::variant<std::reference_wrapper<E>...> const & a,
55 std::variant<std::reference_wrapper<E>...> const & b
56 ) {
57 return std::visit(*this, a, b);
58 }
59
60};
61
62// An equivalent to const_cast for std::reference_wrapper.
63template <typename T>
67
68// A visitor function wrapper for callables with two arguments, where the
69// second is the target of a std::reference_wrapper (as is the case for
70// GenericMap's visitation interface).
71//
72// Construct with the make_refwrap_visitor function.
73template <typename F>
75private:
76 F _func;
77public:
78
79 explicit refwrap_visitor(F && func) : _func(std::forward(func)) {}
80
81 template <typename K, typename V>
82 auto operator()(K const & key, std::reference_wrapper<V> const & value) const {
83 return _func(key, value.get());
84 }
85
86};
87
88// Specialization of refwrap_visitor for functions passed by lvalue reference.
89//
90// This is necessary because you can't perfectly forward an argument that you
91// need to capture in order to use it multiple times; if it's an rvalue
92// reference, it needs to be moved into a value (not just passed along into
93// another function as an rvalue reference). Without a specialization, lvalue
94// references end up being coped, so modifications to the callable would not be
95// reflected to the caller.
96template <typename F>
97class refwrap_visitor<F&> {
98private:
100public:
101
102 explicit refwrap_visitor(F & func) : _func(std::ref(func)) {}
103
104 template <typename K, typename V>
105 auto operator()(K const & key, std::reference_wrapper<V> const & value) const {
106 return _func(key, value.get());
107 }
108
109};
110
111// Helper function to create a refwrap_visitor instance without explicitly
112// specifying the type of the wrapped callable.
113template <typename F>
115 return refwrap_visitor<F>(std::forward<F>(func));
116}
117
118} // namespace detail
119} // namespace typehandling
120} // namespace afw
121} // namespace lsst
122
123#endif
table::Key< int > b
auto operator()(K const &key, std::reference_wrapper< V > const &value) const
auto operator()(K const &key, std::reference_wrapper< V > const &value) const
std::reference_wrapper< T > refwrap_const_cast(std::reference_wrapper< T const > const &r)
refwrap_visitor< F > make_refwrap_visitor(F &&func)
STL namespace.
T ref(T... args)
bool operator()(std::reference_wrapper< T > a, std::reference_wrapper< U > b)
bool operator()(std::reference_wrapper< T > a, std::reference_wrapper< T > b)
bool operator()(std::variant< std::reference_wrapper< E >... > const &a, std::variant< std::reference_wrapper< E >... > const &b)