1 // Copyright 2023 The Abseil Authors
2 //
3 // Licensed under the Apache License, Version 2.0 (the "License");
4 // you may not use this file except in compliance with the License.
5 // You may obtain a copy of the License at
6 //
7 // https://www.apache.org/licenses/LICENSE-2.0
8 //
9 // Unless required by applicable law or agreed to in writing, software
10 // distributed under the License is distributed on an "AS IS" BASIS,
11 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
12 // See the License for the specific language governing permissions and
13 // limitations under the License.
14
15 // The IfConstexpr and IfConstexprElse utilities in this file are meant to be
16 // used to emulate `if constexpr` in pre-C++17 mode in library implementation.
17 // The motivation is to allow for avoiding complex SFINAE.
18 //
19 // The functions passed in must depend on the type(s) of the object(s) that
20 // require SFINAE. For example:
21 // template<typename T>
22 // int MaybeFoo(T& t) {
23 // if constexpr (HasFoo<T>::value) return t.foo();
24 // return 0;
25 // }
26 //
27 // can be written in pre-C++17 as:
28 //
29 // template<typename T>
30 // int MaybeFoo(T& t) {
31 // int i = 0;
32 // absl::utility_internal::IfConstexpr<HasFoo<T>::value>(
33 // [&](const auto& fooer) { i = fooer.foo(); }, t);
34 // return i;
35 // }
36
37 #ifndef ABSL_UTILITY_INTERNAL_IF_CONSTEXPR_H_
38 #define ABSL_UTILITY_INTERNAL_IF_CONSTEXPR_H_
39
40 #include <tuple>
41 #include <utility>
42
43 #include "absl/base/config.h"
44
45 namespace absl {
46 ABSL_NAMESPACE_BEGIN
47
48 namespace utility_internal {
49
50 template <bool condition, typename TrueFunc, typename FalseFunc,
51 typename... Args>
IfConstexprElse(TrueFunc && true_func,FalseFunc && false_func,Args &&...args)52 auto IfConstexprElse(TrueFunc&& true_func, FalseFunc&& false_func,
53 Args&&... args) {
54 return std::get<condition>(std::forward_as_tuple(
55 std::forward<FalseFunc>(false_func), std::forward<TrueFunc>(true_func)))(
56 std::forward<Args>(args)...);
57 }
58
59 template <bool condition, typename Func, typename... Args>
IfConstexpr(Func && func,Args &&...args)60 void IfConstexpr(Func&& func, Args&&... args) {
61 IfConstexprElse<condition>(std::forward<Func>(func), [](auto&&...){},
62 std::forward<Args>(args)...);
63 }
64
65 } // namespace utility_internal
66
67 ABSL_NAMESPACE_END
68 } // namespace absl
69
70 #endif // ABSL_UTILITY_INTERNAL_IF_CONSTEXPR_H_
71