string_constant.h (2439B)
1 // Copyright 2020 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 #ifndef ABSL_STRINGS_INTERNAL_STRING_CONSTANT_H_ 16 #define ABSL_STRINGS_INTERNAL_STRING_CONSTANT_H_ 17 18 #include "absl/meta/type_traits.h" 19 #include "absl/strings/string_view.h" 20 21 namespace absl { 22 ABSL_NAMESPACE_BEGIN 23 namespace strings_internal { 24 25 // StringConstant<T> represents a compile time string constant. 26 // It can be accessed via its `absl::string_view value` static member. 27 // It is guaranteed that the `string_view` returned has constant `.data()`, 28 // constant `.size()` and constant `value[i]` for all `0 <= i < .size()` 29 // 30 // The `T` is an opaque type. It is guaranteed that different string constants 31 // will have different values of `T`. This allows users to associate the string 32 // constant with other static state at compile time. 33 // 34 // Instances should be made using the `MakeStringConstant()` factory function 35 // below. 36 template <typename T> 37 struct StringConstant { 38 private: 39 static constexpr bool TryConstexprEval(absl::string_view view) { 40 return view.empty() || 2 * view[0] != 1; 41 } 42 43 public: 44 static constexpr absl::string_view value = T{}(); 45 constexpr absl::string_view operator()() const { return value; } 46 47 // Check to be sure `view` points to constant data. 48 // Otherwise, it can't be constant evaluated. 49 static_assert(TryConstexprEval(value), 50 "The input string_view must point to constant data."); 51 }; 52 53 // Factory function for `StringConstant` instances. 54 // It supports callables that have a constexpr default constructor and a 55 // constexpr operator(). 56 // It must return an `absl::string_view` or `const char*` pointing to constant 57 // data. This is validated at compile time. 58 template <typename T> 59 constexpr StringConstant<T> MakeStringConstant(T) { 60 return {}; 61 } 62 63 } // namespace strings_internal 64 ABSL_NAMESPACE_END 65 } // namespace absl 66 67 #endif // ABSL_STRINGS_INTERNAL_STRING_CONSTANT_H_