// Copyright 2020 The Abseil Authors. // // Licensed under the Apache License, Version 2.0 (the "License"); // you may not use this file except in compliance with the License. // You may obtain a copy of the License at // // https://www.apache.org/licenses/LICENSE-2.0 // // Unless required by applicable law or agreed to in writing, software // distributed under the License is distributed on an "AS IS" BASIS, // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. // See the License for the specific language governing permissions and // limitations under the License. #ifndef ABSL_STRINGS_INTERNAL_STRING_CONSTANT_H_ #define ABSL_STRINGS_INTERNAL_STRING_CONSTANT_H_ #include "absl/meta/type_traits.h" #include "absl/strings/string_view.h" namespace absl { ABSL_NAMESPACE_BEGIN namespace strings_internal { // StringConstant<T> represents a compile time string constant. // It can be accessed via its `absl::string_view value` static member. // It is guaranteed that the `string_view` returned has constant `.data()`, // constant `.size()` and constant `value[i]` for all `0 <= i < .size()` // // The `T` is an opaque type. It is guaranteed that different string constants // will have different values of `T`. This allows users to associate the string // constant with other static state at compile time. // // Instances should be made using the `MakeStringConstant()` factory function // below. template <typename T> struct StringConstant { static constexpr absl::string_view value = T{}(); constexpr absl::string_view operator()() const { return value; } // Check to be sure `view` points to constant data. // Otherwise, it can't be constant evaluated. static_assert(value.empty() || 2 * value[0] != 1, "The input string_view must point to constant data."); }; template <typename T> constexpr absl::string_view StringConstant<T>::value; // NOLINT // Factory function for `StringConstant` instances. // It supports callables that have a constexpr default constructor and a // constexpr operator(). // It must return an `absl::string_view` or `const char*` pointing to constant // data. This is validated at compile time. template <typename T> constexpr StringConstant<T> MakeStringConstant(T) { return {}; } } // namespace strings_internal ABSL_NAMESPACE_END } // namespace absl #endif // ABSL_STRINGS_INTERNAL_STRING_CONSTANT_H_