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