65 lines
2.3 KiB
C
65 lines
2.3 KiB
C
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// 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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static constexpr absl::string_view value = T{}();
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constexpr absl::string_view operator()() const { return value; }
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// Check to be sure `view` points to constant data.
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// Otherwise, it can't be constant evaluated.
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static_assert(value.empty() || 2 * value[0] != 1,
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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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