Abseil Common Libraries (C++) (grcp 依赖)
https://abseil.io/
You can not select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
72 lines
2.5 KiB
72 lines
2.5 KiB
// 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 { |
|
private: |
|
static constexpr bool TryConstexprEval(absl::string_view view) { |
|
return view.empty() || 2 * view[0] != 1; |
|
} |
|
|
|
public: |
|
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(TryConstexprEval(value), |
|
"The input string_view must point to constant data."); |
|
}; |
|
|
|
#ifdef ABSL_INTERNAL_NEED_REDUNDANT_CONSTEXPR_DECL |
|
template <typename T> |
|
constexpr absl::string_view StringConstant<T>::value; |
|
#endif |
|
|
|
// 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_
|
|
|