Abseil Common Libraries (C++) (grcp 依赖)
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70 lines
2.5 KiB
70 lines
2.5 KiB
7 years ago
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// Copyright 2017 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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// http://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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//
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#ifndef ABSL_STRINGS_INTERNAL_RESIZE_UNINITIALIZED_H_
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#define ABSL_STRINGS_INTERNAL_RESIZE_UNINITIALIZED_H_
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#include <string>
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#include <utility>
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#include "absl/base/port.h"
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#include "absl/meta/type_traits.h" // for void_t
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namespace absl {
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namespace strings_internal {
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// Is a subclass of true_type or false_type, depending on whether or not
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// T has a resize_uninitialized member.
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template <typename T, typename = void>
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struct HasResizeUninitialized : std::false_type {};
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template <typename T>
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struct HasResizeUninitialized<
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T, absl::void_t<decltype(std::declval<T>().resize_uninitialized(237))>>
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: std::true_type {};
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template <typename string_type>
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void ResizeUninit(string_type* s, size_t new_size, std::true_type) {
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s->resize_uninitialized(new_size);
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}
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template <typename string_type>
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void ResizeUninit(string_type* s, size_t new_size, std::false_type) {
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s->resize(new_size);
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}
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// Returns true if the std::string implementation supports a resize where
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// the new characters added to the std::string are left untouched.
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//
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// (A better name might be "STLStringSupportsUninitializedResize", alluding to
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// the previous function.)
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template <typename string_type>
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inline constexpr bool STLStringSupportsNontrashingResize(string_type*) {
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return HasResizeUninitialized<string_type>();
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}
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// Like str->resize(new_size), except any new characters added to "*str" as a
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// result of resizing may be left uninitialized, rather than being filled with
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// '0' bytes. Typically used when code is then going to overwrite the backing
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// store of the std::string with known data. Uses a Google extension to std::string.
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template <typename string_type, typename = void>
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inline void STLStringResizeUninitialized(string_type* s, size_t new_size) {
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ResizeUninit(s, new_size, HasResizeUninitialized<string_type>());
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}
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} // namespace strings_internal
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} // namespace absl
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#endif // ABSL_STRINGS_INTERNAL_RESIZE_UNINITIALIZED_H_
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