72 lines
2.6 KiB
C++
72 lines
2.6 KiB
C++
//
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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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// 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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//
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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 <type_traits>
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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_default_init member.
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template <typename string_type, typename = void>
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struct ResizeUninitializedTraits {
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using HasMember = std::false_type;
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static void Resize(string_type* s, size_t new_size) { s->resize(new_size); }
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};
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// __resize_default_init is provided by libc++ >= 8.0 and by Google's internal
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// ::string implementation.
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template <typename string_type>
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struct ResizeUninitializedTraits<
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string_type, absl::void_t<decltype(std::declval<string_type&>()
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.__resize_default_init(237))> > {
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using HasMember = std::true_type;
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static void Resize(string_type* s, size_t new_size) {
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s->__resize_default_init(new_size);
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}
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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 ResizeUninitializedTraits<string_type>::HasMember::value;
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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.
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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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ResizeUninitializedTraits<string_type>::Resize(s, new_size);
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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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