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- //
- // Copyright 2017 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_RESIZE_UNINITIALIZED_H_
- #define ABSL_STRINGS_INTERNAL_RESIZE_UNINITIALIZED_H_
- #include <algorithm>
- #include <string>
- #include <type_traits>
- #include <utility>
- #include "absl/base/port.h"
- #include "absl/meta/type_traits.h" // for void_t
- namespace absl {
- ABSL_NAMESPACE_BEGIN
- namespace strings_internal {
- // In this type trait, we look for a __resize_default_init member function, and
- // we use it if available, otherwise, we use resize. We provide HasMember to
- // indicate whether __resize_default_init is present.
- template <typename string_type, typename = void>
- struct ResizeUninitializedTraits {
- using HasMember = std::false_type;
- static void Resize(string_type* s, size_t new_size) { s->resize(new_size); }
- };
- // __resize_default_init is provided by libc++ >= 8.0
- template <typename string_type>
- struct ResizeUninitializedTraits<
- string_type, absl::void_t<decltype(std::declval<string_type&>()
- .__resize_default_init(237))> > {
- using HasMember = std::true_type;
- static void Resize(string_type* s, size_t new_size) {
- s->__resize_default_init(new_size);
- }
- };
- // Returns true if the std::string implementation supports a resize where
- // the new characters added to the std::string are left untouched.
- //
- // (A better name might be "STLStringSupportsUninitializedResize", alluding to
- // the previous function.)
- template <typename string_type>
- inline constexpr bool STLStringSupportsNontrashingResize(string_type*) {
- return ResizeUninitializedTraits<string_type>::HasMember::value;
- }
- // Like str->resize(new_size), except any new characters added to "*str" as a
- // result of resizing may be left uninitialized, rather than being filled with
- // '0' bytes. Typically used when code is then going to overwrite the backing
- // store of the std::string with known data.
- template <typename string_type, typename = void>
- inline void STLStringResizeUninitialized(string_type* s, size_t new_size) {
- ResizeUninitializedTraits<string_type>::Resize(s, new_size);
- }
- // Used to ensure exponential growth so that the amortized complexity of
- // increasing the string size by a small amount is O(1), in contrast to
- // O(str->size()) in the case of precise growth.
- template <typename string_type>
- void STLStringReserveAmortized(string_type* s, size_t new_size) {
- const size_t cap = s->capacity();
- if (new_size > cap) {
- // Make sure to always grow by at least a factor of 2x.
- s->reserve((std::max)(new_size, 2 * cap));
- }
- }
- // In this type trait, we look for an __append_default_init member function, and
- // we use it if available, otherwise, we use append.
- template <typename string_type, typename = void>
- struct AppendUninitializedTraits {
- static void Append(string_type* s, size_t n) {
- s->append(n, typename string_type::value_type());
- }
- };
- template <typename string_type>
- struct AppendUninitializedTraits<
- string_type, absl::void_t<decltype(std::declval<string_type&>()
- .__append_default_init(237))> > {
- static void Append(string_type* s, size_t n) {
- s->__append_default_init(n);
- }
- };
- // Like STLStringResizeUninitialized(str, new_size), except guaranteed to use
- // exponential growth so that the amortized complexity of increasing the string
- // size by a small amount is O(1), in contrast to O(str->size()) in the case of
- // precise growth.
- template <typename string_type>
- void STLStringResizeUninitializedAmortized(string_type* s, size_t new_size) {
- const size_t size = s->size();
- if (new_size > size) {
- AppendUninitializedTraits<string_type>::Append(s, new_size - size);
- } else {
- s->erase(new_size);
- }
- }
- } // namespace strings_internal
- ABSL_NAMESPACE_END
- } // namespace absl
- #endif // ABSL_STRINGS_INTERNAL_RESIZE_UNINITIALIZED_H_
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