// // 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_