resize_uninitialized.h (4426B)
1 // 2 // Copyright 2017 The Abseil Authors. 3 // 4 // Licensed under the Apache License, Version 2.0 (the "License"); 5 // you may not use this file except in compliance with the License. 6 // You may obtain a copy of the License at 7 // 8 // https://www.apache.org/licenses/LICENSE-2.0 9 // 10 // Unless required by applicable law or agreed to in writing, software 11 // distributed under the License is distributed on an "AS IS" BASIS, 12 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 13 // See the License for the specific language governing permissions and 14 // limitations under the License. 15 // 16 17 #ifndef ABSL_STRINGS_INTERNAL_RESIZE_UNINITIALIZED_H_ 18 #define ABSL_STRINGS_INTERNAL_RESIZE_UNINITIALIZED_H_ 19 20 #include <algorithm> 21 #include <string> 22 #include <type_traits> 23 #include <utility> 24 25 #include "absl/base/port.h" 26 #include "absl/meta/type_traits.h" // for void_t 27 28 namespace absl { 29 ABSL_NAMESPACE_BEGIN 30 namespace strings_internal { 31 32 // In this type trait, we look for a __resize_default_init member function, and 33 // we use it if available, otherwise, we use resize. We provide HasMember to 34 // indicate whether __resize_default_init is present. 35 template <typename string_type, typename = void> 36 struct ResizeUninitializedTraits { 37 using HasMember = std::false_type; 38 static void Resize(string_type* s, size_t new_size) { s->resize(new_size); } 39 }; 40 41 // __resize_default_init is provided by libc++ >= 8.0 42 template <typename string_type> 43 struct ResizeUninitializedTraits< 44 string_type, absl::void_t<decltype(std::declval<string_type&>() 45 .__resize_default_init(237))> > { 46 using HasMember = std::true_type; 47 static void Resize(string_type* s, size_t new_size) { 48 s->__resize_default_init(new_size); 49 } 50 }; 51 52 // Returns true if the std::string implementation supports a resize where 53 // the new characters added to the std::string are left untouched. 54 // 55 // (A better name might be "STLStringSupportsUninitializedResize", alluding to 56 // the previous function.) 57 template <typename string_type> 58 inline constexpr bool STLStringSupportsNontrashingResize(string_type*) { 59 return ResizeUninitializedTraits<string_type>::HasMember::value; 60 } 61 62 // Like str->resize(new_size), except any new characters added to "*str" as a 63 // result of resizing may be left uninitialized, rather than being filled with 64 // '0' bytes. Typically used when code is then going to overwrite the backing 65 // store of the std::string with known data. 66 template <typename string_type, typename = void> 67 inline void STLStringResizeUninitialized(string_type* s, size_t new_size) { 68 ResizeUninitializedTraits<string_type>::Resize(s, new_size); 69 } 70 71 // Used to ensure exponential growth so that the amortized complexity of 72 // increasing the string size by a small amount is O(1), in contrast to 73 // O(str->size()) in the case of precise growth. 74 template <typename string_type> 75 void STLStringReserveAmortized(string_type* s, size_t new_size) { 76 const size_t cap = s->capacity(); 77 if (new_size > cap) { 78 // Make sure to always grow by at least a factor of 2x. 79 s->reserve((std::max)(new_size, 2 * cap)); 80 } 81 } 82 83 // In this type trait, we look for an __append_default_init member function, and 84 // we use it if available, otherwise, we use append. 85 template <typename string_type, typename = void> 86 struct AppendUninitializedTraits { 87 static void Append(string_type* s, size_t n) { 88 s->append(n, typename string_type::value_type()); 89 } 90 }; 91 92 template <typename string_type> 93 struct AppendUninitializedTraits< 94 string_type, absl::void_t<decltype(std::declval<string_type&>() 95 .__append_default_init(237))> > { 96 static void Append(string_type* s, size_t n) { 97 s->__append_default_init(n); 98 } 99 }; 100 101 // Like STLStringResizeUninitialized(str, new_size), except guaranteed to use 102 // exponential growth so that the amortized complexity of increasing the string 103 // size by a small amount is O(1), in contrast to O(str->size()) in the case of 104 // precise growth. 105 template <typename string_type> 106 void STLStringResizeUninitializedAmortized(string_type* s, size_t new_size) { 107 const size_t size = s->size(); 108 if (new_size > size) { 109 AppendUninitializedTraits<string_type>::Append(s, new_size - size); 110 } else { 111 s->erase(new_size); 112 } 113 } 114 115 } // namespace strings_internal 116 ABSL_NAMESPACE_END 117 } // namespace absl 118 119 #endif // ABSL_STRINGS_INTERNAL_RESIZE_UNINITIALIZED_H_