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 <string> 21 #include <type_traits> 22 #include <utility> 23 24 #include "absl/base/port.h" 25 #include "absl/meta/type_traits.h" // for void_t 26 27 namespace absl { 28 ABSL_NAMESPACE_BEGIN 29 namespace strings_internal { 30 31 // Is a subclass of true_type or false_type, depending on whether or not 32 // T has a __resize_default_init member. 33 template <typename string_type, typename = void> 34 struct ResizeUninitializedTraits { 35 using HasMember = std::false_type; ResizeResizeUninitializedTraits36 static void Resize(string_type* s, size_t new_size) { s->resize(new_size); } 37 }; 38 39 // __resize_default_init is provided by libc++ >= 8.0 40 template <typename string_type> 41 struct ResizeUninitializedTraits< 42 string_type, absl::void_t<decltype(std::declval<string_type&>() 43 .__resize_default_init(237))> > { 44 using HasMember = std::true_type; 45 static void Resize(string_type* s, size_t new_size) { 46 s->__resize_default_init(new_size); 47 } 48 }; 49 50 // Returns true if the std::string implementation supports a resize where 51 // the new characters added to the std::string are left untouched. 52 // 53 // (A better name might be "STLStringSupportsUninitializedResize", alluding to 54 // the previous function.) 55 template <typename string_type> 56 inline constexpr bool STLStringSupportsNontrashingResize(string_type*) { 57 return ResizeUninitializedTraits<string_type>::HasMember::value; 58 } 59 60 // Like str->resize(new_size), except any new characters added to "*str" as a 61 // result of resizing may be left uninitialized, rather than being filled with 62 // '0' bytes. Typically used when code is then going to overwrite the backing 63 // store of the std::string with known data. 64 template <typename string_type, typename = void> 65 inline void STLStringResizeUninitialized(string_type* s, size_t new_size) { 66 ResizeUninitializedTraits<string_type>::Resize(s, new_size); 67 } 68 69 } // namespace strings_internal 70 ABSL_NAMESPACE_END 71 } // namespace absl 72 73 #endif // ABSL_STRINGS_INTERNAL_RESIZE_UNINITIALIZED_H_ 74