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
27namespace absl {
28namespace strings_internal {
29
30// Is a subclass of true_type or false_type, depending on whether or not
31// T has a __resize_default_init member.
32template <typename string_type, typename = void>
33struct ResizeUninitializedTraits {
34 using HasMember = std::false_type;
35 static void Resize(string_type* s, size_t new_size) { s->resize(new_size); }
36};
37
38// __resize_default_init is provided by libc++ >= 8.0 and by Google's internal
39// ::string implementation.
40template <typename string_type>
41struct 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.)
55template <typename string_type>
56inline 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.
64template <typename string_type, typename = void>
65inline void STLStringResizeUninitialized(string_type* s, size_t new_size) {
66 ResizeUninitializedTraits<string_type>::Resize(s, new_size);
67}
68
69} // namespace strings_internal
70} // namespace absl
71
72#endif // ABSL_STRINGS_INTERNAL_RESIZE_UNINITIALIZED_H_
73