chromium/third_party/abseil-cpp/absl/strings/internal/resize_uninitialized.h

//
// 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 { … };

// __resize_default_init is provided by libc++ >= 8.0
ResizeUninitializedTraits<string_type, absl::void_t<decltype(std::declval<string_type &>().__resize_default_init(237))>>;

// 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*) { … }

// 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) { … }

// 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) { … }

// 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 { … };

AppendUninitializedTraits<string_type, absl::void_t<decltype(std::declval<string_type &>().__append_default_init(237))>>;

// 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) { … }

}  // namespace strings_internal
ABSL_NAMESPACE_END
}  // namespace absl

#endif  // ABSL_STRINGS_INTERNAL_RESIZE_UNINITIALIZED_H_