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// Copyright 2011 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#ifndef BASE_AUTO_RESET_H_
#define BASE_AUTO_RESET_H_
#include <utility>
#include "base/check_op.h"
#include "base/memory/raw_ptr_exclusion.h"
// base::AutoReset<> is useful for setting a variable to a new value only within
// a particular scope. An base::AutoReset<> object resets a variable to its
// original value upon destruction, making it an alternative to writing
// "var = false;" or "var = old_val;" at all of a block's exit points.
//
// This should be obvious, but note that an base::AutoReset<> instance should
// have a shorter lifetime than its scoped_variable, to prevent invalid memory
// writes when the base::AutoReset<> object is destroyed.
namespace base {
template <typename T>
class [[maybe_unused, nodiscard]] AutoReset {
public:
template <typename U>
AutoReset(T* scoped_variable, U&& new_value)
: scoped_variable_(scoped_variable),
original_value_(
std::exchange(*scoped_variable_, std::forward<U>(new_value))) {}
// A constructor that's useful for asserting the old value of
// `scoped_variable`, especially when it's inconvenient to check this before
// constructing the AutoReset object (e.g. in a class member initializer
// list).
template <typename U>
AutoReset(T* scoped_variable, U&& new_value, const T& expected_old_value)
: AutoReset(scoped_variable, new_value) {
DCHECK_EQ(original_value_, expected_old_value);
}
AutoReset(AutoReset&& other)
: scoped_variable_(std::exchange(other.scoped_variable_, nullptr)),
original_value_(std::move(other.original_value_)) {}
AutoReset& operator=(AutoReset&& rhs) {
scoped_variable_ = std::exchange(rhs.scoped_variable_, nullptr);
original_value_ = std::move(rhs.original_value_);
return *this;
}
~AutoReset() {
if (scoped_variable_) {
*scoped_variable_ = std::move(original_value_);
}
}
private:
// `scoped_variable_` is not a raw_ptr<T> for performance reasons: Large
// number of non-PartitionAlloc pointees + AutoReset is typically short-lived
// (e.g. allocated on the stack).
RAW_PTR_EXCLUSION T* scoped_variable_;
T original_value_;
};
} // namespace base
#endif // BASE_AUTO_RESET_H_
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