1 // Copyright 2011 The Chromium Authors
2 // Use of this source code is governed by a BSD-style license that can be
3 // found in the LICENSE file.
5 #ifndef BASE_AUTO_RESET_H_
6 #define BASE_AUTO_RESET_H_
10 #include "base/check_op.h"
11 #include "base/memory/raw_ptr_exclusion.h"
13 // base::AutoReset<> is useful for setting a variable to a new value only within
14 // a particular scope. An base::AutoReset<> object resets a variable to its
15 // original value upon destruction, making it an alternative to writing
16 // "var = false;" or "var = old_val;" at all of a block's exit points.
18 // This should be obvious, but note that an base::AutoReset<> instance should
19 // have a shorter lifetime than its scoped_variable, to prevent invalid memory
20 // writes when the base::AutoReset<> object is destroyed.
25 class [[maybe_unused, nodiscard]] AutoReset {
28 AutoReset(T* scoped_variable, U&& new_value)
29 : scoped_variable_(scoped_variable),
31 std::exchange(*scoped_variable_, std::forward<U>(new_value))) {}
33 // A constructor that's useful for asserting the old value of
34 // `scoped_variable`, especially when it's inconvenient to check this before
35 // constructing the AutoReset object (e.g. in a class member initializer
38 AutoReset(T* scoped_variable, U&& new_value, const T& expected_old_value)
39 : AutoReset(scoped_variable, new_value) {
40 DCHECK_EQ(original_value_, expected_old_value);
43 AutoReset(AutoReset&& other)
44 : scoped_variable_(std::exchange(other.scoped_variable_, nullptr)),
45 original_value_(std::move(other.original_value_)) {}
47 AutoReset& operator=(AutoReset&& rhs) {
48 scoped_variable_ = std::exchange(rhs.scoped_variable_, nullptr);
49 original_value_ = std::move(rhs.original_value_);
55 *scoped_variable_ = std::move(original_value_);
59 // `scoped_variable_` is not a raw_ptr<T> for performance reasons: Large
60 // number of non-PartitionAlloc pointees + AutoReset is typically short-lived
61 // (e.g. allocated on the stack).
62 RAW_PTR_EXCLUSION T* scoped_variable_;
69 #endif // BASE_AUTO_RESET_H_