auto_reset.h (2333B)
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. 4 5 #ifndef BASE_AUTO_RESET_H_ 6 #define BASE_AUTO_RESET_H_ 7 8 #include <utility> 9 10 #include "base/check_op.h" 11 #include "base/memory/raw_ptr_exclusion.h" 12 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. 17 // 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. 21 22 namespace base { 23 24 template <typename T> 25 class [[maybe_unused, nodiscard]] AutoReset { 26 public: 27 template <typename U> 28 AutoReset(T* scoped_variable, U&& new_value) 29 : scoped_variable_(scoped_variable), 30 original_value_( 31 std::exchange(*scoped_variable_, std::forward<U>(new_value))) {} 32 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 36 // list). 37 template <typename U> 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); 41 } 42 43 AutoReset(AutoReset&& other) 44 : scoped_variable_(std::exchange(other.scoped_variable_, nullptr)), 45 original_value_(std::move(other.original_value_)) {} 46 47 AutoReset& operator=(AutoReset&& rhs) { 48 scoped_variable_ = std::exchange(rhs.scoped_variable_, nullptr); 49 original_value_ = std::move(rhs.original_value_); 50 return *this; 51 } 52 53 ~AutoReset() { 54 if (scoped_variable_) 55 *scoped_variable_ = std::move(original_value_); 56 } 57 58 private: 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_; 63 64 T original_value_; 65 }; 66 67 } // namespace base 68 69 #endif // BASE_AUTO_RESET_H_