src/utils/scoped_assignment.h

// Copyright 2021 The Tint 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
//
//     http://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 SRC_UTILS_SCOPED_ASSIGNMENT_H_
#define SRC_UTILS_SCOPED_ASSIGNMENT_H_

#include <type_traits>

#include "src/utils/concat.h"

namespace tint {
namespace utils {

/// Helper class that temporarily assigns a value to a variable for the lifetime
/// of the ScopedAssignment object. Once the ScopedAssignment is destructed, the
/// original value is restored.
template <typename T>
class ScopedAssignment {
 public:
  /// Constructor
  /// @param var the variable to temporarily assign a new value to
  /// @param val the value to assign to `ref` for the lifetime of this
  /// ScopedAssignment.
  ScopedAssignment(T& var, T val) : ref_(var) {
    old_value_ = var;
    var = val;
  }

  /// Destructor
  /// Restores the original value of the variable.
  ~ScopedAssignment() { ref_ = old_value_; }

 private:
  ScopedAssignment(const ScopedAssignment&) = delete;
  ScopedAssignment& operator=(const ScopedAssignment&) = delete;

  T& ref_;
  T old_value_;
};

}  // namespace utils
}  // namespace tint

/// TINT_SCOPED_ASSIGNMENT(var, val) assigns `val` to `var`, and automatically
/// restores the original value of `var` when exiting the current lexical scope.
#define TINT_SCOPED_ASSIGNMENT(var, val)                                  \
  ::tint::utils::ScopedAssignment<std::remove_reference_t<decltype(var)>> \
  TINT_CONCAT(tint_scoped_assignment_, __COUNTER__) {                     \
    var, val                                                              \
  }

#endif  //  SRC_UTILS_SCOPED_ASSIGNMENT_H_