Let a variable to a value temporarily.

scoped_save.cc

/*
 * This snippet uses C++11 features. For C++03, see the previous revision.
 *
 * Replace idioms like
 *
 *   struct Foo {
 *     int a;
 *   };
 *
 *   void func(Foo *f) {
 *     int save_a = f->a;
 *     f->a = 1;
 *     // other stuff
 *     f->a = save_a;
 *   }
 *
 * with
 *
 *   void func(Foo *f) {
 *     scoped_save(f->a, 1);
 *     // other stuff
 *   }
 *
 * pros:
 *   - Simplifies the code in case there are many returns.
 *   - Easily enhances exception safety.
 */

/**
 * scoped_save(x) saves the current value of x. The value of x will be restored
 * at the end of the scope.
 *
 * scoped_save(x, a) saves the current value of x and sets x = a. The value of
 * x will be restored at the end of the scope.
 */
#define scoped_save(...) \
  scoped_save_class<decltype(scoped_save_first(__VA_ARGS__))> \
    scoped_save_concat(scoped_save_var_, __LINE__)(__VA_ARGS__)

#define scoped_save_first(...) scoped_save_first2(__VA_ARGS__, x)
#define scoped_save_first2(first, ...) first
#define scoped_save_concat(a, b) scoped_save_concat2(a, b)
#define scoped_save_concat2(a, b) a ## b

template <typename T> class scoped_save_class {
 public:
  scoped_save_class(T &ref) : ref_(ref), old_value_(ref) {}

  scoped_save_class(T &ref, const T &value) : ref_(ref), old_value_(ref) {
    ref = value;
  }

  ~scoped_save_class() {
    ref_ = old_value_;
  }

 private:
  T &ref_;
  T old_value_;
};