scope_guard.cpp

#ifndef TOKEN_PASTE
#define TOKEN_PASTE(x, y) x##y
#define TOKEN_PASTE2(x, y) TOKEN_PASTE(x, y)
#endif

template <typename L>
struct scope_guard
{
        L l_;
        bool invoked_{false};

        scope_guard(L &&l)
            : l_(std::move(l))
        {
        }

        ~scope_guard()
        {
                invoke();
        }

        void invoke()
        {
                if (!invoked_)
                {
                        l_();
                        invoked_ = false;
                }
        }

        void cancel()
        {
                invoked_ = true;
        }
};

template <typename T>
inline auto make_scope_guard(T &&l)
{
        return scope_guard<T>(std::forward<T>(l));
}

#define DEFER(...) auto TOKEN_PASTE2(__deferred, __COUNTER__) = make_scope_guard([&] { __VA_ARGS__ });

int main()
{
  int fd = open("/proc/self/pagemap", O_RDONLY|O_LARGEFILE);
  
  // DEFER(close(fd););
  // or 
  // auto g = make_guard([fd]{ close(fd); });
  //
  // potentially g.cancel() or g.invoke() if needed
  
  
  return 0;
 }