template<class T>
   struct scoped_replace {
       scoped_replace(T& ref, T new_val) 
          : ref(ref), old_val(ref) { 
         ref =  std::move(new_val); 
       }
       ~scoped_replace() { ref = std::move(old_val); }
       T& ref;
       T old_val;
   };

   thread_local int val = 0;
   int foo() {
      return val;
   }
  
   int bar() {
      scoped_replace _(val, 42);
      return foo();
   }
   int main() {
      foo(); // returns 0
      bar(); // returns 42;
      foo(); // still returns 0
   }