Implementing Visitor Pattern while allowing different return types of functions

Question

I am trying to implement the Visitor Pattern for an object structure which has methods with different return types (string, signed int, unsigned int, etc).

Now, in the object hierarchy I have added an Accept method with the following signature (using C++):

void Accept(Visitor *);

I am unable to figure out how I can use the same interface (with void return type ) while at the same time allowing my concrete methods to have different return types.

Answer 1

The Accept method in the type hierarchy is just a dispatcher, and has no return type. If what you want is the visitation to produce a value the simplest way would be to add that as part of the state of the visitor:

struct times2 : visitor {
   double value;
   times2() : value() {}
   void operator()( int x ) { value = x * 2; }
   void operator()( double x ) { value = x * 2; }
};

object o;
times2 v;
o.accept( v );
std::cout << "Result is " << v.value << std::endl;

Then again, the specific details of the visitor will vary with your implementation, but the idea is that you can store the result in the visitor rather than return it.