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/* Boost.Flyweight example of flyweight-based memoization.
*
* Copyright 2006-2008 Joaquin M Lopez Munoz.
* Distributed under the Boost Software License, Version 1.0.
* (See accompanying file LICENSE_1_0.txt or copy at
* http://www.boost.org/LICENSE_1_0.txt)
*
* See http://www.boost.org/libs/flyweight for library home page.
*/
#include <boost/flyweight.hpp>
#include <boost/flyweight/key_value.hpp>
#include <boost/flyweight/no_tracking.hpp>
#include <boost/noncopyable.hpp>
#include <iostream>
using namespace boost::flyweights;
/* Memoized calculation of Fibonacci numbers */
/* This class takes an int n and calculates F(n) at construction time */
struct compute_fibonacci;
/* A Fibonacci number can be modeled as a key-value flyweight
* We choose the no_tracking policy so that the calculations
* persist for future use throughout the program. See
* Tutorial: Configuring Boost.Flyweight: Tracking policies for
* further information on tracking policies.
*/
typedef flyweight<key_value<int,compute_fibonacci>,no_tracking> fibonacci;
/* Implementation of compute_fibonacci. Note that the construction
* of compute_fibonacci(n) uses fibonacci(n-1) and fibonacci(n-2),
* which effectively memoizes the computation.
*/
struct compute_fibonacci:private boost::noncopyable
{
compute_fibonacci(int n):
result(n==0?0:n==1?1:fibonacci(n-2).get()+fibonacci(n-1).get())
{}
operator int()const{return result;}
int result;
};
int main()
{
/* list some Fibonacci numbers */
for(int n=0;n<40;++n){
std::cout<<"F("<<n<<")="<<fibonacci(n)<<std::endl;
}
return 0;
}
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