duo4.hpp

C++ Templates 完全导引范例

/* The following code example is taken from the book
 * "C++ Templates - The Complete Guide"
 * by David Vandevoorde and Nicolai M. Josuttis, Addison-Wesley, 2002
 *
 * (C) Copyright David Vandevoorde and Nicolai M. Josuttis 2002.
 * Permission to copy, use, modify, sell and distribute this software
 * is granted provided this copyright notice appears in all copies.
 * This software is provided "as is" without express or implied
 * warranty, and with no claim as to its suitability for any purpose.
 */
#include "typeop.hpp"

// primary template for value of Nth field of (duo) T
template <int N, typename T>
class DuoValue {
  public:
    static void get(T&) {        // in general, we have no value
    }
    static void get(T const&) {
    }
};

// specialization for 1st field of a plain duo
template <typename A, typename B>
class DuoValue<1, Duo<A, B> > {
  public:
    static A& get(Duo<A, B> &d) { 
        return d.v1();
    }
    static A const& get(Duo<A, B> const &d) {
        return d.v1();
    }
};

// specialization for 2nd field of a plain duo
template <typename A, typename B>
class DuoValue<2, Duo<A, B> > {
  public:
    static B& get(Duo<A, B> &d) { 
        return d.v2(); 
    }
    static B const& get(Duo<A, B> const &d) { 
        return d.v2();
    }
};

// specialization for Nth field of recursive duo
template <int N, typename A, typename B, typename C>
struct DuoValue<N, Duo<A, Duo<B,C> > > {
    static
    typename TypeOp<typename DuoT<N-1, Duo<B,C> >::ResultT>::RefT
    get(Duo<A, Duo<B,C> > &d) {
        return DuoValue<N-1, Duo<B,C> >::get(d.v2());
    }

    static typename TypeOp<typename DuoT<N-1, Duo<B,C>
                          >::ResultT>::RefConstT
    get(Duo<A, Duo<B,C> > const &d) {
        return DuoValue<N-1, Duo<B,C> >::get(d.v2());
    }
};

// specialization for 1st field of recursive duo
template <typename A, typename B, typename C>
class DuoValue<1, Duo<A, Duo<B,C> > > {
  public:
    static A& get(Duo<A, Duo<B,C> > &d) { 
        return d.v1(); 
    }
    static A const& get(Duo<A, Duo<B,C> > const &d) { 
        return d.v1(); 
    }
};

// specialization for 2nd field of recursive duo
template <typename A, typename B, typename C>
class DuoValue<2, Duo<A, Duo<B,C> > > {
  public:
    static B& get(Duo<A, Duo<B,C> > &d) { 
        return d.v2().v1(); 
    }
    static B const& get(Duo<A, Duo<B,C> > const &d) {
        return d.v2().v1();
    }
};