#眉標=Boost
#副標=「Boost技術與應用」系列文章（4）
#大標= Boost.Tuple
　　　兼談Loki.Tuple（上）
#作者=文圖／侯捷



==<反灰>===========
pair<int, string> p = make_pair(4,string("hello"));  //給予初值
p.first = 8;           //直接設定成員變數first
p.second = "world";  //直接設定成員變數second
cout << p.first << ' ' << p.second << endl;  //直接讀取成員變數
==<end>==============




==程式1		<box>===========
template <class T1, class T2>
struct pair {
  typedef T1 first_type;
  typedef T2 second_type;

  T1 first;		// 注意它是 public
  T2 second;		// 注意它是 public
  pair() : first(T1()), second(T2()) {}
  pair(const T1& a, const T2& b) : first(a), second(b) {}

  // 以pair A 為pair B的初值。A,B 的對應元素型別可以不同，只要能轉換就好。
  template <class U1, class U2>
  pair(const pair<U1, U2>& p) : first(p.first), second(p.second) {}
};

template <class T1, class T2>
inline bool operator==(const pair<T1, T2>& x, const pair<T1, T2>& y) { 
  return x.first == y.first && x.second == y.second; 
  // 第一元素和第二元素都相等，才視為相等。
}
template <class T1, class T2>
inline bool operator<(const pair<T1, T2>& x, const pair<T1, T2>& y) { 
  return x.first < y.first || (!(y.first < x.first) && x.second < y.second); 
  // x的第一元素小於y的第一元素，
  // 或x的第一元素不大於y的第一元素而x的第二元素小於y的第二元素，
  // 才被視為x小於y。
}
// 根據兩個數值，製造出一個pair
template <class T1, class T2>
inline pair<T1, T2> make_pair(const T1& x, const T2& y) {
  return pair<T1, T2>(x, y);
}
==<end>==============




==程式2		<box>===========
template <class T1, class T2, class T3>
struct triple {
  typedef T1 first_type;
  typedef T2 second_type;
  typedef T3 third_type;

  T1 first;		// 注意它是 public
  T2 second;		// 注意它是 public
  T3 third;		// 注意它是 public
  triple() : first(T1()), second(T2()), third(T3()) {}
  triple(const T1& a, const T2& b, const T3& c) 
    : first(a), second(b) , third(c) {}
  ...
};
...
// 根據三個值製造出一個triple
template <class T1, class T2, class T3>
inline 
triple<T1,T2,T3> make_triple(const T1& x, const T2& y, const T3& z) {
  return triple<T1,T2,T3>(x, y, z);
}
==<end>==============




==<反灰>===========
using namespace Loki
using namespace Loki::TL;   //TL代表TypeList
...
typedef Tuple<TYPELIST_3(string,double,int)>  MyData;
MyData obj;

//以index設欄位值
Field<0>(obj).value_ = string("jjhou");
Field<1>(obj).value_ = 3.141592653;		
Field<2>(obj).value_ = 928;	

//以index取欄位值，而後列印
name = Field<0>(obj).value_;    
d = Field<1>(obj).value_;		
i = Field<2>(obj).value_;
cout << name << ' ' << d << ' ' << i << endl;	
   //輸出 jjhou 3.14159 928
==<end>==============




==<反灰>===========
template <typename T> 
struct OpNewCreator { 
    T* Create() { /* ... */ } 
};
template <typename T> 
struct MallocCreator { 
    T* Create() { /* ... */ } 
};
template <typename T> 
struct PrototypeCreator { 
    T* Create() { /* ... */ } 
};
==<end>==============




==<反灰>===========
template <template <typename> class CP> 
class WMgr : public CP<W> {  /* ... */   };
==<end>==============



==<反灰>===========
template<typename T,    //增加這個參數
         template <typename> class CP>   
class Mgr : public CP<T>                                
{
public:
  void DoSomething()
  {     //以CP產生T物件。CP和T是兩個獨立自由度
        T* p= CP<T>().Create();
        ...
        delete p;
  }
};
==<end>==============



==<反灰>===========
  Mgr<W,MallocCreator> wMgr;
  W* pw = wMgr.Create();   //喚起MallocCreator<W>::Create()
  delete pw;                    

  Mgr<G,OpNewCreator> gMgr;
  G* pg = gMgr.Create();   //喚起OpNewCreator<G>::Create()
  delete pg;                   
==<end>==============



==<反灰>===========
  WMgr< MallocCreator<W> > wMgr;
  W* pw = wMgr.Create();  //喚起MallocCreator<W>::Create()
  delete pw;

  GMgr< OpNewCreator<G> > gMgr;
  G* pg = gMgr.Create();  //喚起OpNewCreator<G>::Create()
  delete pg;
==<end>==============



==<反灰>===========
//註：以下位於namespace Loki::TL 內。TL代表 "TypeList"
template <class T, class U>
struct Typelist
{
    typedef T Head;
    typedef U Tail;
};
==<end>==============




==<反灰>===========
typedef Typelist<signed char,
           Typelist<short int,	
             Typelist<int, 
              Typelist<long int, NullType> > > >
	      SignedIntegrals;
==<end>==============




==<反灰>===========
class NullType {};  	// 見loki的NullType.h
==<end>==============




==<反灰>===========
//註：以下位於namespace Loki::TL 內。TL代表 "TypeList"
#define TYPELIST_1(T1) Typelist<T1, NullType>
#define TYPELIST_2(T1,T2) Typelist<T1, TYPELIST_1(T2) >
#define TYPELIST_3(T1,T2,T3) Typelist<T1, TYPELIST_2(T2,T3) >
...
#define TYPELIST_50(...) ...
// Loki 把Typelists的長度擴充至 50
==<end>==============



==<反灰>===========
typedef TYPELIST_4(signed char, short int, int, long int)
	SignedIntegrals;
==<end>==============



==<反灰>===========
//泛化
template <class TList, unsigned int index>
struct TypeAt;

//偏特化：當索引為0，第0個位置上的型別是Head — 那便是結果。
template <class Head, class Tail>
struct TypeAt<Typelist<Head, Tail>, 0>
{
     typedef Head Result;
};

//偏特化：當索引不為0，第i個型別就是Tail的第i-1個型別
template <class Head, class Tail, unsigned int i>
struct TypeAt<Typelist<Head, Tail>, i>
{
     typedef typename TypeAt<Tail, i-1>::Result Result;
};
==<end>==============




==<反灰>===========
template <class T>
struct Holder
{
    T value_;
};
==<end>==============



==<反灰>===========
typedef GetScatterHierarchy<
              TYPELIST_3(int, string, W), 
              Holder> ABC;
ABC abc;
==<end>==============




==程式3	Field()以索引i處理第i個元素；返回左值，所以元素既可讀又可寫。<box>===========
//註：摘自Loki的HierarchyGenerator.h
template <class TList, template<class> class Unit>
Unit<typename TList::Head>& 
FieldHelper(GenScatterHierarchy<TList, Unit>& obj, 
			    Int2Type<0>)  
{
    GenScatterHierarchy<typename TList::Head, Unit>& leftBase = obj;
		return leftBase;
}

template <unsigned int i, class TList, template<class> class Unit>
Unit<typename TypeAt<TList, i>::Result>&    
FieldHelper(GenScatterHierarchy<TList, Unit>& obj, 
	            Int2Type<i>)
{
   GenScatterHierarchy<typename TList::Tail, Unit>& rightBase = obj;
	return FieldHelper(rightBase, Int2Type<i-1>());
}

template <unsigned int i, class TList, template<class> class Unit>
Unit<typename TypeAt<TList, i>::Result>&
Field (GenScatterHierarchy<TList, Unit>& obj)
{
	return FieldHelper(obj, Int2Type<i>());
}

//註：為滿足上述程式碼，需要一個能夠將數值轉化為型別的裝置如下。
//以下摘自Loki的ListManip.h
template <int v>
struct Int2Type
{
    enum { value = v };
};
==<end>==============