templatezhou.cpp

template实例 可以测试编译器支持的标准程度

#include <stdio.h> 

struct NullType{};

template<int i> struct Int{enum{value=i};}; 
//class的常量是int 
template<typename L, typename R> 
struct Pair 
{ 
    typedef L Left; 
    typedef R Right; 
}; 

#define IntToPair(x) Pair<Int<x>, NullType> 
/*  Here are some facilities to manipulate int lists    */ 

/*  1.  Get the length of an int list   */ 
template<typename T> 
struct Length 
{ 
    enum{result = 0}; 
}; 

template<typename L, typename R> 
struct Length<Pair<L, R> > 
{ 
    enum{result = 1+Length<R>::result }; 
}; 
/*  2.  Display an int list */ 
template<typename T> 
struct Display 
{ 
    void show() 
    { 
        printf("Unknown"); 
    } 
}; 

template<> 
struct Display<NullType> //特例化的标志，类名后有<> 
{ 
    void show() 
    { 
        printf("Null"); 
    } 
}; 

template<int i> 
struct Display<Int<i> > 
{ 
    void show() 
    { 
        printf("%d", i); 
    } 
}; 

template<typename L, typename R> 
struct Display<Pair<L, R> > 
{ 
    void show() 
    { 
        printf("( "); 
        Display<L>().show(); 
        printf("  "); 
        Display<R>().show(); 
        printf(") "); 
    } 
}; 

/*  3.  Add an int value to the end of an int list  */ 
template<typename T, int i> 
struct Append 
{ 
    typedef IntToPair(i) Type; 
}; 

template<typename L, typename R, int i> 
struct Append<Pair<L, R>, i> 
{ 
    typedef Pair<L, typename Append<R, i>::Type> Type; 
}; 

/*  4.  Reverse an int list */ 
template<typename T> 
struct Reverse 
{ 
    typedef NullType Type; 
}; 

template<int l, typename R> 
struct Reverse<Pair<Int<l>, R> > 
{ 
    typedef typename Append<typename Reverse<R>::Type, l>::Type Type; 
}; 

/*  5.  Concatenate two int lists   */ 
template<typename L, typename R> 
struct Concatenate 
{ 
    typedef Pair<L, Pair<R, NullType> > Type; 
}; 

template<typename R> 
struct Concatenate<NullType, R> 
{ 
    typedef Pair<R, NullType> Type; 
}; 

template<typename L, typename R> 
struct Concatenate<NullType, Pair<L,R> > 
{ 
    typedef Pair<L, R> Type; 
}; 

template<typename L1, typename R1, typename L2, typename R2> 
struct Concatenate<Pair<L1, R1>, Pair<L2, R2> > 
{ 
    typedef Pair<L1, typename Concatenate<R1, Pair<L2, R2> >::Type> Type; 
}; 

/*  6.  Insert an int to a SORTED int list  */ 
template<bool, typename T, typename F> 
struct If 
{ 
    typedef T Type; 
}; 

template<typename T, typename F> 
struct If<false, T, F> 
{ 
    typedef F Type; 
}; 

template<typename T, int i> 
struct Insert 
{ 
    typedef IntToPair(i) Type; 
}; 

template<int l, typename R, int i> 
struct Insert<Pair<Int<l>, R>, i > 
{ 
    typedef typename If< l<i, 
        Pair<Int<l>, typename Insert<R, i>::Type>, 
        Pair<Int<i>, Pair<Int<l>, R> > >::Type Type; 
}; 

/*  7.  Sort an int list    */ 
template<typename T> 
struct Sort 
{ 
    typedef NullType Type; 
}; 

template<int l, typename R> 
struct Sort<Pair<Int<l>, R> > 
{ 
    typedef typename Insert<typename Sort<R>::Type, l>::Type Type; 
}; 

/*  8.  Some macros to simplify the operations  */ 
#define pair(a, b) Pair<a, b> 
#define cons(i, T) Pair<Int<i>, T > 
#define length(list) Length<list>::result 
#define assign(name, type) typedef type name 
#define append(list, i) Append<list, i>::Type 
#define reverse(list) Reverse<list>::Type 
#define concat(a, b) Concatenate<a, b>::Type 
#define insert(list, i) Insert<list, i>::Type 
#define sort(list) Sort<list>::Type 
#define show(list)                                   \
    do{ printf(#list ": \n");                        \
    Display<list >().show();                         \
    printf("\n"); }while(0) 

assign(MyPhoneNum, cons(1, cons(3, cons(9, cons(5, cons(1, cons(0, 
          cons(7, cons(1,cons(7, cons(8, IntToPair(4)))))))))))); 

int main() 
{ 
    show(MyPhoneNum); 
    show(cons(9, MyPhoneNum)); 
    show(append(MyPhoneNum, 888)); 
    show(sort(MyPhoneNum)); 
    show(insert(sort(MyPhoneNum), 6)); 

    assign(ReverseNum, reverse(MyPhoneNum)); 
    show(ReverseNum); 
    show(pair(MyPhoneNum, MyPhoneNum)); 
    show(concat(MyPhoneNum, ReverseNum)); 

    printf("Numbers: %d\n", length(MyPhoneNum)); 
    return 0; 
}