typelist.h

Ｃ＋＋程序设计新思维中提到的TypeList 实现品

//##############################################################################
//The dnc Library
//Copyright (c) 2005 Dreamsoft 赵纯华
//提供Loki程序库的TypeList功能
//
//	TypeList是整个dnc程序库的基础,为dnc库的实现提供最为基础的设施.这里的代码来自
//Loki程序库,约有改动.
//##############################################################################
//
//Last update: 2005-11-9

#ifndef __DNC_TYPELIST_H__
#define __DNC_TYPELIST_H__

#ifndef __DNC_NULLTYPE_H__
#include "nulltype.h"
#endif

namespace dnc{

//------------------------------------------------------------------------------
// class template TypeList
// The building block of typelists of any length
// Use it through the dnc_typelist_NN macros
// Defines nested types:
//     Head (first element, a non-typelist type by convention)
//     Tail (second element, can be another typelist)
//------------------------------------------------------------------------------

    template <class T, class U>
    struct TypeList
    {
       typedef T Head;
       typedef U Tail;
    };

    namespace TL
    {
//------------------------------------------------------------------------------
// class template Length
// Computes the length of a typelist
// Invocation (TList is a typelist):
// Length<TList>::value
// returns a compile-time constant containing the length of TList, not counting
//     the end terminator (which by convention is NullType)
//------------------------------------------------------------------------------

        template <class TList> struct Length;
        template <> struct Length<NullType>
        {
            enum { value = 0 };
        };
        
        template <class T, class U>
        struct Length< TypeList<T, U> >
        {
            enum { value = 1 + Length<U>::value };
        };

//------------------------------------------------------------------------------
// class template TypeAt
// Finds the type at a given index in a typelist
// Invocation (TList is a typelist and index is a compile-time integral 
//     constant):
// TypeAt<TList, index>::Result
// returns the type in position 'index' in TList
// If you pass an out-of-bounds index, the result is a compile-time error
//------------------------------------------------------------------------------

        template <class TList, unsigned int index> struct TypeAt;
        
        template <class Head, class Tail>
        struct TypeAt<TypeList<Head, Tail>, 0>
        {
            typedef Head Result;
        };

        template <class Head, class Tail, unsigned int i>
        struct TypeAt<TypeList<Head, Tail>, i>
        {
            typedef typename TypeAt<Tail, i - 1>::Result Result;
        };

//------------------------------------------------------------------------------
// class template TypeAtNonStrict
// Finds the type at a given index in a typelist
// Invocations (TList is a typelist and index is a compile-time integral 
//     constant):
// a) TypeAt<TList, index>::Result
// returns the type in position 'index' in TList, or NullType if index is 
//     out-of-bounds
// b) TypeAt<TList, index, D>::Result
// returns the type in position 'index' in TList, or D if index is out-of-bounds
//------------------------------------------------------------------------------

        template <class TList, unsigned int index,
            typename DefaultType = NullType>
        struct TypeAtNonStrict
        {
            typedef DefaultType Result;
        };
        
        template <class Head, class Tail, typename DefaultType>
        struct TypeAtNonStrict<TypeList<Head, Tail>, 0, DefaultType>
        {
            typedef Head Result;
        };
        
        template <class Head, class Tail, unsigned int i, typename DefaultType>
        struct TypeAtNonStrict<TypeList<Head, Tail>, i, DefaultType>
        {
            typedef typename 
                TypeAtNonStrict<Tail, i - 1, DefaultType>::Result Result;
        };

//------------------------------------------------------------------------------
// class template IndexOf
// Finds the index of a type in a typelist
// Invocation (TList is a typelist and T is a type):
// IndexOf<TList, T>::value
// returns the position of T in TList, or NullType if T is not found in TList
//------------------------------------------------------------------------------

        template <class TList, class T> struct IndexOf;
        
        template <class T>
        struct IndexOf<NullType, T>
        {
            enum { value = -1 };
        };
        
        template <class T, class Tail>
        struct IndexOf<TypeList<T, Tail>, T>
        {
            enum { value = 0 };
        };
        
        template <class Head, class Tail, class T>
        struct IndexOf<TypeList<Head, Tail>, T>
        {
        private:
            enum { temp = IndexOf<Tail, T>::value };
        public:
            enum { value = (temp == -1 ? -1 : 1 + temp) };
        };

//------------------------------------------------------------------------------
// class template Append
// Appends a type or a typelist to another
// Invocation (TList is a typelist and T is either a type or a typelist):
// Append<TList, T>::Result
// returns a typelist that is TList followed by T and NullType-terminated
//------------------------------------------------------------------------------

        template <class TList, class T> struct Append;
        
        template <> struct Append<NullType, NullType>
        {
            typedef NullType Result;
        };
        
        template <class T> struct Append<NullType, T>
        {
            typedef TypeList<T,NullType> Result;
        };
        
        template <class Head, class Tail>
        struct Append<NullType, TypeList<Head, Tail> >
        {
            typedef TypeList<Head, Tail> Result;
        };
        
        template <class Head, class Tail, class T>
        struct Append<TypeList<Head, Tail>, T>
        {
            typedef TypeList<Head, 
                    typename Append<Tail, T>::Result>
                Result;
        };

		
        
//------------------------------------------------------------------------------
// class template Erase
// Erases the first occurence, if any, of a type in a typelist
// Invocation (TList is a typelist and T is a type):
// Erase<TList, T>::Result
// returns a typelist that is TList without the first occurence of T
//------------------------------------------------------------------------------

        template <class TList, class T> struct Erase;
        
        template <class T>                         // Specialization 1
        struct Erase<NullType, T>
        {
            typedef NullType Result;
        };

        template <class T, class Tail>             // Specialization 2
        struct Erase<TypeList<T, Tail>, T>
        {
            typedef Tail Result;
        };

        template <class Head, class Tail, class T> // Specialization 3
        struct Erase<TypeList<Head, Tail>, T>
        {
            typedef TypeList<Head, 
                    typename Erase<Tail, T>::Result>
                Result;
        };

//------------------------------------------------------------------------------
// class template EraseAll
// Erases all first occurences, if any, of a type in a typelist
// Invocation (TList is a typelist and T is a type):
// EraseAll<TList, T>::Result
// returns a typelist that is TList without any occurence of T
//------------------------------------------------------------------------------

        template <class TList, class T> struct EraseAll;
        template <class T>
        struct EraseAll<NullType, T>
        {
            typedef NullType Result;
        };