Modern C++ Design 学习笔记第3章

1.Length

template <class T, class U>
struct Typelist
{
    typedef T Head;
    typedef U Tail;
};
class NullType {};
template <class TList> struct Length;
//特化
template <> struct Length<NullType>
{
    enum { value = 0 };
};
//递归式
template <class T, class U>
struct Length< Typelist<T, U> >
{
    enum { value = 1 + Length<U>::value };
};

#define LOKI_TYPELIST_1(T1) Typelist<T1, NullType>

#define LOKI_TYPELIST_2(T1, T2) Typelist<T1, LOKI_TYPELIST_1(T2) >

#define LOKI_TYPELIST_3(T1, T2, T3) Typelist<T1, LOKI_TYPELIST_2(T2, T3) >

int main() 
{ 
    //3
    std::size_t n=Length<LOKI_TYPELIST_3(int,int,int)>::value;
}

2.typedef typename的定义

见此

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;
};

http://blog.163.com/cp7618@yeah/blog/static/70234777201122111034365/

http://www.cnblogs.com/carekee/articles/1748675.html

typename告诉编译器, 后面的消息XXX::type是一个已经存在于某处的类型的名字(type name)

3.TypeAtNonStrict

加了一个默认值DefaultType

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;
};

4.IndexOf

也是递归式,定义了一个temp临时变量

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) };
};

5.Append

6.Erase