C++ Templates (给模板参数命名)

 

书中的例子实现手法使用多重/虚拟继承, 实现手法感觉比较诡秘. 但是至少告诉我是可以实现的.

于是干脆自己也练了练手,  博君一笑. 只在VC7.1下测试过, VC6也许可能可以迂回实现, 但是估计工作量太大.

1. 首先需要一个基本的 If 语句. 

template <bool, class T, class U>
struct if_
{
    typedef T type;
};

template<class T, class U>
struct if_<false,  T,  U>
{
    typedef U type;
};

2. 然后使用一个 type_selector meta-function,N表示第几个默认参数(注意我的默认Policy参数DefaultPolicyArgs里面有一个meta data, 为0.如果是用户定义的Policy, 那么形如Policy2_is的模板类里面有一个meta data为2. 这个数字主要是用于定位.

最后的DefaultType是当扫描一遍, 发现没有任何对应N位置的自定义Policy参数, 那么就取这个为默认值, 结束递归.(下面的4个void的特化版本就是干这个的)

template< 
    int N,
    class T1,
    class T2,
    class T3,
    class T4,
    class DefaultType>
struct type_selector
{
    typedef typename if_ <
        (T1::value == N), 
        T1, 
        type_selector<N, T2, T3, T4, void, DefaultType> >::type eval_type;

    typedef typename eval_type::type type;
};

//shift以后, 如果都是默认值, 递归会来到这里, 结束.
template<
    int N,
    class DefaultType>
struct type_selector<N, void, void, void, void, DefaultType>
{
    typedef DefaultType type;
};

struct DefaultPolicy1 {};
struct DefaultPolicy2 {};
struct DefaultPolicy3 {
  public:
    static void doPrint() {
        std::cout << "DefaultPolicy3::doPrint()/n";
    }
};
class DefaultPolicy4 {};

struct  DefaultPolicyArgs {
    static const int value = 0;
};


template <typename Policy>
struct Policy1_is
{
    typedef Policy type;
    static const int value = 1;
};

template <typename Policy>
struct Policy2_is
{
    typedef Policy type;
    static const int value = 2;
};


template <typename Policy>
struct Policy3_is
{
    typedef Policy type;
    static const int value = 3;
};


template <typename Policy>
struct Policy4_is
{
    typedef Policy type;
    static const int value = 4;
};


template<class T1, class T2, class T3, class T4>
struct PolicySelector
{
    typedef typename type_selector<1, T1, T2, T3, T4, DefaultPolicy1>::type P1;
    typedef typename type_selector<2, T1, T2, T3, T4, DefaultPolicy2>::type P2;
    typedef typename type_selector<3, T1, T2, T3, T4, DefaultPolicy3>::type P3;
    typedef typename type_selector<4, T1, T2, T3, T4, DefaultPolicy4>::type P4;
};
    

template <typename T1 = DefaultPolicyArgs,
          typename T2 = DefaultPolicyArgs,
          typename T3 = DefaultPolicyArgs,
          typename T4 = DefaultPolicyArgs>
class BreadSlicer {
    typedef typename PolicySelector<T1, T2, T3, T4> Policies;
    
  public:
    void print () {
        std::cout << typeid(Policies::P3).name() << std::endl;
        Policies::P3::doPrint();
    }
    void  print_2()
    {
        std::cout << typeid(Policies::P2).name() << std::endl;
        Policies::P2::print_2();
    }
    //...
};


//下面的就是测试代码了.

class CustomPolicy {
  public:    
    static void doPrint() {
        std::cout << "CustomPolicy::doPrint()/n";
    }
};

class CustomPolicy2 {
public:
    static void print_2()
    {
        std::cout << "Here is CustomPolicy2 instance" << std::endl;
    }
};
    
int main()
{
    BreadSlicer<> bc1;
    bc1.print();

    BreadSlicer< Policy3_is<CustomPolicy>,
        Policy2_is<CustomPolicy2> > bc2;
    
    bc2.print();
    return 0;
}