看C++ Templates 16.1 Named Template Arguments
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;
}
