我可能弄错了,但它不是类似于您正在寻找的下面的代码吗?
#include <iostream>
#include <utility>
class nill
{
};
template<typename KEY, typename VALUE, VALUE v>
class parameter
{
public:
typedef KEY key_type;
typedef VALUE value_type;
static constexpr VALUE value = v;
};
template<typename ...>
class parameter_pack
{
};
template<typename KEY, typename P>
class get_param;
template<typename KEY, typename H, typename ...TAIL>
class get_param<KEY, parameter_pack<H, TAIL...>>
{
private:
typedef typename get_param<KEY, H>::type result;
public:
typedef typename std::conditional<std::is_same<result, nill>::value,
typename get_param<KEY, parameter_pack<TAIL...>>::type, result>::type
type;
};
template<typename KEY>
class get_param<KEY, parameter_pack<>>
{
public:
typedef nill type;
};
template<typename KEY, typename K, typename V, V v>
class get_param<KEY, parameter<K, V, v>>
{
public:
typedef typename std::conditional<std::is_same<K, KEY>::value,
parameter<K, V, v> , nill>::type type;
};
template<unsigned T>
class tag
{
};
int main()
{
typedef parameter_pack
<
parameter_pack
<
parameter<tag<0>, int, 0>
>,
parameter_pack
<
parameter<tag<4>, int, 42>,
parameter<tag<2>, int, 1>,
parameter_pack
<
parameter<tag<3>, int, 100>,
parameter<tag<4>, int, 5>,
parameter<tag<0>, int, 33>,
parameter<tag<2>, int, 666>
>
>,
parameter<tag<1>, int, -1>
> pack;
typedef typename get_param<tag<4> , pack>::type param;
std::cout << param::value << '\n';
return 0;
}
输出:
42
由 GCC 4.6 编译
编辑以允许运行时值修改:
对于值的运行时修改,只需将参数类修改为以下内容:
template<typename KEY, typename VALUE>
class parameter
{
public:
typedef KEY key_type;
typedef VALUE value_type;
static VALUE value;
};
template<typename KEY, typename VALUE>
VALUE parameter<KEY, VALUE>::value;
它不再有一个常量值成员。
请注意,该值是一个静态变量,这意味着使用特定键更改一个条目的值会更改所有其他条目的值。为了克服这个问题,可以简单地修改“标记系统”,以便可以将不同类型的两个标记视为相同的键,然后允许用户指定应该使用哪个元函数 get_param 来正确识别所需的键(而不是总是使用 std::is_same)。因此,现在允许在运行时修改值的完整示例如下所示:
#include <iostream>
#include <utility>
class nill
{
};
template<typename KEY, typename VALUE>
class parameter
{
public:
typedef KEY key_type;
typedef VALUE value_type;
static VALUE value;
};
template<typename KEY, typename VALUE>
VALUE parameter<KEY, VALUE>::value;
template<typename ...>
class parameter_pack
{
};
template<template<typename> class MF, typename P>
class get_param;
template<template<typename> class MF, typename H, typename ...TAIL>
class get_param<MF, parameter_pack<H, TAIL...>>
{
private:
typedef typename get_param<MF, H>::type result;
public:
typedef typename std::conditional<std::is_same<result, nill>::value,
typename get_param<MF, parameter_pack<TAIL...>>::type, result>::type
type;
};
template<template<typename> class MF>
class get_param<MF, parameter_pack<>>
{
public:
typedef nill type;
};
template<template<typename> class MF, typename K, typename V>
class get_param<MF, parameter<K, V>>
{
public:
typedef typename std::conditional<MF<K>::value,
parameter<K, V> , nill>::type type;
};
template<unsigned T, unsigned U = 0>
class tag
{
};
template<typename K1, typename K2>
class compare_tag;
template<unsigned T1, unsigned U1, unsigned T2, unsigned U2>
class compare_tag<tag<T1, U1>, tag<T2, U2>>
{
public:
static constexpr bool value = T1 == T2;
};
template<typename T>
class find4 : public compare_tag<T, tag<4>>
{};
template<typename T>
class find2 : public compare_tag<T, tag<2>>
{};
template<typename T>
class find1 : public compare_tag<T, tag<1>>
{};
int main()
{
typedef parameter_pack
<
parameter_pack
<
parameter<tag<0, 0>, int>
>,
parameter_pack
<
parameter<tag<4, 0>, int>,
parameter<tag<2, 0>, int>,
parameter_pack
<
parameter<tag<3, 0>, int>,
parameter<tag<4, 1>, int>,
parameter<tag<0, 1>, int>,
parameter<tag<2, 1>, int>
>
>,
parameter<tag<1, 0>, int>
> pack;
std::cin >> get_param<find4, pack>::type::value;
std::cin >> get_param<find2, pack>::type::value;
std::cin >> get_param<find1, pack>::type::value;
std::cout << get_param<find4, pack>::type::value << '\n';
std::cout << get_param<find2, pack>::type::value << '\n';
std::cout << get_param<find1, pack>::type::value << '\n';
return 0;
}
输出:
42
666
0
42
666
0