如何创建类型列表的笛卡尔积？ [英] How to create the Cartesian product of a type list?

问题描述

我想使用可变参数模板创建一个类型列表的叉积。

I'd like to create the cross product of a list of types using variadic templates.

这里是我到目前为止：

#include <iostream>
#include <typeinfo>
#include <cxxabi.h>

template<typename...> struct type_list {};

template<typename T1, typename T2> struct type_pair {};

template<typename T, typename... Rest>
  struct row
{
  typedef type_list<type_pair<T,Rest>...> type;
};

template<typename... T>
  struct cross_product
{
  typedef type_list<typename row<T,T...>::type...> type;
};

int main()
{
  int s;
  typedef cross_product<int, float, short>::type result;
  std::cout << abi::__cxa_demangle(typeid(result).name(), 0, 0, &s) << std::endl;

  return 0;
}

此程式输出：

$ g++ -std=c++0x cross_product.cpp ; ./a.out 
type_list<type_list<type_pair<int, int>, type_pair<int, float>, type_pair<int, short> >, type_list<type_pair<float, int>, type_pair<float, float>, type_pair<float, short> >, type_list<type_pair<short, int>, type_pair<short, float>, type_pair<short, short> > >

但我想输出：

type_list<type_pair<int,int>, type_pair<int,float>, type_pair<int,short>, type_pair<float,int>,...>

也就是说，没有嵌套的 type_list 。

That is, without the nested type_lists.

如果没有行帮助，有没有直接的方法，或者解决方案解开 c>

Is there a direct way to do this without the row helper, or should the solution "unwrap" the nested type_lists somehow?

推荐答案

不知怎的，我的大脑被炸了：I认为我使用的代码比所需的更多，但至少，它有所需的结果（虽然我没有修复内存泄漏）：

Somehow my brain is fried: I think I'm using more code than is needed but, at least, it has the desired results (although I didn't fix the memory leak):

#include <iostream>
#include <typeinfo>
#include <cxxabi.h>

template<typename...> struct type_list {};

template<typename T1, typename T2> struct type_pair {};

template<typename T, typename... Rest>
  struct row
{
  typedef type_list<type_pair<T,Rest>...> type;
};

template <typename... T> struct concat;
template <typename... S, typename... T>
struct concat<type_list<S...>, type_list<T...>>
{
    typedef type_list<S..., T...> type;
};

template <typename... T>
struct expand
{
    typedef type_list<T...> type;
};
template <> struct expand<> { typedef type_list<> type; };
template <typename... T, typename... L>
struct expand<type_list<T...>, L...>
{
    typedef typename concat<typename expand<T...>::type, typename expand<L...>::type>::type type;
};

template<typename... T>
  struct cross_product
{
    typedef typename expand<type_list<typename row<T,T...>::type...>>::type type;

};

int main()
{
  int s;
  typedef cross_product<int, float, short>::type result;
  std::cout << abi::__cxa_demangle(typeid(result).name(), 0, 0, &s) << std::endl;

  return 0;
}

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