拆分可变参数模板参数

问题描述

如何将可变参数模板参数分成两半？例如：

How do I split variadic template arguments in two halves? Something like:

template <int d> struct a {
  std::array <int, d> p, q;
  template <typename ... T> a (T ... t) : p ({half of t...}), q ({other half of t...}) {}
};

推荐答案

我们仍然缺乏很多帮助来操纵可变参数包（或我不知道他们）。

We still lack a lot of helpers to manipulate variadic parameter packs (or I am not aware of them). Until a nice Boost library bring them to us, we can still write our own.

例如，如果你愿意将你的数组初始化推迟到构造函数体中，那么，可以创建和使用将部分参数包复制到输出迭代器的命令：

For example, if you are willing to postpone your arrays initialization to the constructor body, you can create and use a fonction that copies part of the parameter pack to an output iterator:

#include <array>
#include <cassert>
#include <iostream>

// Copy n values from the parameter pack to an output iterator
template < typename OutputIterator >
void copy_n( size_t n, OutputIterator )
{
  assert ( n == 0 );
}

template < typename OutputIterator, typename T, typename... Args >
void copy_n( size_t n, OutputIterator out, const T & value, Args... args )
{
  if ( n > 0 )
  {
    *out = value;
    copy_n( n - 1, ++out, args... );
  }
}

// Copy n values from the parameter pack to an output iterator, starting at
// the "beginth" element
template < typename OutputIterator >
void copy_range( size_t begin, size_t size, OutputIterator out )
{
  assert( size == 0 );
}


template < typename OutputIterator, typename T, typename... Args >
void copy_range( size_t begin, size_t size, OutputIterator out, T value, Args... args )
{
  if ( begin == 0 )
  {
    copy_n( size, out, value, args... );
  }
  else
  {
    copy_range( begin - 1, size, out, args... );
  }
}


template < int N >
struct DoubleArray
{
  std::array< int, N > p;
  std::array< int, N > q;

  template < typename... Args >
  DoubleArray ( Args... args )
  {
    copy_range( 0, N, p.begin(), args... );
    copy_range( N, N, q.begin(), args... );
  }

};

int main()
{
  DoubleArray<3> mya(1, 2, 3, 4, 5, 6);
  std::cout << mya.p[0] << mya.p[2] << std::endl;
  std::cout << mya.q[0] << mya.q[2] << std::endl;
}

// Ouput:
// 13
// 46

正如你所看到的，你可以（不是这样）轻松地创建自己的算法来操纵参数包;所有需要的是一个很好的递归和模式匹配的理解（像做模板MetaProgramming时一样...）。

As you can see, you can (not so) easily create your own algorithms to manipulate parameter packs; all is needed is a good understanding of recursion and pattern matching (as always when doing Template MetaProgramming...).

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