Question

看到这个问题，Best STL transform - like template function for ternary operators，认为对于踢腿和傻笑进行一次改编是很酷的。

所以我把它扔在一起，似乎工作......有一个空洞。我的preincrement方法对我来说很奇怪。没有拆包，我无法弄清楚如何处理。我希望语法类似(++args)...（类似于(*args)...的deference语法），但会返回错误。有谁知道怎么做？

#include <iostream>
#include <vector>

inline void preincrement(){}

template <typename T,typename... Ts>
inline void preincrement(T& t,Ts&... ts)
{
  ++t;
  preincrement(ts...);
}
template <class OutputIterator, class NArryOperator, class InputIterator, class... InputIterators>
OutputIterator transform_n(OutputIterator out_iter, NArryOperator oper, InputIterator iter1begin, InputIterator iter1end,InputIterators... args)
{
  for (; iter1begin != iter1end; ++out_iter,++iter1begin,preincrement(args...))
  {
    *out_iter = oper(*iter1begin,(*args)...);
  }
  return out_iter;
}

template <typename T>
struct noop
{
  T operator()(const T& val){return val;}
};

template <typename Ot,typename T,typename... Ts>
struct nsum
{
  Ot operator()(const T& t,const Ts&... ts)
  {
    return (Ot)t+nsum<Ot,Ts...>()(ts...);
  }
};

template <typename Ot, typename T>
struct nsum<Ot,T>
{
  Ot operator()(const T& t)
  {
    return (Ot)t;
  }
};

int main(int argc,char** argv)
{
  std::vector<int> rng;
  for (int i = 0; i < 10; i++) {rng.push_back(i);}
  std::vector<int> rng1 = rng;
  std::vector<int> rng2 = rng;
  std::vector<int> rng3 = rng;
  std::vector<float> out(rng.size());
  auto beg = out.begin();
  auto end = transform_n(beg,nsum<double,int,int,int,int>(),
  rng.begin(),rng.end(),
  rng1.begin(),
  rng2.begin(),
  rng3.begin());

  for (auto i = beg; i != end; ++i)
  {
    std::cout << *i << std::endl;
  }
}

Answer 1

直接使用后增量的解决方案：

template <class OutputIterator, class NArryOperator,
          class InputIterator, class... InputIterators>
OutputIterator transform_n(OutputIterator out_iter, NArryOperator oper,
                           InputIterator iter1begin, InputIterator iter1end,
                           InputIterators... args)
{
  while(iter1begin != iter1end)
  {
    *out_iter++ = oper(*iter1begin++, (*args++)...);
  }
  return out_iter;
}

关于递增for语句的“increment-expression”：

你不能扩展for-statement的“increment-expression”;包扩展仅限于几个上下文，因此operator,重载不能应用[temp.variadic] / 4。通常的技巧是必需的，例如扩展函数调用（使用的是什么）或使用虚拟数组：

using iarr = int[];
for (; iter1begin != iter1end;
     ++out_iter,++iter1begin,(void)iarr{0, (++args,void(),0)...})

如何对可变参数模板参数进行预增量？

1 个答案: