如何使用SFINAE启用返回“自动"的成员函数 [英] how to SFINAE for enabling member function returning `auto`

问题描述

在模板元编程中，可以对返回类型使用SFINAE来选择某个模板成员函数，即

In template meta programming, one can use SFINAE on the return type to choose a certain template member function, i.e.

 template<int N> struct A {
   int sum() const noexcept
   { return _sum<N-1>(); }
 private:
   int _data[N];
   template<int I> typename std::enable_if< I,int>::type _sum() const noexcept
   { return _sum<I-1>() + _data[I]; }
   template<int I> typename std::enable_if<!I,int>::type _sum() const noexcept
   { return _data[I]; }
};

但是，如果有问题的函数(在上面的示例中为_sum())具有自动检测到的返回类型(例如在此示例中为_func())，则此方法将不起作用

However, this won't work if the function in question (_sum() in above example) has auto-detected return type, such as _func() in this example

template<int N> class A
{
   /* ... */
private:
  // how to make SFINAE work for _func() ?
  template<int I, typename BinaryOp, typename UnaryFunc>
  auto _func(BinaryOp op, UnaryFunc f) const noexcept -> decltype(f(_data[0]))
  { return op(_func<I-1>(op,f),f(_data[I])); }
};

要在这里获得SFINAE还可以做些什么?

What else can be done to get SFINAE here?

推荐答案

在DavidRodríguez-dribeas之后，以下代码按预期工作:

Following David Rodríguez - dribeas, the following code worked as intended:

template<int N> class A
{
  int min_abs() const noexcept
  {
    return _func<N-1>([](int x, int y)->int { return std::min(x,y); },
                      [](int x)->int { return std::abs(x); });
  }
private:
  int _data[N];

  template<int I, typename BinaryOp, typename UnaryFunc>
  auto _func(BinaryOp op, UnaryFunc f) const noexcept
    -> typename std::enable_if< I>,decltype(f(_data[0]))>::type
  { return op(_func<I-1>(op,f),f(_data[I])); }

  template<int I, typename BinaryOp, typename UnaryFunc>
  auto _func(BinaryOp op, UnaryFunc f) const noexcept
    -> typename std::enable_if<!I>,decltype(f(_data[0]))>::type
  { return f(_data[I]); }
};

严格来说，我们还必须确保二进制运算符op返回正确的类型.为了使答案简洁明了，我将其留给读者自己弄清楚...

strictly speaking, we must also ensure that the binary operator op returns the correct type. For simplicity and brevity of the answer, I leave that for the reader to figure out ...

这篇关于如何使用SFINAE启用返回“自动"的成员函数的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！