Class 构造函数的模板参数包扩展

Question

我想制作一个 class 模板 RestrictedInteger，它只能用编译时已知的某些值来构造。这是我可以手动完成的方式：

// Wrapper
template<int... Is> using IntList = std::integer_sequence<int, Is...>;

// This is my class
template<class intList> class RestrictedInteger;
template<int I1>
class RestrictedInteger<IntList<I1>> {
  const int _i;
public:
  constexpr RestrictedInteger(std::integral_constant<int, I1>) : _i(I1) {}
};
//[...]
template<int I1, I2, I3>
class RestrictedInteger<IntList<I1, I2, I3>> {
  const int _i;
public:
  constexpr RestrictedInteger(std::integral_constant<int, I1>) : _i(I1) {}
  constexpr RestrictedInteger(std::integral_constant<int, I2>) : _i(I2) {}
  constexpr RestrictedInteger(std::integral_constant<int, I3>) : _i(I3) {}
};
//[...] (and so on)

当然，我想改用可变参数模板。 如果这是合法的:

template<int... Is>
class RestrictedInteger<IntList<Is...>> {
  int _i;
public:
  constexpr RestrictedInteger(std::integral_constant<int, Is>) : _i(Is) {}... // ERROR
}

因为我使用的是 C++17，所以我认为它会像这样工作：

template<int... Is>
class RestrictedInteger<IntList<Is...>> {
  int _i;
public:
  template<int I>
  constexpr RestrictedInteger(std::enable_if_t<...||(I==Is), std::integral_constant<int, I>>) : _i(I) {} // syntax error: '...' (Visual Stuio 2019)
};

但显然不是。

有解决此问题的巧妙方法吗？

Answer 1

如果编译失败是一个选项（您不需要编译器来查找其他重载）- 您可以将 static_assert 放入构造函数中：

#include <type_traits>
#include <utility>

template<int... Is> using IntList = std::integer_sequence<int, Is...>;

template<class intList> class RestrictedInteger;

template<int... Is>
class RestrictedInteger<IntList<Is...>> {
private:
  const int _i;
public:
  template <int I>
  constexpr RestrictedInteger(std::integral_constant<int, I>) : _i(I) 
  {
      static_assert(((I == Is) || ...), "Invalid value");
  }
};

int main()
{
    RestrictedInteger<IntList<1, 2, 3>> i = std::integral_constant<int, 3>();
    RestrictedInteger<IntList<1, 2, 3>> ii = std::integral_constant<int, 6>(); // fails
}

或使用 std::enable_if 的更详细的解决方案：

#include <type_traits>
#include <utility>

template<int... Is> using IntList = std::integer_sequence<int, Is...>;

template<class intList> class RestrictedInteger;

template<int... Is>
class RestrictedInteger<IntList<Is...>> {
private:
  const int _i;
public:
  template <int I, typename std::enable_if_t<((I == Is) || ...)>* = nullptr>
  constexpr RestrictedInteger(std::integral_constant<int, I>) : _i(I) 
  {
  }
};

int main()
{
    RestrictedInteger<IntList<1, 2, 3>> i = std::integral_constant<int, 3>();
    RestrictedInteger<IntList<1, 2, 3>> ii = std::integral_constant<int, 6>(); // fails
}

Class 构造函数的模板参数包扩展

Class template parameter pack expansion for constructors

c++

template-meta-programming

variadic-templates