我们不能从initializer_list创建std :: array，但是可以使用带有可变参数的辅助函数来创建它吗？

Question

Please consider the following vector class:

template<typename T, class Tuple = std::vector<T>>
class vector
{
public:
    using size_type = typename Tuple::size_type;

    template<class T = Tuple, class = std::enable_if_t<std::is_constructible<T, size_type>::value>>
    vector(size_type n)
        : m_elements(n)
    { }
    template<class T = Tuple, class = std::enable_if_t<std::is_constructible<T, std::initializer_list<T>>::value>>
    vector(std::initializer_list<T> init)
        : m_elements(init)
    { }

private:
    Tuple m_elements;
}; // class vector

template<typename T, std::size_t N>
using static_vector = vector<T, std::array<T, N>>;

template<typename T>
static_vector<T, /* N */> make_static_vector(T... elements) { /* ... */ }

The problem is that I want to use std::array for Tuple , but std::array is not constructible from an initialiter_list . That's why I've disabled the corresponding constructor of vector in that case. (I know that I could use std::copy(init.begin(), init.end(), m_elements.begin() in either case, but that would yield other problems.)

Since I would like to do something like static_vector<double> x = { 1, 2, 3 }; I thought my best option (feel free to correct me) to provide a helper function like make_static_vector . (That's not as beautiful as an initializer_list , so, again, please feel free to come up with a more beautiful solution.)

I want to write auto x = make_static_vector<double>(1, 2, 3); . How do I need to implement it?

Answer 1

A simple form looks like this:

template <typename T,typename... Elements>
static_vector<T, sizeof...(Elements)>
  make_static_vector(Elements... elements)
{
    return static_vector<T,sizeof...(Elements)>(elements...);
}

You will also need to make a variadic constructor for your vector:

template <typename... Args>
vector(Args... init)
    : m_elements{static_cast<T>(init)...}
{ }

The static_cast is necessary if you want to avoid narrowing errors using ints where doubles are required.

If you want to use perfect forwarding, it's a bit more to type, but it's the same idea.