解压缩多个参数包以初始化2d数组

Question

我正在尝试解压缩多个参数包，以便arr[i][j] = X(i,j)。然而，我在下面的当前方法中出现了以下错误。是否有使用C++14/17实现上述目标的方法？

// Non copyable and default constructable
struct X
{
X(const X&) = delete;
X(int x, int y) : _x(x), _y(y)
{}
int _x;
int _y;
};
X getX(int i, int j)
{
return X(i,j);
}

template <std::size_t ...Xidx, std::size_t ... Yidx>
auto make_2d_array(std::index_sequence<Xidx...>, std::index_sequence<Yidx...>)
{
std::array<std::array<X,sizeof...(Xidx)>, sizeof...(Yidx)> arr = {getX(Xidx, Yidx)...};
return arr;
}
static constexpr std::size_t M = 5;
static constexpr std::size_t N = 6;
int main()
{
std::array<std::array<X,M>, N> arr = make_2d_array(std::make_index_sequence<M>{}, std::make_index_sequence<N>{});
}

错误

<source>:26:87: error: pack expansion contains parameter packs 'Xidx' and 'Yidx' that have different lengths (5 vs. 6)
std::array<std::array<X,sizeof...(Xidx)>, sizeof...(Yidx)> arr = {getX(Xidx, Yidx)...};
     ~~~~  ~~~~ ^
<source>:35:42: note: in instantiation of function template specialization 'make_2d_array<0, 1, 2, 3, 4, 0, 1, 2, 3, 4, 5>' requested here
std::array<std::array<X,M>, N> arr = make_2d_array(std::make_index_sequence<M>{}, std::make_index_sequence<N>{});
^
1 error generated.
Compiler returned: 1

Answer 1

我会做这个

template<std::size_t... XIs, std::size_t... YIs>
std::array<std::array<X, sizeof...(XIs)>, sizeof...(YIs)> make_2d_array(std::index_sequence<XIs...>, std::index_sequence<YIs...>)
{
return {
[](auto YI) -> std::array<X, sizeof...(XIs)> {
return {getX(XIs, YI)...};
}(YIs)...
};
}

是的，有点可怕。原理很简单，我们需要在语法上充分分离XIs和YIs，使每个...展开只打开其中一个，并且我们需要两个展开来完成"；产品"；而不是"；拉链"；。