C++ template using multidimensional arrays

Question

Is it possible in C++ to write this:

template <typename T, size_t dim, size_t len>
using A = /* something */;

that will make these two lines equivalent:

/* 1 */ A<int, 3, 5> a;   A<char, 5, 3> c;

/* 2 */ int a[5][5][5];   char c[3][3][3][3][3];

?

Please explain more about "equivalent". Maybe show some lines of code you want to work and what effect you want them to have. — Yunnosch
– Yunnosch, Commented Nov 18, 2017 at 7:45

Brian Bi · Accepted Answer · 2017-11-18 07:49:39Z

5

Is it possible? Yes, you can do arbitrarily complex calculations at compile-time, so there certainly is a solution.

The most straightforward way is to just use template recursion, like so:

template <class T, size_t dim, size_t len>
struct A_helper {
    using type = typename A_helper<T, dim - 1, len>::type[len];
};

template <class T, size_t len>
struct A_helper<T, 0, len> {
    using type = T;
};

template <class T, size_t dim, size_t len>
using A = typename A_helper<T, dim, len>::type;

See it on Coliru: http://coliru.stacked-crooked.com/a/bfc9052b30bce553

answered Nov 18, 2017 at 7:49

Brian Bi

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3 Comments

iBug Over a year ago

Can I directly use template<typename T, size_t dim, size_t len> using A = A<T, dim-1, len>[len]?

iBug Over a year ago

i.e. is a helper class necessary?

Brian Bi Over a year ago

@iBug You need something you can partially specialize in order to provide the base case, and alias templates cannot be (partially or wholly) specialized.

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