Elegantly define multi-dimensional array in modern C++

Question

Defining multi-dimensional array using the T[][][] syntax is easy. However, this creates a raw array type which doesn't fit nicely into modern C++. That's why we have std::array since C++11. But the syntax to define a multi-dimensional array using std::array is quite messy. For example, to define a three-dimensional int array, you would have to write std::array<std::array<std::array<int, 5>, 5>, 5>. The syntax doesn't scale at all. I'm asking for a fix for this issue. Maybe, this issue cannot be fixed using existing utility provided by C++. In that case, I'm happy with a custom tool developed to ease the syntax.

Found a solution myself:

template <typename T, std::size_t n, std::size_t... ns>
struct multi_array {
  using type = std::array<typename multi_array<T, ns...>::type, n>;
};

template <typename T, std::size_t n>
struct multi_array<T, n> {
  using type = std::array<T, n>;
};

template <typename T, std::size_t... ns>
using multi_array_t = typename multi_array<T, ns...>::type;

Wondering whether the implementation can be further simplified.

Azalea

@AngryLettuce I think that would be a little too heavy for simple matters like this. I don't want to pull in a whole Boost just for this. – Azalea

Are typedef's not acceptable: typedef std::array< std::array< std::array<int, 5 > > > threeD; .. threeD arr; arr[0][0][0] ...? This example would have to be modified to allow for different types, obviously, but curious if that might help? – Semaphore

You could use the approach in the answers of the question at #24463856. – Toomer

Also https://mcmap.net/q/535211/-less-verbose-way-to-declare-multidimensional-std-array/636019 – the same question, but tagged c++11. – Yseulta

Note that the memory layout of a nested std::array isn't guaranteed to be the same as that of a C=style array of arrays (at least in C++11, I'm not sure if that got fixed in C++14.) – Pentlandite

@Pentlandite Isn't std::array just a simple wrapper around raw array? – Azalea

@Azalea But the standard allows it to have some padding at the end (or at least it did in C++11.) – Pentlandite

Main difficulty is access using indices. – Duero

@Pentlandite Nested std::array are stored in contiguous mem locations. See https://mcmap.net/q/356473/-does-std-array-of-std-array-have-contiguous-memory – Deafmute

@MandeepSingh Yes. My comment still stands. – Pentlandite

@Pentlandite Ah yes, you are right, read your comment again and you are not talking about contiguous mem location but probably about padding which might occur :) – Deafmute

Answer 1

Refer to Multi-dimensional arrays in C++11

template <class T, std::size_t I, std::size_t... J>
struct MultiDimArray 
{
  using Nested = typename MultiDimArray<T, J...>::type;
  // typedef typename MultiDimArray<T, J...>::type Nested;
  using type = std::array<Nested, I>;
  // typedef std::array<Nested, I> type;
};

template <class T, std::size_t I>
struct MultiDimArray<T, I> 
{
  using type = std::array<T, I>;
  // typedef std::array<T, I> type;
};

MultiDimArray<float, 3, 4, 5, 6, 7>::type floats;

MultiDimArray is a pair of meta-functions to compute nested type for multi-dimensional std::array. The most general MultiDimArray is a variadic template of unsigned integers to pass an arbitrary number of dimensions. The terminating MultiDimArray specialization defines the simplest case of single dimensional std::array.

Dorison

Could the implementation be further simplified? – Azalea

Its usage can be a tiny bit more improved by using template alias on top of it: template <class T, size_t I, size_t... J> using MultiDimArray_t = typename MultiDimArray<T, I, J...>::type; MultiDimArray_t<float, 3, 4, 5, 6, 7> floatArr; – Deafmute