How to declare std::vector with an 'n' dimensional array?

Question

My thoughts are the following:
for e.g. a two-dimensional array:

int a[9][9];
std::vector<int** a> b;

But what if I have

/* I know, it is usually a bad practise to have more than 2 dimensional arrays, but it can happen, when you need it */
int a[3][4][5][6]; 
std::vector<int**** a> b; // ? this just looks bad

That's a vector of pointers, not a multi-dimensional array. Either use a vector of vectors (of vectors...), or a flat vector<int> using arithmetic for multidimensional indexing. — Mike Seymour
– Mike Seymour, Commented May 13, 2015 at 14:47
@Mike Seymour using arithmetic for multidimensional indexing was downvoted by my superiors, because that would make the code hard to read and maintain. But yes, that would be a good solution. — Snowman
– Snowman, Commented May 13, 2015 at 14:49
@Snowman: Wrap the vector in a class, and the arithmetic in an accessor function. Then it can be as easy to read as a(1,2,3) = 42; — Mike Seymour
– Mike Seymour, Commented May 13, 2015 at 14:52

rashmatash · Accepted Answer · 2015-05-13 15:06:48Z

2

Try this:

struct MD_array{ //multidimentional array
   a[3][4][5][6];
};
std::vector<MD_array> b;

Then you can access each array like so:

b[i].a[x][y][z][w] = value;

answered May 13, 2015 at 15:06

rashmatash

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dwcanillas · Accepted Answer · 2015-05-13 15:07:59Z

1

You could also use an alias declaration:

template <typename T, size_t I, size_t J, size_t K, size_t N>
using SomeArr = std::array<std::array<std::array<std::array<T, I>, J>, K>, N>;

int main()
{
    SomeArr<int,3,4,5,6> arr;
    std::vector<SomeArr<int,3,4,5,6>> someVec;
    someVec.push_back(arr);
}

answered May 13, 2015 at 15:07

dwcanillas

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4 Comments

Barry Over a year ago

You could generalize your alias quite a bit. See this question.

dwcanillas Over a year ago

@Barry nice, thanks. Still on VS2012 at work so I havent learned all the stuff that comes with 2013/2015

dwcanillas Over a year ago

@Barry why were structs used in that answer? Is it not possible to be done using only alias declarations?

Barry Over a year ago

Can you think of a way to? I'm fairly sure it's not possible.

Vlad from Moscow · Accepted Answer · 2015-05-13 15:05:23Z

1

You can do this

int a[3][4][5][6]; 
std::vector<int**** a> b;

in two ways like this

int a[3][4][5][6]; 
std::vector<int ( * )[4][5][6]> b; 

b.push_back( a );

and like this

int a[3][4][5][6]; 
std::vector<int ( * )[3][4][5][6]> b; 

b.push_back( &a );

Though it is not clear what you are trying to achieve.:)

edited May 13, 2015 at 15:05

answered May 13, 2015 at 15:02

Vlad from Moscow

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1 Comment

Snowman Over a year ago

What Mike Seymour said in the comments, is what I'd like to achieve :) I just wanted to know, what is the best practise in this case ( I am student, and want to learn the tricks of the trade ). I am already implementing it: "Wrap the vector in a class, and the arithmetic in an accessor function. Then it can be as easy to read as a(1,2,3) = 42"

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