Statically declared 2-D array C++ as data member of a class

Question

I am trying to create a class grid which contains the data members unsigned NR, unsigned NC and it should also contain a 2D array double Coordiantes[NR][NC]. I wish to initialize the data members NR and NC through the class constructor. I am trying to avoid the dynamic allocation of the 2-D array as I prefer contiguous memory allocation so as to avoid the cache misses as much as possible.

I am not sure if it is possible but any inputs would be helpful.

I believe NR and NC must be known at compile time in order to create a static array like this. Have you looked at http://stackoverflow.com/questions/21943621/how-to-create-a-contiguous-2d-array-in-c? — Daniel B., Mar 21 '16 at 05:03
`vector< vector >` would solve the purpose, but I recently came to know accessing `vector< vector > A` is quite slow as compared to `double A[m][n]`. — user3496912, Mar 21 '16 at 05:04
@DanielB. That answer is syntactically correct, but is not idiomatic c++. Better use the RAII idiom. — StoryTeller - Unslander Monica, Mar 21 '16 at 05:23
True, the top answer there is not raii, but the less upvoted (go figure) answers are more in line with what probably ought to be done. — Daniel B., Mar 21 '16 at 16:30

score 15 · Accepted Answer · answered Mar 21 '16 at 05:44

15

class Array2D {
public:
    vector<int> v;
    int nc;
    Array2D(int NR, int NC) : v(NR*NC), nc(NC) {}
    int* operator[](int r) { return &v[r*nc]; }
};

int main()
{
    Array2D array2d(2, 3);
    array2d[0][0] = 1;
    array2d[1][2] = 6;
}

This allows you to create a class that will function like a 2D array. It's fast and the data is contiguous.

answered Mar 21 '16 at 05:44

doug

3,840
1
14
18

This is the cleanest implementation I've seen. Especially nice is the usage of the **built-in operator []** to access the contents without the need to implement it specifically for the second dimension. – SomeWittyUsername Mar 21 '16 at 07:05
@SomeWittyUsername - Thanks! Probably helped that I was originally a C programmer. I do love modern c++ though. – doug Mar 21 '16 at 07:23
Interesting but somewhat risky - with `int*` there is no way to assert a validity of `a[i][j]` expression. – Evg Oct 25 '22 at 02:47
1

@Evg So true. I actually prefer the (r,c) approach and it's easy to bounds conditionally check, It's a minimalist approach with conventional brackets. Simplest thing i could think of. I currently use a ptr, row, and col. All consts in a constexpr class. An even smaller object footprint but very fast. And const correct thanks to c++20. – doug Oct 25 '22 at 03:03
If you can stretch to C++20, returning a `std::span` from the array operator would allow bounds checking. Not zero overhead though, since the extent is dynamic. – Useless Dec 16 '22 at 15:31

StoryTeller - Unslander Monica · Answer 2 · 2016-03-21T05:46:52.673

2

I recommend you simply use std::vector<double>(NC*NR).
And index into it by coord.at(i*NC + j) (or coord[i*NC + j], when you code is correct and you want to glean performance out of it).

You'll get contiguous memory and cache friendly loops without doing your own memory allocation. Always prefer RAII when possible.

edited Mar 21 '16 at 05:46

answered Mar 21 '16 at 05:22

StoryTeller - Unslander Monica

165,132
21
377
458

Statically declared 2-D array C++ as data member of a class

2 Answers2

Linked