what is the best modern c++ approach to construct and manipulate a 2d array [closed]

Question

As in the title, I got to find tons of solutions doing this. Most people would say std::array<4, std::array<4, int>> or std::vector<std::vector<int>> are good, which, however, I find looks teidous and hard to extend to multi-dimensional arrays. Also, I fear that these arrays are actually object-arrays which is likely to be with some overhead (I guess so, not really certain). So what is the best way to do this ?

Answer 1

Some points to get you started.

First, std::array vs std::vector. This is easy. If you know at compile time the size of your 2d array then definitely std::array else std::vector.

std::vector<std::vector<T>> is ok for something that you need to cook up quickly and you use seldom in pieces of code that are not performance critical⁽¹⁾.

The big downside to std::vector<std::vector<T>> is the memory layout. You have double indirection and each line is allocated separately, wreaking havoc on your cache so it's a definitely a no no in performance critical code ⁽¹⁾. For performance critical code you need to use a mature math library. Experts can write performance oriented code order of magnitude better than you or me and another important advantage of such library is that it passed the test of time.

If for whatever reason you want to write this yourself then the solution is to flatten the matrix. I.e. use std::vector<T>(numRows * numColumns) and then access the elements with the formula: (i, j) -> v[i * numColumns + j]. And wrap that in a nice interface. How complex the interface is and what kind of operations you allow on it it's totally up to you.

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⁽¹⁾ I would like to point out that most often especially novice programmers completely misunderstand performance concerns (it's completely understandable due to lack of experience). First there are some few generally acceptable practices that apply to any situation. Then comes the analysis of the algorithm and the data types used. But other than that you first write code for readability and then, if performance is a concern, you profile your code and start optimizing where the profiler tells you.