static constexpr array in C++?

Question

In the following, I would like to define valid neighbours as a class member attribute:

struct Chessboard
{ 
    ...

    vector<vector<Cell>> cells;

    static int constexpr neighbours[][2] = {
         {-1, -1}, {-1, 0}, {-1, 1},
         {0, -1},  {0, 0},  {0, 1},
         {1, 1},   {1, 0},  {1, 1}
    };

    bool isNeighbour() {
        for (int row = 0; row < nRows; row++) {
            for (int col = 0; col < nCols; col++) {
                for (const int *d : neighbours) { // <---- Error!
                    if (cells[row][col].isMagic) {
                        return true;
                    }
                }
            }
        }
    }
};

Unfortunately, I get an undefined reference to Chessboard::neighbours` during the link process.

What would be the correct way of doing it?

Looking at this question it seems static members should be declared inside the .cpp file, but in my case, I get the exact same behaviour.

For instance, in Python I can declare static members inside my class. Why isn't allowed in C++?

You have to declare it inside the `struct Chessboard` and define it outside. — πάντα ῥεῖ, Oct 08 '20 at 21:13
You still need a definition of `neighbours` somewhere outside of your class. — n. m. could be an AI, Oct 08 '20 at 21:14
Isn't possible to have a static class member the same as in Python? — nowox, Oct 08 '20 at 21:28
@nowox _"Why both?"_ Because its required by the compiler. Alternatively you may add `inline`, if you have c++17. Whatever a _"SSOT"_ means, examples can be found in the duplicate. — πάντα ῥεῖ, Oct 09 '20 at 08:19

static constexpr array in C++?

0 Answers0