Data structure for a crossword puzzle grid?

Question

I'd like to design a crossword puzzle editor in C++. It is a grid of blocks, each block containing a letter (or being black between two words), possibly a number and a thick or thin border line. The block is therefore a container class for them. The grid is a container of blocks. But how would I structure the grid?

1. A raw 2d array: Block grid[row][column]?
2. Vector of Vectors: vector<vector<Block>>?
3. Two vectors, one for the rows and one for the columns: vector<Block> row; vector<Block> column vector<Block> row; vector<Block> column ?
4. A map, which keys are the row/column pairs and the values are the blocks: map<int[2], Block> ?

Answer 1

By default, plain static/dynamic arrays (or their wrappers) are the most preferable: they are the most comfortable for both the programmer (random access API etc) and the processor (memory locality etc).

The easiest-to-implement Block layout in an array/a vector is [first row Blocks..., second row Blocks..., etc] - a 1D array which acts as a 2D array. It can be indexed like crossword[x + y * crossword.width()] , which isn't pretty, so you might want to use a library/self-written wrapper with API like crossword(x, y) which performs that xy -to- i -index conversion under the hood.

Maybe something like this:

class Crossword {
    std::vector<Block> raw;
    size_t length{}; // can't name it "width" because there's a "width()" member function below

public:
    Crossword() {}
    Crossword(size_t x, size_t y): raw(x * y), length{x} {}
    Crossword(Crossword&& other) noexcept { *this = std::move(other); }
    Crossword& operator=(Crossword&& other) noexcept {
        std::swap(raw, other.raw);
        std::swap(length, other.length);
        return *this;
    }

    auto size() const { return raw.size(); }
    auto width() const { return length; }
    auto height() const { return size() / length; }
    auto& operator()(size_t x, size_t y) const { return raw[x + y * length]; }
    auto& operator()(size_t x, size_t y) { return raw[x + y * length]; }
};