Can anyone explain what "std::vector<std::vector<char>> board(3, std::vector<char>(3, ' '));" means

Question

I came across this code for building a Tic-Tac-Toe game, and have no idea what it means. Can anyone explain in simple terms?

std::vector<std::vector<char>> board(3, std::vector<char>(3, ' '));

context:

void drawBoard(const std::vector<std::vector<char>> &board)
{
    for (const auto &row : board)
    {
        for (char cell : row)
        {
            std::cout << cell << " ";
        }
        std::cout << std::endl;
    }
}

int main()
{
    std::vector<std::vector<char>> board(3, std::vector<char>(3, ' '));
    char currentPlayer = 'X';
    int row, col;
    bool gameOver = false;

Answer 1

The line is using one of std::vectors constructors, twice:

constexpr vector( size_type count,
                  const T& value,
                  const Allocator& alloc = Allocator() );

This constructor is used to initialize the vector with count number of copies of value.

So this ...

std::vector<char>(3, ' ')

... constructs a vector of char with three elements, all being initialized to ' ' (space).

In this ...

std::vector<std::vector<char>> board(3, std::vector<char>(3, ' '));
//                                   ^                    ^   ^
//                                   |                    |   |
//                                   |               ^    +-1-+
//                                   |               |
//                                   +-------2-------+

... the value_type is std::vector<char> and it makes three copies (2 in the graph above) of std::vector<char>(3, ' ') (1 in the graph above) which makes a 3x3 game board of char all initialized to ' '.