无法将二维数组传递给 C++ 中的辅助函数

Question

I was learning c++ and implementing the game of life when I created a helper function to print the current board which is a 2d array. I cannot seem to pass the array into the function as I get an error, "Candidate function not viable: no known conversion from 'char [rows][cols]' to 'char (*)[cols]' for 3rd argument." I am using Xcode as an ide if that helps.

void printArray(int rows, int cols, char board[rows][cols]){
    for(int r = 0; r < rows; r++){
        for(int c = 0; c < cols; c++){
            cout << board[r][c];
            cout << " ";
        }
        cout << "\n";
    }
}

int main(){
     char board[5][5];
    for(int r = 0; r < 5; r++){
        for(int c = 0; c < 5; c++){
            board[r][c] = 0;
        }
    }
    printArray(5, 5, board);
    return 0;
}

I've tried switching up the parameter to different things such as char **board, char board[][cols], char (*board)[cols]. Even casting my input board which leads to other errors.

Answer 1

If you want to pass 2d arrays to a function there is a special syntax. Unfortunately, the other previous 2 answers do not answer fully correctly.

You can pass by reference or by pointer. The array dimensions must be compile time constants. That is a requirement from C++.

Please see:

constexpr size_t NumberOfRows = 3;
constexpr size_t NumberOfColumns = 4;

// Typedef for easier usage
using IntMatrix2d = int[NumberOfRows][NumberOfColumns];

//Solution 1 ------
// Pass by reference
void function1(int(&matrix)[NumberOfRows][NumberOfColumns])  {}

// Pass by pointer
void function2(int(*m)[NumberOfRows][NumberOfColumns])  {}

//Solution 2 ------
// Pass by reference
void function3(IntMatrix2d& matrix) {}

// Pass by pointer 
void function4(IntMatrix2d* matrix) {}


int main()
{
    // Solution 1
    // Handwritten matrix. Dimension is compile time constant
    int matrix1[NumberOfRows][NumberOfColumns];

    // Pass by reference
    function1(matrix1);

    // Pass by pointer
    function2(&matrix1);

    // Solution 2 -----
    IntMatrix2d matrix2;

    // Pass by reference
    function3(matrix2);

    // Pass by pointer
    function4(&matrix2);

    return 0;
}

If you typedef or use using for your type definition, then it gets rather intuitive.

Answer 2

If you are not very comfortable with pointers then there are some easy ways to do the task

1. You have to define the 2d array size by default , before passing array to the function so that the size doesn't seem to be unknown to the function.

#include <iostream>

const std::size_t rows=5;
const std::size_t cols=5;

void printArray(char board[rows][cols]) {
    for (int r = 0; r < rows; r++) {
        for (int c = 0; c < cols; c++) {
            std::cout << board[r][c];
            std::cout << " ";
        }
        std::cout << "\n";
    }
}

int main() {
    char board[rows][cols];
    for (int r = 0; r < rows; r++) {
        for (int c = 0; c < cols; c++) {
            board[r][c] = '0';
        }
    }
    printArray(board);
    return 0;
}

2. Use vector . Make your board a vector.

#include <iostream>
#include <vector>
void printArray(std::vector<std::vector<char>> &board) {
    for (int r = 0; r < board.size(); r++) {
        for (int c = 0; c < board[0].size(); c++) {
            std::cout << board[r][c];
            std::cout << " ";
        }
        std::cout << "\n";
    }
}

int main() {
    std::vector<std::vector<char>> board(rows, std::vector<char>(cols, '0'));
    printArray(board);
}

Answer 3

I encountered this problem while doing a project for a class. To work around it, I made a double pointer array, and then used passed it to the function to manipulate it.

int** createArr(){

    int** pixels = 0;
    pixels = new int*[numrows];

    for (int row = 0; row < numrows; row++){
    pixels[row] = new int[numcols];

            for (int col = 0; col < numcols; col++){
                ss >> pixels[row][col];

            }
    }
    return pixels;
}

int** newArr = createArr(); //calls function to create array

func(newArr); //where func is a function that modifies the array.

Don't forget to delete your arrays at the end to avoid memory leaks. Hope this helps.