c++ : pass 2d-array by reference?

Question

I'm getting back into c++ and am having trouble figuring out how to pass a 2D-array to a function. The code below is my current attempt, I've been able to pass vector strings by reference by using:

vector<string> g_dictionary;
getDictionaryFromFile(g_dictionary, "d.txt");
...
void getDictionaryFromFile(vector<string> &g_dictionary, string fileName){..}

But when I try to do the same thing with my 2d-array like so below, I get an error on the line "solve_point(boardEx);"said a reference of type char & cannot be initialized with a value of type boardEx[5][4]

#include <stdio.h>   
#include <string>
using namespace std;

void solve_point(char* &board){ 
    printf("solve_point\n");
    //board[2][2] = 'c';
}

int main(){
    char boardEx[5][4];
    solve_point(boardEx);
}

Answer 1

The type char*& is a reference to a pointer. A "2d" array decays to a pointer to an array.

For your array boardEx it will decay to the type char(*)[4] which needs to be the type your function accepts:

void solve_point(char (*board)[4]) { ... }

---

Or you can use templates to deduce the array dimensions

template<size_t M, size_t N>
void solve_point(char (&board)[M][N]) { ... }

Or use std::array :

std::array<std::array<char, 5>, 4> boardEx;

...

void solve_point(std::array<std::array<char, 5>, 4> const& board) { ... }

Or use std::vector :

std::vector<std::vector<char>> boardEx(5, std::vector<char>(4));

...

void solve_point(std::vector<std::vector<char> const& board) { ... }

Considering the edit of the question, the solution using std::vector is the only portable and standard solution possible.

Answer 2

A reference to a 2D array like you have can be defined using:

char (&ref)[5][4] = boardEx;

You can change the function to use the same syntax.

void solve_point(char (&board)[5][4]){ 
    printf("solve_point\n");
    //board[2][2] = 'c';
}

---

For a dynamically allocated array, it will be better to use std::vector .

int width = 7;
int height = 9;
char boardEx[width][height];

is supported by some compilers as an extension but it is not standard C++. Instead, use:

int width = 7;
int height = 9;
std::vecotr<std::vector<char>> boardEx(width, std::vector(height));

Update solve_point accordingly.

Answer 3

You can declare the function that accepts the array either by value or by reference.

For example (by value)

void solve_point( char ( *board )[4] ){ 
    printf("solve_point\n");
    //board[2][2] = 'c';
}

int main(){
    char boardEx[5][4];
    solve_point(boardEx);
}

Or (by reference)

void solve_point(char ( &board )[5][4] ){ 
    printf("solve_point\n");
    //board[2][2] = 'c';
}

int main(){
    char boardEx[5][4];
    solve_point(boardEx);
}

In the both cases you can access an element of the array using an expression like this

board[i][j] = 'c';

Bear in mind that if you have a multidimensional array like for example this

T a[N1][N2][N3];

where T is some type specifier then you can rewrite the declaration the following way

T ( a[N1] )[N2][N3];

Now to get pointer to elements of the array just substitute ( a[N1] ) for example for ( *pa )

T ( *pa )[N2][N3] = a;

To get reference to the array rewrite its declaration like

T ( a )[N1][N2][N3];

and substitute ( a ) for ( &ra ) like

T ( &ra )[N1][N2][N3] = a;

If you want to write a function that accepts two dimensional arrays of different sizes by reference then you can write

template <typename T, size_t M, size_t N>
void solve_point( T ( &board )[M][N] ){ 
    //...
}

int main(){
    char boardEx[5][4];
    solve_point(boardEx);
}