How to pass a stack matrix by reference in C++

Question

My question is simple, if I have a matrix created on the stack rather than on the heap, such as int matrix[10][10], how can I pass it by reference? Or, pass it in a way that it doesn't pass the whole matrix as argument, but just its pointer or reference, or whatever. I use C++11.

void proc(/*WHAT GOES HERE?*/ matrix, int n){    
    matrix[n-1][n-1] = 7;     
}

int main(){

    int matrix[10][10];     

    proc(matrix, 10);       

    return 0;

}

Answer 1

You simply need:

// By reference:
void proc_ref(int (&matrix)[10][10]); // first dimension must have a size of 10

// By pointer:
void proc_ptr(int (*matrix)[10], int n); // n is the size of the first dimension

In the first case, matrix will be a reference to an array of 10 array of 10 ints ("reference to int[10][10]"), in the second case matrix will be a pointer to an array of 10 int ("pointer to int[10]").

In both cases you can use it like you want in proc:

matrix[i][j] = 42;

The second version allows passing matrix of various size such as int[14][10] or int[12][10] (as long as the second dimension as a size of 10). It also allows passing dynamically allocated array of array of 10 int:

int (*p)[10] = new int[42][10];
proc_ref (p); // Error
proc_ptr (p, 42); // Ok

int m[24][10];
proc_ref (p); // Error
proc_ptr (p, 24); // Ok

If you want to only allow square matrix declared with automatic storage duration, use the reference versions.

Note: You have to specify the second dimension of your matrix at compile time. If you want to be "generic" you could use a template:

template <size_t N>
void proc (int (&matrix)[N][N]);

Also, if you are using c++11, you should use std::array which is much more convenient while still doing exactly what you want (no dynamic allocation):

template <typename T, size_t N>
using matrix_t = std::array<std::array<T, N>, N>;

template <typename T, size_t N>
void proc (matrix_t<T, N> &matrix) {
    matrix[N - 1][N - 1] = 7;
}

int main () {
    matrix_t<int, 10> matrix;
    proc(matrix);
}

Answer 2

Array might decay to pointer. You can declare the parameter type as a pointer (to array) like:

void proc(int (*matrix)[10], int n){    
    matrix[n-1][n-1] = 7;     
}

Note the dimension won't be reserved when array decaying to pointer, means you might pass int [11][10] to proc() in this case.

If you don't want this, you can declare the parameter type as reference like:

void proc(int (&matrix)[10][10], int n){    
    matrix[n-1][n-1] = 7;     
}

Only int[10][10] could be passed here.