CODEWITHSUNDEEP
c++functionmultidimensional-array
user153245
user153245

Reputation: 287

C++ generalize two dimension array in function

I want to write a function in C++ that take a matrix of any size and print it.

my code is as follows and it works. But my questions:

1) Is it the best practice? 2) Why casting using (int *) works but static_cast<int *> does not?

Thanks

#include <iostream>

using namespace std;

void print_matrix(int *, int, int);

int main()
{
    int mat[3][3] = {{1, 2, 3},
                    {4, 5, 6},
                    {7, 8, 9}
    };

    int mat2[5][5] = {{1, 2, 3, 40, 50},
                    {4, 5, 6, 70, 80},
                    {7, 8, 9, 10, 20},
                    {17, 18, 19, 10, 20},
                    {71, 81, 91, 110, 120}
    };

    print_matrix((int *)mat2, 5, 5);
    cout << endl;

    print_matrix((int *)(mat), 3, 3);
    cout << endl;

//    static cast does not work:
//    error: invalid static_cast from type 'int [3][3]' to type 'int*'|
//    print_matrix(static_cast<int *>(mat), 3, 3);

    return 0;
}

void print_matrix(int *matPtr, int row, int col)
{
    for(int i = 0; i < row; i++) {
        for(int j = 0; j < col; j++)
            cout << *(matPtr + col * i + j) << '\t';
        cout << endl;
    }
}

Upvotes: 0

Views: 99

Answers (1)

R Sahu
R Sahu

Reputation: 206717

Is it the best practice?

No, it is not.

It will be better to use a function template.

template <size_t NR, size_t NC>
void print_matrix(int (&matrix)[NR][NC])
{
   ...
}

and call it as:

print_matrix(mat2); // NR and NC are deduced by the compiler

Why casting using (int *) works but static_cast<int *> does not?

Given 

int mat[3][3] = { ... };

the following pointers have the same numerical value even though the pointers are of different types.

int (*p1)[3][3] = &mat;
int (*p2)[3] = &mat[0];
int *p3 = &mat[0][0];

Because of this coincidence, use of (int*) works.

Using static_cast<int*> does not work because when a 2D array decays to a pointer, it does not decay to int* -- mat decays to int (*)[3] (pointer to an "array of 3 ints") and mat2 decays to int (*)[5] (pointer to an "array of 5 ints"). They cannot be cast to int* in general.

Upvotes: 1

Related Questions