Returning a pointer to a 2-D array from a function

Question

In order to understand how pointer work I wrote this function which has to return a 3*3 matrix.

int** Matrix::getMatrix(){
    cout<<"The  matrix is: \n";
    int (*p)[3]=m;

    for(int i=0;i<n;i++){
        for(int j=0;j<n;j++){
        cout<<m[i][j]<<"\t";
        }
        cout<<"\n";
    }
    return p;

}

Here m is a 3*3 array.But at the line return p; it gives the error return value type does not match function type.

With p am I not returning a pointer to a 3*3 matrix.?What's wrong with this.Can someone please help me to correct this.

I believe, you should be returning a type `int*`, which would return the value of p (ie., address of m) instead of `int**`. — Venkata Krishna, Dec 29 '14 at 10:05

Jarod42 · Accepted Answer · 2014-12-29T13:14:58.360

2

int (*)[3] and int** are not the same type:

int** is a pointer to a pointer to int
int (*)[3] is a pointer of an array of 3 int.

Even if int [3] may decay to int*, pointer on there different type are also different.

The correct syntax to return int (*)[3] would be:

int (*Matrix::getMatrix())[3];

or with typedef:

using int3 = int[3];

int3* Matrix::getMatrix();

And as m is int[3][3], you may even return reference (int(&)[3][3]):

int (&Matrix::getMatrix())[3][3];

and with typedef:

using mat3 = int[3][3];
mat3& Matrix::getMatrix();

It would be more intuitive with std::array or std::vector

edited Dec 29 '14 at 13:14

answered Dec 29 '14 at 10:22

Jarod42

203,559
14
181
302

In the first line you are missing an asterix in `int**`. – cmaster - reinstate monica Dec 29 '14 at 10:26
`auto Matrix::getMatrix() -> decltype(m)` (plus something to make it a reference) – M.M Dec 29 '14 at 10:33
@MattMcNabb: `decltype(m)&` is the way. Even in C++14 with `auto` ( `int(*)[3]`) or `decltype(auto)` (`int[3][3]` that you can't return) doesn't improve that. – Jarod42 Dec 29 '14 at 10:54
@Jarod42 What is the difference of `int (*)[3]` and `int**`.I don't know much about pointers.Still beginning to learn it. Also what is meant by `int (*Matrix::getMatrix())[3]; `.What does `*Matrix` mean?Pointer to a class? – clarkson Dec 29 '14 at 13:06
@clarkson: I edited the answer for clarification of the difference. – Jarod42 Dec 29 '14 at 13:22
@clarkson: `int(*f())[3]` is the (unintuitive) syntax to return pointer on array of 3 int. I would recommend to not use it. Prefer the syntax with `typedef` or appropriate structure (generic `std::array` or a dedicated class of your own). – Jarod42 Dec 29 '14 at 13:23

Returning a pointer to a 2-D array from a function

1 Answers1