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
– Venkata Krishna, Commented Dec 29, 2014 at 10:05

Jarod42 · Accepted Answer · 2014-12-29 13:14:58Z

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, 2014 at 13:14

answered Dec 29, 2014 at 10:22

Jarod42

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6 Comments

cmaster - reinstate monica Over a year ago

In the first line you are missing an asterix in int**.

M.M Over a year ago

auto Matrix::getMatrix() -> decltype(m) (plus something to make it a reference)

Jarod42 Over a year ago

@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.

clarkson Over a year ago

@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?

Jarod42 Over a year ago

@clarkson: I edited the answer for clarification of the difference.

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