Passing a 2D array to a C++ function

I have a function which I want to take, as a parameter, a 2D array of variable size.

So far I have this:

void myFunction(double** myArray){
     myArray[x][y] = 5;
     etc...
}

And I have declared an array elsewhere in my code:

double anArray[10][10];

However, calling myFunction(anArray) gives me an error.

I do not want to copy the array when I pass it in. Any changes made in myFunction should alter the state of anArray. If I understand correctly, I only want to pass in as an argument a pointer to a 2D array. The function needs to accept arrays of different sizes also. So for example, [10][10] and [5][5]. How can I do this?

There are three ways to pass a 2D array to a function:

1. The parameter is a 2D array

   int array[10][10];
   void passFunc(int a[][10])
   {
       // ...
   }
   passFunc(array);
   

2. The parameter is an array containing pointers

   int *array[10];
   for(int i = 0; i < 10; i++)
       array[i] = new int[10];
   void passFunc(int *a[10]) //Array containing pointers
   {
       // ...
   }
   passFunc(array);
   

3. The parameter is a pointer to a pointer

   int **array;
   array = new int *[10];
   for(int i = 0; i <10; i++)
       array[i] = new int[10];
   void passFunc(int **a)
   {
       // ...
   }
   passFunc(array);
   

Fixed Size

1. Pass by reference

template <size_t rows, size_t cols>
void process_2d_array_template(int (&array)[rows][cols])
{
    std::cout << __func__ << std::endl;
    for (size_t i = 0; i < rows; ++i)
    {
        std::cout << i << ": ";
        for (size_t j = 0; j < cols; ++j)
            std::cout << array[i][j] << '\t';
        std::cout << std::endl;
    }
}

In C++ passing the array by reference without losing the dimension information is probably the safest, since one needn't worry about the caller passing an incorrect dimension (compiler flags when mismatching). However, this isn't possible with dynamic (freestore) arrays; it works for automatic (usually stack-living) arrays only i.e. the dimensionality should be known at compile time.

2. Pass by pointer

void process_2d_array_pointer(int (*array)[5][10])
{
    std::cout << __func__ << std::endl;
    for (size_t i = 0; i < 5; ++i)
    {
        std::cout << i << ": ";
        for (size_t j = 0; j < 10; ++j)
            std::cout << (*array)[i][j] << '\t';
        std::cout << std::endl;
    }    
}

The C equivalent of the previous method is passing the array by pointer. This should not be confused with passing by the array's decayed pointer type (3), which is the common, popular method, albeit less safe than this one but more flexible. Like (1), use this method when all the dimensions of the array is fixed and known at compile-time. Note that when calling the function the array's address should be passed process_2d_array_pointer(&a) and not the address of the first element by decay process_2d_array_pointer(a).

Variable Size

These are inherited from C but are less safe, the compiler has no way of checking, guaranteeing that the caller is passing the required dimensions. The function only banks on what the caller passes in as the dimension(s). These are more flexible than the above ones since arrays of different lengths can be passed to them invariably.

It is to be remembered that there's no such thing as passing an array directly to a function in C [while in C++ they can be passed as a reference (1)]; (2) is passing a pointer to the array and not the array itself. Always passing an array as-is becomes a pointer-copy operation which is facilitated by array's nature of decaying into a pointer.

3. Pass by (value) a pointer to the decayed type

// int array[][10] is just fancy notation for the same thing
void process_2d_array(int (*array)[10], size_t rows)
{
    std::cout << __func__ << std::endl;
    for (size_t i = 0; i < rows; ++i)
    {
        std::cout << i << ": ";
        for (size_t j = 0; j < 10; ++j)
            std::cout << array[i][j] << '\t';
        std::cout << std::endl;
    }
}

Although int array[][10] is allowed, I'd not recommend it over the above syntax since the above syntax makes it clear that the identifier array is a single pointer to an array of 10 integers, while this syntax looks like it's a 2D array but is the same pointer to an array of 10 integers. Here we know the number of elements in a single row (i.e. the column size, 10 here) but the number of rows is unknown and hence to be passed as an argument. In this case there's some safety since the compiler can flag when a pointer to an array with second dimension not equal to 10 is passed. The first dimension is the varying part and can be omitted. See here for the rationale on why only the first dimension is allowed to be omitted.

4. Pass by pointer to a pointer

// int *array[10] is just fancy notation for the same thing
void process_pointer_2_pointer(int **array, size_t rows, size_t cols)
{
    std::cout << __func__ << std::endl;
    for (size_t i = 0; i < rows; ++i)
    {
        std::cout << i << ": ";
        for (size_t j = 0; j < cols; ++j)
            std::cout << array[i][j] << '\t';
        std::cout << std::endl;
    }
}

Again there's an alternative syntax of int *array[10] which is the same as int **array. In this syntax the [10] is ignored as it decays into a pointer thereby becoming int **array. Perhaps it is just a cue to the caller that the passed array should have at least 10 columns, even then row count is required. In any case the compiler doesn't flag for any length/size violations (it only checks if the type passed is a pointer to pointer), hence requiring both row and column counts as parameter makes sense here.

Note: (4) is the least safest option since it hardly has any type check and the most inconvenient. One cannot legitimately pass a 2D array to this function; C-FAQ condemns the usual workaround of doing int x[5][10]; process_pointer_2_pointer((int**)&x[0][0], 5, 10); as it may potentially lead to undefined behaviour due to array flattening. The right way of passing an array in this method brings us to the inconvenient part i.e. we need an additional (surrogate) array of pointers with each of its element pointing to the respective row of the actual, to-be-passed array; this surrogate is then passed to the function (see below); all this for getting the same job done as the above methods which are more safer, cleaner and perhaps faster.

Here's a driver program to test the above functions:

#include <iostream>

// copy above functions here

int main()
{
    int a[5][10] = { { } };
    process_2d_array_template(a);
    process_2d_array_pointer(&a);    // <-- notice the unusual usage of addressof (&) operator on an array
    process_2d_array(a, 5);
    // works since a's first dimension decays into a pointer thereby becoming int (*)[10]

    int *b[5];  // surrogate
    for (size_t i = 0; i < 5; ++i)
    {
        b[i] = a[i];
    }
    // another popular way to define b: here the 2D arrays dims may be non-const, runtime var
    // int **b = new int*[5];
    // for (size_t i = 0; i < 5; ++i) b[i] = new int[10];
    process_pointer_2_pointer(b, 5, 10);
    // process_2d_array(b, 5);
    // doesn't work since b's first dimension decays into a pointer thereby becoming int**
}

A modification to shengy's first suggestion, you can use templates to make the function accept a multi-dimensional array variable (instead of storing an array of pointers that have to be managed and deleted):

template <size_t size_x, size_t size_y>
void func(double (&arr)[size_x][size_y])
{
    printf("%p\n", &arr);
}

int main()
{
    double a1[10][10];
    double a2[5][5];
    
    printf("%p\n%p\n\n", &a1, &a2);
    func(a1);
    func(a2);
    
    return 0;
}

The print statements are there to show that the arrays are getting passed by reference (by displaying the variables' addresses)

Surprised that no one mentioned this yet, but you can simply template on anything 2D supporting [][] semantics.

template <typename TwoD>
void myFunction(TwoD& myArray){
     myArray[x][y] = 5;
     etc...
}

// call with
double anArray[10][10];
myFunction(anArray);

It works with any 2D "array-like" datastructure, such as std::vector<std::vector<T>>, or a user defined type to maximize code reuse.

You can create a function template like this:

template<int R, int C>
void myFunction(double (&myArray)[R][C])
{
    myArray[x][y] = 5;
    etc...
}

Then you have both dimension sizes via R and C. A different function will be created for each array size, so if your function is large and you call it with a variety of different array sizes, this may be costly. You could use it as a wrapper over a function like this though:

void myFunction(double * arr, int R, int C)
{
    arr[x * C + y] = 5;
    etc...
}

It treats the array as one dimensional, and uses arithmetic to figure out the offsets of the indexes. In this case, you would define the template like this:

template<int C, int R>
void myFunction(double (&myArray)[R][C])
{
    myFunction(*myArray, R, C);
}

anArray[10][10] is not a pointer to a pointer, it is a contiguous chunk of memory suitable for storing 100 values of type double, which compiler knows how to address because you specified the dimensions. You need to pass it to a function as an array. You can omit the size of the initial dimension, as follows:

void f(double p[][10]) {
}

However, this will not let you pass arrays with the last dimension other than ten.

The best solution in C++ is to use std::vector<std::vector<double> >: it is nearly as efficient, and significantly more convenient.

Single dimensional array decays to a pointer pointer pointing to the first element in the array. While a 2D array decays to a pointer pointing to first row. So, the function prototype should be -

void myFunction(double (*myArray) [10]);

I would prefer std::vector over raw arrays.

Here is a vector of vectors matrix example

#include <iostream>
#include <vector>
using namespace std;

typedef vector< vector<int> > Matrix;

void print(Matrix& m)
{
   int M=m.size();
   int N=m[0].size();
   for(int i=0; i<M; i++) {
      for(int j=0; j<N; j++)
         cout << m[i][j] << " ";
      cout << endl;
   }
   cout << endl;
}


int main()
{
    Matrix m = { {1,2,3,4},
                 {5,6,7,8},
                 {9,1,2,3} };
    print(m);

    //To initialize a 3 x 4 matrix with 0:
    Matrix n( 3,vector<int>(4,0));
    print(n);
    return 0;
}

output:

1 2 3 4
5 6 7 8
9 1 2 3

0 0 0 0
0 0 0 0
0 0 0 0

You can do something like this...

#include<iostream>

using namespace std;

//for changing values in 2D array
void myFunc(double *a,int rows,int cols){
    for(int i=0;i<rows;i++){
	    for(int j=0;j<cols;j++){
		    *(a+ i*rows + j)+=10.0;
	    }
    }
}

//for printing 2D array,similar to myFunc
void printArray(double *a,int rows,int cols){
    cout<<"Printing your array...\n";
    for(int i=0;i<rows;i++){
	    for(int j=0;j<cols;j++){
		    cout<<*(a+ i*rows + j)<<"  ";
	    }
	cout<<"\n";
    }
}

int main(){
    //declare and initialize your array
    double a[2][2]={{1.5 , 2.5},{3.5 , 4.5}};

    //the 1st argument is the address of the first row i.e
    //the first 1D array
    //the 2nd argument is the no of rows of your array
    //the 3rd argument is the no of columns of your array
    myFunc(a[0],2,2);

    //same way as myFunc
    printArray(a[0],2,2);

    return 0;
}

Your output will be as follows...

11.5  12.5
13.5  14.5

We can use several ways to pass a 2D array to a function:

• Using single pointer we have to typecast the 2D array.

   #include<bits/stdc++.h>
   using namespace std;
  
  
   void func(int *arr, int m, int n)
   {
       for (int i=0; i<m; i++)
       {
          for (int j=0; j<n; j++)
          {
             cout<<*((arr+i*n) + j)<<" ";
          }
          cout<<endl;
       }
   }
  
   int main()
   {
       int m = 3, n = 3;
       int arr[m][n] = {{1, 2, 3}, {4, 5, 6}, {7, 8, 9}};
       func((int *)arr, m, n);
       return 0;
   }
  

• Using double pointer In this way, we also typecast the 2d array

   #include<bits/stdc++.h>
   using namespace std;
  
  void func(int **arr, int row, int col)
  {
     for (int i=0; i<row; i++)
     {
        for(int j=0 ; j<col; j++)
        {
          cout<<arr[i][j]<<" ";
        }
        printf("\n");
     }
  }
  
  int main()
  {
    int row, colum;
    cin>>row>>colum;
    int** arr = new int*[row];
  
    for(int i=0; i<row; i++)
    {
       arr[i] = new int[colum];
    }
  
    for(int i=0; i<row; i++)
    {
        for(int j=0; j<colum; j++)
        {
           cin>>arr[i][j];
        }
    }
    func(arr, row, colum);
  
    return 0;
  }
  

One important thing for passing multidimensional arrays is:

• First array dimension need not be specified.
• Second(any any further)dimension must be specified.

1.When only second dimension is available globally (either as a macro or as a global constant)

const int N = 3;

void print(int arr[][N], int m)
{
int i, j;
for (i = 0; i < m; i++)
  for (j = 0; j < N; j++)
    printf("%d ", arr[i][j]);
}

int main()
{
int arr[][3] = {{1, 2, 3}, {4, 5, 6}, {7, 8, 9}};
print(arr, 3);
return 0;
}

2.Using a single pointer: In this method,we must typecast the 2D array when passing to function.

void print(int *arr, int m, int n)
{
int i, j;
for (i = 0; i < m; i++)
  for (j = 0; j < n; j++)
    printf("%d ", *((arr+i*n) + j));
 }

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

// We can also use "print(&arr[0][0], m, n);"
print((int *)arr, m, n);
return 0;
}

#include <iostream>

using std::cout;

template <typename table>
void PrintArray(table& a) {
    const size_t rows = sizeof(a) / sizeof(a[0]);
    const size_t cols = sizeof(a[0]) / sizeof(a[0][0]);
    cout << "Array has " << rows << " rows and " << cols << " columns.\n\n";
    for (size_t i = 0; i < rows; i++) {
        for (size_t j = 0; j < cols; j++) {
            cout << a[i][j] << ' ';
        }
        cout << '\n';
    }
}

int main()
{
    size_t a[5][8]{}; // you can change the type and size of the 2D array
    PrintArray(a);
    return 0;
}

You can use template facility in C++ to do this. I did something like this :

template<typename T, size_t col>
T process(T a[][col], size_t row) {
...
}

the problem with this approach is that for every value of col which you provide, the a new function definition is instantiated using the template. so,

int some_mat[3][3], another_mat[4,5];
process(some_mat, 3);
process(another_mat, 4);

instantiates the template twice to produce 2 function definitions (one where col = 3 and one where col = 5).

If you want to pass int a[2][3] to void func(int** pp) you need auxiliary steps as follows.

int a[2][3];
int* p[2] = {a[0],a[1]};
int** pp = p;

func(pp);

As the first [2] can be implicitly specified, it can be simplified further as.

int a[][3];
int* p[] = {a[0],a[1]};
int** pp = p;

func(pp);

In the case you want to pass a dynamic sized 2-d array to a function, using some pointers could work for you.

void func1(int *arr, int n, int m){
    ...
    int i_j_the_element = arr[i * m + j];  // use the idiom of i * m + j for arr[i][j] 
    ...
}

void func2(){
    ...
    int arr[n][m];
    ...
    func1(&(arr[0][0]), n, m);
}

You are allowed to omit the leftmost dimension and so you end up with two options:

void f1(double a[][2][3]) { ... }

void f2(double (*a)[2][3]) { ... }

double a[1][2][3];

f1(a); // ok
f2(a); // ok 

This is the same with pointers:

// compilation error: cannot convert ‘double (*)[2][3]’ to ‘double***’ 
// double ***p1 = a;

// compilation error: cannot convert ‘double (*)[2][3]’ to ‘double (**)[3]’
// double (**p2)[3] = a;

double (*p3)[2][3] = a; // ok

// compilation error: array of pointers != pointer to array
// double *p4[2][3] = a;

double (*p5)[3] = a[0]; // ok

double *p6 = a[0][1]; // ok

The decay of an N dimensional array to a pointer to N-1 dimensional array is allowed by C++ standard, since you can lose the leftmost dimension and still being able to correctly access array elements with N-1 dimension information.

Details in here

Though, arrays and pointers are not the same: an array can decay into a pointer, but a pointer doesn't carry state about the size/configuration of the data to which it points.

A char ** is a pointer to a memory block containing character pointers, which themselves point to memory blocks of characters. A char [][] is a single memory block which contains characters. This has an impact on how the compiler translate the code and how the final performance will be.

Source