Returning two-dimensional array in C++

**Drogin** · 06-18-2008

Hi!
I want to make a get() function for a two-dimensional array in my class.
Like this:

Code:

class MyClass {
  public:
  getArray();  // <-----How do I define and decleare this function?

  private:
  int array[5][5];
};

So, I would like to use the function like this:

Code:

int main() {
MyClass foo;

// *Decleare the array somehow* <-------how would I do this?
array = foo.getArray();
return 0;
}

**lordmule** · 06-18-2008

I'm guessing you want to use something like:

Code:

array[1][3] = 40;

I may not know how to achieve that, but you could instead. workaround is simply using int* as the return type/declaration type. Then add getWidth() and getHeight() functions.

Code:

array[1 * getHeight() + 3]

Assuming you have differing dimensions:

Code:

int arr[H][W];
arr[y][x] = v;

then the native index:

Code:

arr[ y * H + W ] = v;

**Elysia** · 06-18-2008

std::vector< std::vector<type> > foo()

**lordmule** · 06-18-2008

Actually, there was another post just now, with a similar problem, so with no life of mine i made a sample for you:

Code:

typedef int array2d [5][5];

array2d arr;

array2d* get_arr()
{
	return &arr; 
}

int main()
{
	int i;
	int *t;

	t = (int*)(arr);
	for (i = 0; i < 25; i++) {
		t[i] = i;
	}
	printf("%d\n", arr[3][2]);

	array2d *a1;
	a1 = get_arr();

	printf("%d\n", (*a1)[3][2]);

	return 0;
}

thank this guy King Mir:
http://cboard.cprogramming.com/showthread.php?t=103106

**Elysia** · 06-18-2008

Use a vector. It's far easier.
No global variables either.
This is C++, not C...

**anon** · 06-18-2008

A vector of vectors may have too much overhead to be a replacement for small multidimensional stack array. (I believe there are cases where it is justified not to want dynamic allocations.)

By the way, do you want to return a copy of the array?

In that case, it might be meaningful to use an out parameter (the contents are copied to a suitable array provided by the caller).

If you don't want a copy, then you might try returning a pointer to pointer (int**).

-----
Best advice depends on what exactly you are doing this for...

**matsp** · 06-18-2008

Originally Posted by anon

A vector of vectors may have too much overhead to be a replacement for small multidimensional stack array. (I believe there are cases where it is justified not to want dynamic allocations.)

By the way, do you want to return a copy of the array?

In that case, it might be meaningful to use an out parameter (the contents are copied to a suitable array provided by the caller).

If you don't want a copy, then you might try returning a pointer to pointer (int**).

-----
Best advice depends on what exactly you are doing this for...

Returning a pointer to pointer won't work, since you loose the dimension of the array. You will need to indicate at least the last dimension of the array.

gcc (and g++) doesn't seem to allow functions returning arrays.

--
Mats

**Drogin** · 06-18-2008

I can give you a very simple Java-verison of what I want.
I will comment it well, so non-java people can read it.

So, I would like a C++ verison of this java-code:

Code:

class MyClass {
    private int[][] iPointer = new int[5][5]; // This makes a new array of int, and let iPointer point to it

    // This is the get-function. It returns a pointer to the array
    public int[][] getArray() {
        return iPointer;
    }
}


class MainClass {
    public static void main(String[] args) { // This is the java int main()
        MyClass myclass = new MyClass(); // Run the above class.
        int[][] anotherPointer; // Decleare a pointer that should be pointing to a 2-dimensional int array
        anotherPointer = myclass.getArray(); // Set the pointer to point to the array returned


        // Then, I want to be able to do this, on the array returned.
        // (This pointer points to the iPointer member in myclass)
        anotherPointer[0][0]=5;
        anotherPointer[0][1]=3;
    }
}

**matsp** · 06-18-2008

You should be able to return a pointer to an array in a class. Let me work on it for few minutes.

Not very pretty:

Code:

#include <iostream>

typedef int myarray[5][5];

class myclass
{
    myarray arr;

public:
    myclass() {
	for(int i = 0; i < 5; i++)
	{
	    for(int j = 0; j < 5; j++)
		arr[j][i] = i * j;
	}
    }
    friend std::ostream &operator<<(std::ostream &os, const myclass &c);
    myarray * getArray()
	{
	    return &arr;
	}


};

std::ostream &operator<<(std::ostream &os, const myclass &c) 
{
    for(int i = 0; i < 5; i++)
    {
	for(int j = 0; j < 5; j++)
	    os << c.arr[j][i] << " ";
	
	os << std::endl;
    }
    
    return os;
}



int main()
{
    myclass c;
    int (*p)[5][5];
   
    std::cout << c;
  
    p = c.getArray();
    for(int i = 0; i < 5; i++)
    {
	for(int j = 0; j < 5; j++)
	    std::cout << (*p)[j][i] << " ";
	std::cout << std::endl;
    }
    return 0;

}

--
Mats

**Elysia** · 06-18-2008

If you want Java approach, it's easy if you use std::vector. You can return std::vector, so essentially, you can return an array that way.

**Drogin** · 06-18-2008

Well, I know.
But there should be a way to do this without vectors too...?
Let's say I would use this code-block inside a game-loop, using vectors. That would hurt. If you catch my drift

**BMJ** · 06-18-2008

Originally Posted by Elysia

If you want Java approach, it's easy if you use std::vector. You can return std::vector, so essentially, you can return an array that way.

Hi Elysia,
This would work well and all, but it really isn't going to help them understand why you can't "return a 2d array" in C++.

When people learn C++ they generally learn about how strings are represented in memory as null-terminated byte arrays before they learn to use the std::string class, right? Same idea here, IMO.

**Drogin** · 06-18-2008

Okay folks, thanks a lot for your time, and the wonderful replies.
laserlight's solution worked fine, so that was all I needed, thanks a ton

To Elysia: I'm sorry I couldnt adress you earlier, I was kinda busy.
I am not anti-vector. Using vectors would surely pose no problem for me for the use I want, and it was an excelent suggestion.

However, I would also like to know how to make it without vectors, in case I would ever need to do that. For example because of memory or speed concerns.
And just out of curiosity. I've been trying for hours to get it to work, so I wanted to see the "correct" solution.

**Elysia** · 06-18-2008

Originally Posted by BMJ

When people learn C++ they generally learn about how strings are represented in memory as null-terminated byte arrays before they learn to use the std::string class, right? Same idea here, IMO.

Oh, but was not under impression that it was to learn, but rather to get it working with a good solution.

Originally Posted by Drogin

To Elysia: I'm sorry I couldnt adress you earlier, I was kinda busy.
I am not anti-vector. Using vectors would surely pose no problem for me for the use I want, and it was an excelent suggestion.

However, I would also like to know how to make it without vectors, in case I would ever need to do that. For example because of memory or speed concerns.
And just out of curiosity. I've been trying for hours to get it to work, so I wanted to see the "correct" solution.

I see.
But you could have mentioned it earlier.
Anyway, no harm done.

**Elysia** · 06-18-2008

Then use references.
Create the vector inside the class and let getArray return a reference to it.

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