C++ Set Array size in constructor, copy a 2D Array

Question

So in my header file for a class Foo I declare a 2D array of ints like so:

int board[][];

I am intentionally leaving out a size because I want to set that in the constructor. board[][] will not change in size once initialized. One of my constructors looks like this:

Foo(int _board[][]);

In this I want to set board[][] to the same size as _board[][] and to also copy the contents. I have tried using this in the .cpp implementation of Foo:

Foo::Foo(int _board[][]){
board[][] = _board[][]; //Does not work as intended
}

However, this does not work as intended. How can I make board[][] be the same size and have the same contents as _board[][] in the constructor?

you cant do it using array without knowing the 2nd dimension size. — sleeping_dragon
– sleeping_dragon, Commented Oct 28, 2012 at 22:46

PiotrNycz · Accepted Answer · 2012-10-28 23:15:25Z

C++ is different than Java. int a[][]; is not allowed as variable type. Some misleading C++ feature is that first size is allowed to be left empty:

int foo(int a[]);
int foo(int a[][3]); 
int foo(int a[][3][4]);

Another misleading C++ feature is that this is allowed in initialization of an array (compiler will count the size):

int a[][] = {{1,2}, {1,2,3,4}, {1}};

which is equivalent to:

int a[3][4] = {{1,2}, {1,2,3,4}, {1}};

For your very case - use std::vector:

std::vector<std::vector<int>> board;
Foo(std::vector<std::vector<int>> board) : board(board) {}

If you can't use std::vector for whatever reason - then only solution is to use int** with both sizes:

int** board;
size_t s1;
size_t s2;
Foo(int** board = NULL, size_t s1 = 0, size_t s2 = 0) : board(board), s1(s1), s2(s2) {}

But be aware that you cannot use this way:

int board[][] = {{1,1,2},{1,2,2}};
Foo foo((int**)board,2,3);

because you must provide a dynamic array:

int** board = new int*[2] { new int[3]{1,1,2}, new int[3]{1,2,2}};

And since that - you have to implement copy constructor, assignment operator and destructor:

Foo(const Foo& other) : TODO { TODO }  
~Foo() { TODO }  
Foo& operator = (Foo other) { TODO }

So, just use std::vector.

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