2D array as instance variable of class - c++

Question

So if I have a class with a 2D array that I want to initialize with two parameters passed into the constructor, how would I do that, I keep running into errors because it won't let me update the two-d array at all in the constructor.
-- Update from the comments:
In my header file I tried both
int array[][]
and
int **array
and then in the .cpp file in the constructor I'm trying to do
array = new int[arg1][arg2]
Neither declaration of the array in the header file worked.

Answer 1

in the constructor I'm trying to do array = new array[arg1][arg2]
You need to specify the array type, like
array = new int[arg1][arg2];
Note that this works in C++11 only - when using older standards, the second array size needs to be const (which is probably not what you want).
There are also some additional articles discussing the same issue:
Multi-Dimensional Arrays
How to "new" a two-dimension array in C++?
Ideally, since you are using C++ anyway, you should use std::vector as proposed in another answer.
Vectors use a lot of overhead though, don't they? I'm trying to keep my memory use light. –
Start with std::vector. Once your application is running properly from a functional perspective, if you are still concerned about memory usage and/or performance, do benchmarking. If you properly encapsulate your 2D array in a class, you can always change the actual implementation of the array with no impact on the code which uses it.
Technically, if you want to make sure that you have one flat memory area which contains your array, you could use a 1-dimensional array to simulate a 2-dimensional array, like in the following code (just to get you the idea, certainly needs some improvement, especially copy construction and assignment operators are missing):
class Array2D {
private:
int *array;
int size1;
public:
Array2D(int arg1, int arg2) {
size1 = arg1;
array = new int[arg1 * arg2];
}
~Array2D() {
delete[] array;
}
int& at(int i1, int i2) {
return array[i1 * size1 + i2];
}
};
int main() {
Array2D array(10, 10);
array.at(2, 2) = 42;
std::cerr << array.at(2, 2);
return 0;
}

Answer 2

Simplest solution would be:
std::vector<std::vector<VALUE>> arr2(X, std::vector<VALUE>(Y));

Answer 3

Here is an 2d array example with bounds check and custom type, based upon the example from Andreas Fester.
#include <stdexcept>
template <typename T>
class Array2D {
private:
T *array;
unsigned int sizeX;
unsigned int sizeY;
public:
Array2D(unsigned int X, unsigned int Y) {
sizeX = X;
sizeY = Y;
array = new T[X * Y];
}
~Array2D() {
delete[] array;
}
T& at(unsigned int X, unsigned int Y) {
if((X > sizeX) || (Y > sizeY))
throw std::out_of_range("Bla bla");
return array[X * sizeX + Y];
}
};
int main() {
double MyValue;
Array2D<double> *MyArray = new Array2D<double>(10, 100);
MyArray->at(1,1) = 10.1;
MyValue = MyArray->at(1,1);
printf("Array value = %3.3f\n", MyValue);
return 0;
}