C++ :: Passing Matrices Into Classes?

I am currently trying to pass a matrix (multidimensional array) from my main into a function in a class. As usual, you have to include all array sizes, except the first. Unfortunately, the size saved under a variable. When I write it in the square brackets, it doesn't work.

Example:

//main
int main () {
int n = 5;
random object;
object.declare_variable(n);
int array [3][n];
object.pass_matrix(array);

[code].....

I also tried, putting the variable 'var' in the public section above the void and declaring it as static, but still there are error messages; fewer, in my case:

...not an integer constant

So what should I do? Id like to repeat that the int n will vary, which is why I can't just directly write down the size.