Initialize 2d Array at compile and allow user input - c++

Question

I am trying to declare an array that is sized at runtime/compile through an overload constructor.
private:
auto** arr = new int[n][n];
overloadConstruct(int n){
arr[n][n] = {0,0};
}
This does not work, it says the second n needs to be constant and auto is not allowed. Any help would be appreciated. I am not sure of all the rules with arrays, especially 2d arrays when doing this. I just need to be able to size a 2d array at runtime/compile through an input.

Answer 1

Use a vector of vectors instead:
#include <iostream>
#include <vector>
class Foo
{
public:
Foo( const int rowCount, const int colCount )
: m_2dArray( rowCount, std::vector<int>( colCount ) ) // initialize the vector
{ // member like this
}
void printArr() const
{
for ( size_t row = 0; row < m_2dArray.size( ); ++row )
{
for ( size_t col = 0; col < m_2dArray[ 0 ].size( ); ++col )
{
std::cout << m_2dArray[ row ][ col ] << " ";
}
std::cout << '\n';
}
}
private:
std::vector< std::vector<int> > m_2dArray; // the vector member
};
int main ()
{
int rowCount { };
int colCount { };
std::cout << "Enter row count: ";
std::cin >> rowCount;
std::cout << "Enter column count: ";
std::cin >> colCount;
std::cout << "\nThe contents of 2D array:\n";
Foo obj( rowCount, colCount ); // construct the object using user's input
obj.printArr( );
}
The output:
Enter row count: 10
Enter column count: 20
The contents of 2D array:
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
As you can see, the user can enter the values at runtime.