Question

When I read some values from the user and I need to create an array of the specific size I do it somehow like this:

#include <iostream>
using namespace std;    
unsigned* numbers;
int main()
{
int a;
cin >> a;
numbers = new unsigned[a];
}

How can I do it with a 2d array(sized a*b read from the user)?

Answer 1

Anything that would look like a 2D-Array in code will not be a physical 2D array in memory, but either one plain block of memory or scattered, depending on how you allocate it.

• You can torture yourself by doing dynamic allocation of N arrays in another dynamic array of N pointers, like nrussel’s answer suggests.
• You can make a vector of vectors instead, like billz and Arun C.B suggest – that would relieve you from managing the allocated memory yourself but still leave you with N+1 scattered allocations which is not very performant.

• Brennan Vincent’s answer suggests allocation of one dynamic array, containing a*b elements, which gives you one continuous block in memory. Combine that with the builtin dynamic memory management of std::vector he mentioned and be happy:

  std::vector<unsigned> matrix(a*b);

If you want the matrix to be convenient to access, wrap the whole thing into a class providing you access to the elements with 2D-coordinates. But please step away from managing the memory yourself. It just hurts you and anyone who has to maintain that code (and search for mem leaks).