There are two types of memory allocations.

1. Static memory allocation

2. Dynamic memory allocation

Static Memory Allocation: Allocating the total memory requirements that a data structure might need all at once without regard for the actual amount needed at execution time.

Dynamic Memory Allocation: The opposite strategy of static memory allocation - Dynamic Memory Allocation, involves allocating memory as-needed.

This is where you allocate the data that you have. This gives you the opportunity to have everything in order.

Memory allocation is not necessary to display a matrix.

Constructors are necessary to initialize classes. It allows to avoid to a lot of problems with unauthorized access of memory.

Dynamic allocation makes possible allocation of memory during execution of program. If you do not use dynamic allocation, all required memory will be allocated during initialization phase (constructors are usually responsible for that). But you can't use more memory. Dynamic allocation was designed to overcome such problems.