What is the difference between these three lines in C++?
int *p= new int;
int *p= new int[10];
int *p = new int[];
We are already dynamically declaring the memory to pointer variable p, why is it required to specifically mention pointer array size?
int *p = new int;
This allocates enough memory for a single object of type int and stores a pointer to it in the pointer-to-int variable p. This means that *p refers to a valid int object.
int *p = new int[10];
This allocates enough contiguous memory for ten objects of type int and stores a pointer to the first int in the pointer-to-int variable p. This means that p[0] through p[9] refer to valid int objects.
int *p = new int[];
This statement is syntactically incorrect. It is not valid C++, and therefore has no meaning.
... why is it required to specifically mention pointer array size?
How else is the memory allocator supposed to know how much memory to allocate if you don't tell it how many ints you need room for?
new for a single object must be matched with delete for a single object, and new[] for an array must be matched with delete[] for an array. new[] stores the array count in such a way that delete[] knows how many objects to destruct (if needed) before freeing the underlying memory block.