What is the role of pointer variable in data structures?

Answer 1

Structures are a type of variable, just as int and char are types of variable. All variables of the same type have one thing in common: their size is always the same. Typically, an int is 4 bytes, and a short is 2 bytes while a char is 1 byte. So a structure with 2 ints is therefore 8 bytes in length. A structure with an int and a char is 5 bytes, but structure member alignment will typically round this up to the nearest 4 bytes, padding any unused bytes (dependant upon your compiler's member alignment options). Regardless, a structure's size is known in advance by the number and type of its member variables. It's size is fixed. This is important when creating arrays of structures because every element in the array must be same size, and the size must be known in advance.

This then makes it impossible to store variable length strings and other dynamic arrays within a structure. That is where pointer variables come in. Since all they store is a memory address, pointer variables are a fixed size (4 bytes on a 32-bit system). It really doesn't matter how much memory is allocated to that memory address, because that memory is not physically part of the structure -- only the pointer variable itself is a member of the structure. The pointer variable simply "points" to the memory address stored in it, and the type of the pointer determines how that memory is accessed, whether it is a pointer to a primitive variable type, an array of char or an array of another structure altogether. Ultimately, the structure is only concerned with the pointer variable, not what it points at.

This then allows your structures to make better use of memory. There is no need to allocate an array far larger than you actually need, simply to cater for the "worst-case".

Other uses for pointers within structures include the ability to point at other structures of the same type, such as is required in linked lists. You cannot do this with references because the reference must be completely defined in advance; a structure that references its own type would require infinite memory because that reference contains another reference, which contains another reference... But it can point at another instance of the structure, including itself, because pointers only need to be declared, not completely defined.

The same applies to classes with member pointer variables. The only difference is that structures have public access by default while classes have private access by default. But classes can point to structures and vice versa. Apart from that, classes are just like any other variable type; they are a fixed size.