C++ Pointers & Arrays

Pointers §

Data & functions used in a program are stored inside the operative memory of the computer.

Every byte of the operative memory has a label, called address, represented by a hexadecimal number, prefixed with 0x. Example: 0x6dfed4. The first byte of the memory has address of 0x000000, the next one’s address is 0x000001, 256th byte of the memory’s address is 0x0000FF =

Variables, created to store addresses, are called pointers. Pointers are defined using a data type and the * operator, placed infront of the name of the variable:

int *myPointer;

The & prefix causes the statement to evaluate to the memory address of the identifier, so long as the identifier is a primitive type.

int x;
&x; // gets the address of x

The address is the first byte of the sequence of bytes required to store the data of the specific type.

double myVar = 4.00;
&d; // 0x0000C9 (201(10)) + 8 bytes = 0x0000D0 (208(10))

The & operator cannot be applied to unnamed constants or an arithmetic operation: &5, &(a-5).

To get the value of the variable the pointer is referrencing to use the * operator:

std::string food = "Pizza"; // Variable declaration  
std::string *ptr = &food;// Pointer declaration  
  
// Reference: Output the memory address of food with the pointer (0x6dfed4)  
std::cout << ptr << "\n";  
  
// Dereference: Output the value of food with the pointer (Pizza)  
std::cout << *ptr << "\n";

If a pointer has the value of 0 or NULL, it does not point to data in the memory.

A pointer can have the value of another pointer, provided they are of the same type.

int *p1 = 0, *p2 = NULL;  
int h = 10;  
p1 = &h; p2 = p1;  
std::cout << *p2 << std::endl;

Pointers can be assigned to data of the type they were defined with, unless they were defined as void. Void pointers can be assigned to pointers of all types, with casting. Pointers of all types can be assigned to void pointers without casting.

void *v;  
int *p;  
int i = 0, j = 1;  
v = &i;  
p = (int *) v;  
std::cout << *p << std::endl;  
p = &j;  
v = p;  
std::cout << *(int *) v << std::endl;

In the constant pointers to constants, the data pointed to by the pointer is constant and cannot be changed. The pointer itself is constant and cannot change and point somewhere else.

const int ci = 3; 
const int *q = &ci;

In constant pointers, the pointer points to a fixed memory location, and the value at that location can be changed because it is a variable, but the pointer will always point to the same location because it is made constant here.

In the pointers to constant, the data pointed by the pointer is constant and cannot be changed. Although, the pointer itself can change and points somewhere else (as the pointer itself is a variable).

We can create a pointer to a pointer that in turn may point to data or another pointer. The syntax simply requires the unary operator * for each level of indirection while declaring the pointer.

char a;
char *b;
char **c;
a = 'g';
b = &a;
c = &b;

Pointer comparison is done by comparing the addresses. Incrementing or decrementing the pointer, changes its address by the specified value multiplied by the number of bytes needed for its data type.

Arrays §

Arrays are made of multiple sequently ordered objects of the same data type. The name of the array is actually a pointer, that points to the first byte of the first element.