Constructors

Arrays & Pointers

An array variable refers to the first indexed variable in an array.

int a[3] = {10, 20, 30};
cout << a << endl; // the address of a[0], e.g., 0xffffcc14
cout << *a << endl; // the value of a[0], i.e., 10

Here, you can see that 'a' (not a[0]) can be treated as a pointer.

Assume that there is alos a pointer, int *p;;

Then, p=a; is legal. It makes p pointing to the same place as 'a'.

To access the three integers in the array, you can use p[0], p[1], p[2]; which is equivalent to *p, *(p+1), *(p+2).

However, a = p; is illegal because the array pointer 'a' is a const pointer. It's value cannnot be changed.

'a' is the address of array a, you cannot change an memory address of a variable. If you want to change the value, you can do *a = *p

The new[] operator is used to dynamically allocate memory to a data object/item.

int *p = new int[4];
p[0] = 10;
p[1] = 20;
p[2] = 30;
p[3] = 40;
cout << p[0] << endl;
delete [] p;
p = NULL;

Alternatively, we can use this syntax in C++ to initialize a dynamic array

int *p {new int[4] {10, 20, 30, 40} };

As shown above, p is pointing to the first integer in the new array.

After delete []p, we should not forget to set p to NULL to avoid dangling pointers.

delete[]p;

delete[4]p;

delete p; this is legal, but will only deallocate memory for p[0] the first element, without deallocating the rest in the array.

A pointer to an array is especially useful when we want a function to return an array.

int *test(){
    int*p = new int[4];
    p[0] = 10; 
    p[1] = 20; 
    p[2] = 30; 
    p[3] = 40; 
    return p; 
}

Note that the array type is not allowed as a return-type of functions.

Be careful! Remember to use delete[] and set the pointer to NULL for the pointer that takes the return value from such function. ###Example

int *a = test(); 
delete [] a; 
a = NULL;