Dynamic allocation

In C++, memory can be allocated at runtime using the new operator. This is known as dynamic allocation. It lets you create data structures whose size is not known until the program runs, such as linked lists and trees.

Unlike static allocation (where the compiler decides the size at compile time), new requests memory from the heap and returns a pointer to it. Every memory block allocated with new must be released with delete when it is no longer needed, otherwise your program leaks memory.

Here is a simple example that allocates an integer dynamically:

int * p = new int;   // allocate one int on the heap
*p = 42;             // store a value through the pointer
printf("%d\n", *p);  // read it back
delete p;            // free the memory

To allocate a whole array dynamically, use new with brackets, and release it with delete[]:

int * arr = new int[5];   // array of 5 ints
arr[0] = 7;
printf("%d\n", arr[0]);
delete[] arr;             // note the brackets

The pointer returned by new is just like any other pointer: you can dereference it with *, access array elements with [], and pass it to functions.

Exercise

Write a program that dynamically allocates an integer, stores the value 5 in it, and prints it on its own line.