Dynamic Memory

When a program needs to store a huge amount of data in memory, or when the amount of data is nondeterministic at compile time, the heap memory segment is used. It is “unlimited” (unlike the stack).

Heap commands in C

The following functions are used for heap management:

• malloc - allocates memory
• calloc - similar to malloc, but a bit more appropriate for arrays
• realloc - resizes allocated memory block; it can be used as a substitute for malloc or free with the right arguments passed in
• free - deallocates memory

Storing data on the heap

Using C:

// allocates a variable on a STACK
int a;

// allocates memory space on a HEAP (for int - 4 bytes) and returns a void pointer to it
int *p = (int*)malloc(sizeof(int));
*p = 10; // storing data on the HEAP

// clear heap allocation
free(p); // the compiler stores somewhere info on how many bytes were allocated

// array on the HEAP
int *p = (int*)malloc(20 * sizeof(int));
*p = 10; // first index (could be p[0] = 10;)
*(p + 1) = 20; // second index

In the example above the result of malloc is cast to int*, because malloc returns a void* pointer.

Using C++:

// allocates a variable on a STACK
int a;

// a scalar on a HEAP
int *p = new int;
*p = 10;
delete p;

// an array on a HEAP
p = new int[20];
delete[] p;