Question

Computer Science Midterm Exam Review Questions

21. How do you dynamically allocate variables in C++?

22. How do you dynamically allocate an array in C++?

23. How do you deallocate variables in C++?

24. How do you deallocate arrays in C++?

25. How do you dynamically allocate memory in C?

26. How do you deallocate memory in C?

Answer 1

In c++ , new and delete operators are used to allocate and de-allocate memory respectively. The new operator denotes a request for memory allocation on the heap. If sufficient memory is available, new operator initializes the memory and returns the address of the newly allocated memory to the pointer variable.

Here, data type could be any built-in data type including array or any user defined data types including class and structure.

21. Syntax to dynamically allocate variables in c++:

data_type pointer_variable = new data_type;

example : int *p = new int;

22. Syntax to dynamically allocate array in c++:

data_type   pointer_variable = new data_type[ size ] ; // size variable specifies the number of elements in an array

int *p = new int[ 5 ] ; // creating an array of 5 elements

23 . Syntax to de-allocate variable in c++:

delete pointer_variable ; // it releases memory pointed by the pointer_variable.

example :

int *p = new int;

delete p; // delete the memory pointed by the pointer p

24. Syntax to de-allocate arrays in c++:

delete[ ]   pointer_variable;   // it will free the entire array pointed by the pointer_variable.

Following   4 library functions are provided by C defined under <stdlib.h> header file to facilitate dynamic memory allocation in C programming :

1. malloc()

2. calloc()

3. free()

4. realloc()

25. malloc() function is used to dynamically allocate a single large block of memory with the specified size. It returns a pointer of type void which can be cast into a pointer of any form.

pointer = (cast_type*) malloc(size in bytes); 

/* dynamically allocating memory using malloc().

Here, sizeof() operator is used to get the size of int in bytes.   */

int *p = (int *) malloc(sizeof(int));

int *p = (int*) malloc( n* sizeof(int)); // dynamically creating an array ( n denotes the number of elements in the array )

26. free() function is used to dynamically de-allocate the memory .

Syntax:

free(pointer_variable);

int *p = (int*) malloc(sizeof(int));

free(p); // deallocating memory pointed by the pointer p .