Wednesday, 7 February 2018

Part-13 : Use of pointers (for dynamic memory allocation in C)

Part-12 : Use of pointers (for dynamic memory allocation in C++)

Dynamic memory allocation/de-allocation
In the C++ programming language, there are two ways that memory gets allocated for data storage:
1.       Static Memory Allocation
The memory for defined variables is allocated by the compiler itself. The exact size and data type of variable to store must be known at the time of compilation. Therefore, standard array declarations need the constant size.
2.       Dynamic Memory Allocation
The memory allocated for the variables during the run time. The dynamically allocated space usually placed in a program segment known as the heap or the free store. In this allocation exact amount of space or number of items does not have to be known by the compiler in advance. Therefore, for dynamic memory allocation, pointers are crucial.

We can dynamically allocate storage space while the program is running, but we cannot create new variable names during the execution.
Therefore to implement the concept of dynamic allocation, it requires two steps:
1.       Creating the dynamic space.
2.       Storing its address in a pointer (so that the space can be accessed)
In the C++, new operator is used to create the dynamic memory space.
De-allocation is the process to clean-up the space being used for variables or other data storage. The compile time variables are automatically de-allocated based on their known extent (this is the same as scope for "automatic" variables). It is the programmer's job to de-allocate dynamically created space, to de-allocate dynamic memory space the delete operator is available in C++ programming language.

6.5.1 new and delete operators
In the C++, for allocation and de-allocation of memory dynamically,  new and delete operators are used.
To allocate space dynamically, use the unary operator new, followed by the type being allocated.

 new int;        // dynamically allocates an int
 new double;     // dynamically allocates a double

These above statements are not very useful by themselves, because the allocated spaces have no names, and the new operator returns the starting address of the allocated space, and this address can be stored in a pointer:

 int * p;        // declare a pointer p
 p = new int;    // dynamically allocate an int and load address into p
 double * d;     // declare a pointer d
 d = new double; // dynamically allocate a double and load address into d

Consider the following example for dynamically allocation and de-allocation of the variables.

Example 6.4 : Program to allocate and de-allocate the dynamic space using pointers.

#include < iostream.h >
void main()
int *p1;

p1 = new int;     // dynamic memory allocation
*p1 = 300;

cout << “\n Value inside the p1 = ” << *p1;
delete p1;        //free the dynamically allocated memorys
Value inside the p1 = 300

Using the new and delete operators, we can create arrays at runtime by dynamic memory allocation and de-allocation. The general form for doing this is:

p_var = new array_type[size];

size specifies the number of elements in the array.

To free the memory of an array we use:

delete[] p_var;

Let see the given example for the above explanation.

Example 6.5 : Program to create dynamic array using pointers.

#include < iostream.h >
void main()
int *p;

p = new int[10];  // dynamic allocation for array of 10 elements

for(int i = 0; i < 10; i++)
p[i] = i;

cout << “\n content of array are : ” << endl;

for(i = 0; i < 10; i++)
cout << p[i] << endl;
delete[] p;       //free the array memories

content of array are :