The document discusses dynamic memory allocation in C and C++. It describes functions like malloc(), calloc(), free(), and realloc() for allocating and freeing memory dynamically. It also covers the new and delete operators for dynamic memory allocation in C++. Issues like dangling pointers and memory leaks that can occur due to improper use of dynamic memory are explained.

1. Programming Fundamentals
Lecture 9
Dynamic memory allocation in C and in C++,
dangling pointer, memory leak

2. Dynamic memory allocation
• Dynamic memory allocation in C/C++ refers to performing memory
allocation manually by programmer.
• The most important use is flexibility provided to programmers. We
are free to allocate and deallocate memory whenever we need and
whenever we don’t need anymore.

3. Dynamic memory allocation
• For normal variables like “int a”, “char str[10]”, etc, memory is
automatically allocated and deallocated. For dynamically allocated
memory it is programmers responsibility to deallocate memory when
no longer needed.

4. malloc- C function supported in C++
• “malloc” or “memory allocation” method in C 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.
• ptr = (cast-type*) malloc(byte-size)
• ptr = (int*) malloc(100 * sizeof(int));
• Since the size of int is 4 bytes, this statement will allocate 400 bytes of
memory. And, the pointer ptr holds the address of the first byte in the
allocated memory.
• Header file: cstdlib

5. int main()
{
// This pointer will hold the base address of the block
created
int* ptr;
int n, i, sum = 0;
// Get the number of elements for the array
cin>>n;
// Dynamically allocate memory using malloc()
ptr = (int*)malloc(n * sizeof(int));
// Check if the memory has been successfully allocated
by malloc or not
if (ptr == NULL) {
cout<<"Memory not allocated.n";
exit();
}
else {
// Memory has been successfully allocated
cout<<"Memory successfully allocated using malloc.n“;
// Get the elements of the array
for (i = 0; i < n; ++i) {
ptr[i] = i + 1;
}
// Print the elements of the array
cout<<"The elements of the array are: ";
for (i = 0; i < n; ++i) {
cout<<ptr[i];
}
}
return 0;
}

6. Calloc
• “calloc” or “contiguous allocation” method in C is used to
dynamically allocate the specified number of blocks of memory of the
specified type. It initializes each block with a default value ‘0’.
• ptr = (cast-type*)calloc(n, element-size);
• ptr = (float*) calloc(25, sizeof(float));
• This statement allocates contiguous space in memory for 25 elements
each with the size of the float

7. free
• “free” method in C is used to dynamically de-allocate the memory.
The memory allocated using functions malloc() and calloc() is not de-
allocated on their own. Hence the free() method is used, whenever
the dynamic memory allocation takes place. It helps to reduce
wastage of memory by freeing it.
• free(ptr);

8. realloc
• “realloc” or “re-allocation” method in C is used to dynamically
change the memory allocation of a previously allocated memory. In
other words, if the memory previously allocated with the help of
malloc or calloc is insufficient, realloc can be used to dynamically re-
allocate memory.
• ptr = realloc(ptr, newSize); where ptr is
reallocated with new size 'newSize'.
// use: taking array size again from user in a program

9. new operator
• The new operator denotes a request for memory allocation. If
sufficient memory is available, new operator initializes the memory
and returns the address of the newly allocated and initialized memory
to the pointer variable.
pointer-variable = new data-type;
int *p = NULL; p = new int;
int *p = new int;

10. Initialize the memory
• pointer-variable = new data-type(value);
• Example: int *p = new int(25); float *q = new
float(75.25);

11. Allocate block of memory
• new operator is also used to allocate a block(an array) of memory of
type data-type.
• pointer-variable = new data-type[size];
• Example: int *p = new int[10]
Dynamically allocates memory for 10 integers continuously of type int
and returns pointer to the first element of the sequence, which is
assigned to p(a pointer). p[0] refers to first element, p[1] refers to
second element and so on.

12. delete operator
• // Release memory pointed by pointer-variable delete
pointer-variable;
• Here, pointer-variable is the pointer that points to the data object created
by new.
delete p;
To free the dynamically allocated array pointed by pointer-variable, use
following form of delete:
// Release block of memory pointed by pointer-variable
delete[] pointer-variable;
Example: // It will free the entire array pointed by p.
delete[] p;

13. // C++ program to illustrate dynamic allocation
// and deallocation of memory using new and delete
#include <iostream>
using namespace std;
int main ()
{
// Pointer initialization to null
int* p = NULL;
// Request memory for the variable using new
operator
p = new int;
if (p==NULL)
cout << "allocation of memory failedn";
else
{
// Store value at allocated address
*p = 29;
cout << "Value of p: " << *p << endl;
}
// Request block of memory using new operator
float *r = new float(75.25);
cout << "Value of r: " << *r << endl;
// Request block of memory of size n
int n = 5;
int *q = new int[n];
if (q==NULL)
cout << "allocation of memory failedn";
else
{ for (int i = 0; i < n; i++)
q[i] = i+1;
cout << "Value store in block of memory: ";
for (int i = 0; i < n; i++)
cout << q[i] << " ";
}
// freed the allocated memory
delete p;
delete r;
// freed the block of allocated memory
delete[] q;
return 0; }

14. Dangling pointer
• A situation when a pointer points to the memory location of the
deallocated memory. It is important to make a pointer NULL after de-
allocating its allocated memory.
• Example of that situation could be when two pointers pointing to
same memory location and one deallocates that memory.

15. Memory leak
• A memory leak occurs in C++ when you allocate memory dynamically
and never give it back.
• The memory leak occurs, when a piece of memory which was
previously allocated by the programmer. Then it is not deallocated
properly by programmer. That memory is no longer in use by the
program. So that place is reserved for no reason. That’s why this is
called the memory leak.
• For the memory leak, some block of memory may have wasted. If the
system has enough memory, in that case also this may slow down the
performance.

16. References
• C++ How to Program
By Deitel & Deitel
• The C++ Programming Language
By Bjarne Stroustrup
• Object oriented programming using C++ by Tasleem Mustafa, Imran Saeed, Tariq Mehmood, Ahsan Raza
• https://www.tutorialspoint.com/cplusplus
• http://ecomputernotes.com/cpp/introduction-to-oop
• http://www.cplusplus.com/doc/tutorial
• http://www.c4learn.com/c-programming/c-dangling-pointer-causes/
• https://www.guru99.com/c-loop-statement.html
• www.w3schools.com
• https://www.geeksforgeeks.org/