Dynamic Memory Allocation in C++

Example program to demonstrate new and delete

/* program to demonstrate new and delete*/
#include  <iostream> 
using namespace std;
int main()
{
    int *p;
    float *q;
    char *r;

    p=new int(10);//allocates 2 bytes and the pass the address to p
    q=new float(1.5);//allocates 4 bytes and the pass the address to q
    r=new char('x');//allocates 1 bytes and the pass the address to r

    cout<<"*p="<<*p<<" *q="<<*q<<" *r="<<*r<<endl;;
    delete p;// release the memory allocated to p 
    cout<<"*p="<<*p<<" *q="<<*q<<" *r="<<*r<<endl;;

return 0;
}

Output:

*p=10 *q=1.5 *r=x
*p=garbage value *q=1.5 *r=x

Normal Array Declaration vs Using new

• Normal arrays are deallocated by the compiler (If the array is local, then deallocated when the function returns or completes).
• However, dynamically allocated arrays always remain there until either they are deallocated by programmer or program terminates.

Program to demonstrate array creation using new

/* Program to demostrate array creation using new */
#include <iostream> 
using namespace std;

int main() 
{
   int n;
   cout << "how many elements?:"; cin >> n;
   
   int *p = new int[n];		// creates n bytes in heap
   cout << "\nenter elelments:";
  
   for (int i = 0; i < n; i++) cin >> p[i];
  
   cout << "\nelements are:";
  
   for (int i = 0; i < n; i++)
    
   cout << *(p + i) << "\t";
  
   delete[]p; //release the memory 
  
   cout <<  "\n" <<  *(p + 0) << "\n";	//garbage value
   cout << *(p + 2) << "\n";	//garbage value
  
  return 0;
}

Output:

how many elements?:2

enter elelments:13 23

elements are:13	23	
1540455798
1585091451

What happens if enough memory is not available at runtime?

• In rare cases required memory may be available at runtime In such case program may  terminate abnormally
• To avoid this we can make use of an exception defined in std::bad_alloc, in which case it returns a NULL pointer and a user-defined message can be displayed to avoid confusion to the programmer
• Therefore, it may be a good idea to check for the pointer variable whether the  required memory is available or not

int *p = new(nothrow) int;
if (!p)
{
cout << "Sorry:Memory not avaialble to allocate\n";
}

Need for dynamic memory allocation

Static Memory Allocation

• Allocating memory at design time or compile time is known as Static memory allocation.
• In Static memory allocation Memory once allocated neither extended or deleted,
• Due to which there is a chance of wastage or shortage of memory
• Non-static and local variables get memory allocated on Stack Area

Example:

int a[5];//allocates 5xsize(int)=10bytes
int c[5]={1,2,3};//4 bytes wasted as space for 2 elements is unused
int b[2]={1,2,3};//need 4 bytes more

• Static memory once allocated cant be deleted by the programmer in between, it is deleted only by the compiler at the end
• In large Programs in real time projects after usage giving the memory back and having free space would enhance the performance