Dynamic Memory Allocation in C++ can be defined to perform memory allocation manually by the programmer. The dynamically allocated memory is allocated on Heap. All variables declared inside the function will take up a memory from the stack. All the local variables get memory allocated on Stack. Static memory allocation consists of allocating memory in compile-time before the program is executed.
Dynamic Memory Allocation in C++
Stack
A stack is the special region of the computer’s memory, where temporary variables are stored. Stack follows the First In Last Out (FILO) data structure. When a function declares a variable, this variable is pushed into the stack. Once the function exits, the variable is popped from the stack.
Heap
A heap is a region of computer’s memory, used for dynamic memory allocation. When you use dynamic allocation, all the created variables are stored into a heap. Heap memory is not managed automatically. When you use dynamic memory allocation, a pointer that is located in stack points to the region of the allocated memory in heap.
New Operator in C++
The new operator in C++ can be used for dynamic memory allocation. Operator new returns the pointer to the newly allocated space. If you want to allocate memory for one single element of a specified data type (it can be a built-in data type, structure or class).
Syntax: new data_type;
In case if you would like to allocate memory in arrays, follow the order as follows:
Syntax: new data_type[size_of_array];
Initialize memory
We can also initialize the memory using the new operator.
Syntax: pointer-variable = new data-type(value);
Example:
int *p = new int(25); float *q = new float(75.25);
Allocate a block of memory
The new operator is also used to allocate a block(an array) of memory of type data-type.
Syntax: pointer-variable = new data-type[size];
Example:
int *p = new int[10]
Example:
int* arr;//pointer to int int n;//number of elements cout << "Please, enter the number of elements for input" << endl; cin >> n; // get n arr = new int[n];//allocate memory for array of int of size n //get user’s input cout<<"Enter " << n << " elements" << endl; //get elements in loop for (int i = 0; i != n; ++i) cin >> arr[i]; cout<<"You entered :" << endl; for (int i = 0; i != n; ++i) cout << "arr[" << i << "] = " << arr[i] << endl;
Output:
Please, enter the number of elements for input
5
Enter five elements
1
2
3
4
0
You entered:
arr[0] = 1
arr[1] = 2
arr[2] = 3
arr[3] = 4
arr[4] = 0
delete Operator in C++
Once you do not need memory allocated by operator new, you have to release it. You can do it by using operator delete:
Syntax:
delete pointer; for single object and
delete[ ] pointer; for an array of objects
Example:
#include<iostream>
using namespace std;
int main ()
{
//Pointer initialization to null
int* p = NULL;
//Request memory for the variable using new operator
p = new(nothrow) int;
if (!p)
cout<<"allocation of memory failed\n";
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(nothrow) int[n];
if (!q)
cout<<"allocation of memory failed\n";
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;
}
Output:
Value of p: 29
Value of r: 75.25
Value store in block of memory: 1 2 3 4 5