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Stack And Heap

There is very basic question that is asked by almost every interviewer who is testing the knowledge of a .NET resource

What is stack?
What is heap?

In this article I will answer this question. But before I start explaining about stack and heap let us look into some basic concepts.

I assume you all are aware of variables in programming language. But what happens behind the scenes when we declare a variable. On variable declaration a memory location is allocated to store the value of variable. So the allocated memory location is having following attributes

Name
Data type
Value

Consider an example in which we are declaring a variable ‘x’ of type integer and set the value of the variable as ‘5’. Following screen shot provides some idea about how value is stored in memory.

Picture showing how variable stores data in the memory

Click to Enlarge

Now we have a fair idea of a variable. Now let’s talk about stack and heap.

Stack

Think of stack as a pile of objects like stack of plates in your office or college cafeteria. In programming, stack is a pile of memory (see the following figure)

Click to Enlarge

Let us see how stack works with your application. Consider the following function

function MyFunction() {
  int arg1 = 5;
  int arg2 = 6;
}

In the function we have created 2 variables and assign value to them. Behind the scenes, a memory is assigned to each variable in stack (See the following figure).

Picture showing how the data is stored in the stack

Click to Enlarge

When the function is exited stack clears all the memory location assigned to the variables.

Stack is having following attributes

Value types (that holds memory and data on same location) are stored in stack.
Stack is fixed sized.
Stack only contains the variables of those functions that are running.

Heap

Heap is for storing reference types. Unlike value type, reference type does not hold memory and data on the same location. Whenever we create an object, the object gets stored in the heap and a reference of heap is stored in stack (see the following screen shot)

Click to Enlarge

Posted By -	Karan Gupta

Posted On -	Monday, May 4, 2015

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