Stack vs Heap: Know the Difference

What is a Stack?

A stack is a special area of computer’s memory which stores temporary variables created by a function. In stack, variables are declared, stored and initialized during runtime.

It is a temporary storage memory. When the computing task is complete, the memory of the variable will be automatically erased. The stack section mostly contains methods, local variable, and reference variables.

In this tutorial, you will learn,

• What is Stack?
• What is Heap?
• Key Differences between Stack and Heap
• Advantages of using Stack
• Advantages of using Heap
• Disadvantages of using Stack
• Disadvantages of using Heap
• When to use the Heap or stack?

What is Heap?

The heap is a memory used by programming languages to store global variables. By default, all global variable are stored in heap memory space. It supports Dynamic memory allocation.

The heap is not managed automatically for you and is not as tightly managed by the CPU. It is more like a free-floating region of memory.

Key Differences between Stack and Heap

Parameter	Stack	Heap
Type of data structures	A stack is a linear data structure.	Heap is a hierarchical data structure.
Access speed	High-speed access	Slower compared to stack
Space management	Space managed efficiently by OS so memory will never become fragmented.	Heap Space not used as efficiently. Memory can become fragmented as blocks of memory first allocated and then freed.
Access	Local variables only	It allows you to access variables globally.
Limit of space size	Limit on stack size dependent on OS.	Does not have a specific limit on memory size.
Resize	Variables cannot be resized	Variables can be resized.
Memory Allocation	Memory is allocated in a contiguous block.	Memory is allocated in any random order.
Allocation and Deallocation	Automatically done by compiler instructions.	It is manually done by the programmer.
Deallocation	Does not require to de-allocate variables.	Explicit de-allocation is needed.
Cost	Less	More
Implementation	A stack can be implemented in 3 ways simple array based, using dynamic memory, and Linked list based.	Heap can be implemented using array and trees.
Main Issue	Shortage of memory	Memory fragmentation
Locality of reference	Automatic compile time instructions.	Adequate
Flexibility	Fixed size	Resizing is possible
Access time	Faster	Slower

Advantages of using Stack

Here, are the pros/benefits of using stack:

• Helps you to manage the data in a Last In First Out(LIFO) method which is not possible with Linked list and array.
• When a function is called the local variables are stored in a stack, and it is automatically destroyed once returned.
• A stack is used when a variable is not used outside that function.
• It allows you to control how memory is allocated and deallocated.
• Stack automatically cleans up the object.
• Not easily corrupted
• Variables cannot be resized.

Advantages of using Heap

Pros/benefit of using heap memory are:

• Heap helps you to find the greatest and minimum number
• Garbage collection runs on the heap memory to free the memory used by the object.
• Heap method also used in the Priority Queue.
• It allows you to access variables globally.
• Heap doesn’t have any limit on memory size.

Disadvantages of using Stack

Cons/Drawbacks of using Stack memory are:

• Stack memory is very limited.
• Creating too many objects on the stack can increase the risk of stack overflow.
• Random access is not possible.
• Variable storage will be overwritten, which sometimes leads to undefined behavior of the function or program.
• The stack will fall outside of the memory area, which might lead to an abnormal termination.

Disadvantages of using Heap

Cons/drawbacks of using Heaps memory are:

• It can provide the maximum memory an OS can provide
• It takes more time to compute.
• Memory management is more complicated in heap memory as it is used globally.
• It takes too much time in execution compared to the stack.

When to use the Heap or stack?

You should use heap when you require to allocate a large block of memory. For example, you want to create a large size array or big structure to keep that variable around a long time then you should allocate it on the heap.

However, If you are working with relatively small variables that are only required until the function using them is alive. Then you need to use the stack, which is faster and easier.