Stack vs Heap: What’s the Difference?

The difference between stack vs heap memory is a common programming question among coders. Lack of knowledge of the two methods of memory allocation can result in misconceptions related to stack and heap. Here is a clear and detailed explanation that can help in case you encounter the stack vs heap question in a C#, C++, or other programming interview, as these memory types work more or less the same across most languages.

The Difference between Stack vs. Heap

Heap is used for dynamic memory allocation while the stack is used for static memory allocation. Both memories are stored in the computer’s Random Access Memory (RAM). The heap is a section of the computer’s memory that is manually managed by the user and partly by the CPU. Stack, on the other hand, is a special area of the computer’s memory that stores temporary variables formed by each function.

How Stack Memory Works

The stack utilizes a last in, first out (LIFO) data structure that is closely monitored and organized by the CPU. When a function calls another function, and the process recurs, the execution of these functions is delayed until the last function returns its value. Therefore, every time a function exits, all the variables transferred to the stack by that function are deleted. When a stack variable exits, the memory section becomes available for other stack variables.

The good thing about using the stack is that you do not have to manage the memory. Memory is allocated automatically, and you don’t have to free it when it is not needed. What’s more, reading from and writing stack variables is very quick since the CPU efficiently organizes stack memory.

Another significant feature of the stack is that the size of variables that can be stored on it is limited since its size varies with the size of the OS. The stack variables are available when the function that created them is running.

How Heap Memory Works

The heap memory section is larger compared to the stack. To allocate memory on the heap, you must use calloc() or malloc(), both of which are built-in C functions. After allocating memory on the heap, you are supposed to use free() to retain the memory when you are no longer need it. Failure to do so can cause a memory leak to your program. When discarded memory is not released, memory in the heap will be written off, making it unavailable for other processes.

Unlike the stack, you have to manage the memory (allotting and freeing variables). Its memory has no size limit and is relatively slow to read from and to write on since pointers are needed to access memory on the heap. The heap variables can be accessed by any function, anywhere in the program.

Note: if you want to find out more about the questions you might encounter during a C++ interview, you might also be interested in our guide to the most common CPP interview questions and their answers. 

Conclusion

And so, the big stack vs heap dilemma comes to an end. The heap is ideal for storing a large block of memory, for a long time while the stack can be used to store relatively small variables. Knowing the difference between heap vs stack is essential if you want to prove your skills in the programming field. We hope that this piece of information helps you as you prepare for your interview. Share your thoughts with us if the answer was helpful.