Memory management

Heap vs Stack

What is Heap?

  • an area of memory used for dynamic allocation.

  • memory that is not manage automatically

  • if fail to dealocate memory, you will end up with memory leak

  • does not have size restrictions on variable size

What is Stack?

  • the stack is where memory is allocated for automatic variables within functions

  • "LIFO" (last in first out) data structure

  • memory is managed automatically

  • there is a limit on the size of variables

  • stack overflow occurs if call stack pointer exceeds stack boundary

When is heap and stack used?

  • Memory space for objects is always allocated in heap. 

  • Built-in datatypes like int, double, float and parameters to methods are allocated on the stack.

  • Even though objects are held on heap, references to them are also variables and they are placed on stack.

How to store object in heap?

  • When you create an object using the new operator,

myobj = new Object();

  • it allocates memory for the myobj object on the heap.

  • When you do a string initialization,

String myString; 

  • it is a reference to an object so it will be created using new and hence it will be placed on the heap.

  • keyword "static" create variable in heap memory

Can an object store in stack?

  • If you create an object inside a function without using the “new” operator then this will create and store the object on the stack, and not on the heap.

  • The stack memory space is used when you declare automatic variables.

Example:

When to use?

Heap

  • if you need to allocate a large block of memory

  • if you need to keep the variable around a long time

  • if you need variables like arrays and structs that can change size dynamically

Stack

  • if you are dealing with realtively small variables that only need to persist as long as the function using them is alive

Summary

No. Heap Stack
1. manually deleted by programmer automatically deallocate
2. no limit on memory size limit on stack size
3. slower access fast access
4. global variable local variable only
5. memory leak stack overflow
6. initialized at runtime initialized before runtime
7. live from start to the end of application short-lived

Thank you.

See you again next week

Heap vs Stack

By shirlin1028

Heap vs Stack

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