Sunday, 4 September 2016

Storage Bindings For Variables

Storage Bindings For Variables

This post is meant to describe variables into four different categories based on their Storage Bindings. The Binding categories will be
  • Static Variables
  • Stack Dynamic variables
  • Explicit Heap Dynamic variables
  • Implicit Heap Dynamic Variables

Static Variables
Static variables are bound to memory cells sometime before execution and remain bound to the same memory cells until program execution terminates. This means values are assigned to the memory cells of a variable before the execution of a program begins. Global variables are usually static variables although a lot of languages allow subprograms to have static variables called “local static variables”. Class variables in a lot of language implementations are usually static variables. Class Variables are created sometime before program execution begins.
-Static Variables are fast. They can be addressed directly unlike other variables that require indirect addressing which is usually slower.
-Static Variables are history sensitive. They always remember.
Static variables reduce programming flexibility. They do not support recursion.
Example of a Static Variable In Java
class StaticVariableEg 
   static int count=0; //count is a static variable 
   public void increment() 
   public static void main(String args[]) 
      StaticVariableEg obj=new StaticVariableEg(); 
The Implementation of static class variables in Java is similar to that of C# and C++.
C and C++ have a static specifier that can be used in variable definitions in a function to make the variable static.
Stack Dynamic Variables
In stack dynamic variables, storage bindings are created during the “elaboration” of a variable, Not when execution starts but when execution gets to the variable(elaboration). Stack Dynamic variables are created at run time when execution has begun. Like static variables, deallocation cannot be done until execution is complete. In stack dynamic variables both allocation and deallocation is done from the run time stack
Stack Dynamic variables are more flexible than static variables. They can be used in recursive subprograms.
Stack Dynamic variables are not history sensitive. Their values can permanently change during execution.
Stack Dynamic variables are slower than static variables because of the incurred overhead in allocation and deallocation of variables and also it requires indirect addressing.
In Java, C++, and C#, variables defined in methods are by default Stack dynamic
class StackDynamicEg 
     void counter() 
     { int count = 0; //Stack Dynamic variable 
Explicit Heap Dynamic Variables
In Explicit Heap Dynamic variables allocation and deallocation are done at run time by explicit instructions written by the programmer. Allocation and Deallocation of explicit Heap Dynamic variables is done from the heap. They are created with Pointers and Reference variables.
They are very memory prudent.
They can be used to create dynamic data structures like Linked Lists and Trees that can grow and shrink during execution
They require pointers which are relatively more complex to implement
They are slower than static variables because of the incurred overhead in allocation and deallocation of variables
In C++
int *intnode; // Create a pointer 
intnode = new int; // Create the explicit heap-dynamic variable 
delete intnode; // Deallocate the heap-dynamic variable
In Java all objects are explicit heap dynamic variables and are accessed through reference variables. In contrast to C++, Java does not allow explicit deallocation of variables, deallocation is done implicitly by the garbage collector.
Implicit Heap Dynamic Variables
In Implicit Heap Dynamic Variables, allocation and deallocation are done only when the variable is assigned a value at runtime. Allocation and deallocation are done at runtime from the Heap.
  • They are very flexible
  • They make optimum use of memory
  • They are very slow
  • They are not history sensitive
In Python
str = ["Java", "Python", "C++", "Delphi", "Fortran"] #implicit heap dynamic variable 'str' is created at runtime 
print str 
str = 5 #implicit heap dynamic variable which was previously a list of length 5 is now an integer 
print str

Storage Bindings For Variables

Please, this topic is open for discussion. Let us talk more on implementation in other languages I didn't mention or challenges encountered in implementation. I recommend Concepts of Programming Languages (11th Edition) 11th Edition by Robert W. Sebesta to anyone who wants an elaborate explanation of the topic. It is a really great book.

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My name is Solomon Ubani, I'm a computer scientist. I have few years of programming experience. I am happy to learn from you.