Storage Class C / C++ Brush up

C language

Storage Class
storage classes tell you about for things

1. initial value
2. meomory location
3. scope of variable
4. lifetime of variable

1)auto -meomory create at run time

-declare inside main( )

-in variable default is auto 

1. //even if u don't write auto then also program works same
2. #include<stdio.h>
3. #include<conio.h>
4.  
5. int main()
6. {
7.     double goa;
8.     auto int i;          //here i don't have any value therefore it will print some garbage value 
9.     printf("i=%d",i);
10. goa=addi();
11.  printf("goa i=%d",goa);
12. getch();
13. return 0;
14. }
15. 
    
16. 
    
17. int addi()     /
18. {
19.     auto int i ;   
20.     printf("add i=%d",i);  //now here i value will change as this is auto and scope of that i ended
21.     return i;
22. }
23. /*u know how scope overs ? i tell u this code works in stack area of ram and when scope overs
24. then in stack meomory previous i value get poped i.e it get deleted.
25. and Now as soon as i get initialised in addi() at that time lifetime of previous i variable get 
26. over */

summary

1. initial value-garbage value
2. meomory location-stack (in every case meomory created in stack at compile time)
3. scope of variable-Local
4. lifetime of variable-Till control remain with that function

2)register -meomory create at compile time

-store in register and not in ram 

-for very fast access suppose you have make counter

1. #include <stdio.h>
2. #include <conio.h>
3. //register storage class ..very fast storage class use in cpu
4. 
   
5. int main()
6.    {
7.           
8.    reg int i;   
9.    
10.    if(i--)
11.    {
12.           printf("%d",i);
13.           main();
14. }
15.    
16.    
17.    
18.    getch();
19.    return 0;
20.           }

summary

-same summary as auto the only difference here accessing is through cpu rather than ram due to which it is a quick access

initial value-garbage value

memory location-stack

scope of variable-Local

lifetime of variable-Till control remain with that function

3)static-meomory create at runtime

1. #include <stdio.h>
2. #include <conio.h>
3. 
   
4.                                                               //no i outside
5. int main()
6.    {
7.           
8.    static int i=3;   
9.    
10.    if(i--)
11.    {
12.           printf("%d",i);
13.           main();
14. }
15.    
16.    
17.    
18.    getch();
19.    return 0;
20.           }

in above program your output will be 210 but now in above program if u don't write keyword static i.e if u write code

1. #include <stdio.h>
2. #include <conio.h>
3. 
   
4.                                                               
5. int main()
6.    {
7.           
8.    int i=3;   
9.    
10.    if(i--)
11.    {
12.           printf("%d",i);
13.           main();
14. }
15.    
16.    
17.    
18.    getch();
19.    return 0;
20.           }

then you will get 22222222........infinite times as in that case there will be just int which is auto storage class as we know . so, in this case meomory will be created at runtime .

IN COMPILER PROGRAM FIRST COMPILED THEN COMES ON RUNTIME

SO U CAN SAY STATIC NOT SUPPORT RECURSION

summary

1. initial value-0 and not garbage value
2. memory location-stack 
3. scope of variable-Local
4. lifetime of variable-Till control remain with that function

4)extern-meomory create at runtime

With this storage class you can access same common variable in any function all across program

1. #include <stdio.h>
2. #include <conio.h>
3. 
   
4. int i=3;                                                                  //i declare outside here
5. int main()
6.    {
7.           
8.    extern int i;   //this statement means don't create seprate meom for i here as i is already 
9.                          //created globally
10. //let if u write here i=5 then it will change i value all across program.   
11.    if(i--)
12.    {
13.           printf("%d",i);
14.           main();
15. }
16.    
17.    
18.    
19.    getch();
20.    return 0;
21.           }

HERE IN CASE IF U DON'T INITIALIZE YOUR VARIABLE GLOBALLY THEN U WILL GET AN ERROR.

summary

initial value-garbage value

memory location-stack

scope of variable-Local

lifetime of variable-Till control remain with that function