CSC447

Concepts of Programming Languages

L-Values

Instructor: James Riely

Variables

  • Given declarations such as: 
    
int x = 0;
              
    
var x:Int = 0
              
    
(define x 0)
  • What does x mean in? 
    
x = x + 1;
              
    
(set! x (+ x 1))

R-Mode and L-Mode

L-Values

  • Expression for which l-mode evaluation succeeds
  • Effectively, has an address
  • In C, take the address of l-values 
    
int x = 5;
int y = 6;
int *p = &x;
int *q = &y;
int **r = &p;
r = &q;
q = p;
**r = 7;

Not L-Values

  • L-mode evaluation sometimes disallowed
  • In C 
    
int x = 5;
int y = 6;
int *p = &(x + y); /* not allowed */
(x + y) = 7;       /* not allowed */
  • (x + y) not an l-value
    • although it might be stored temporarily

Further L-Values

  • L-values may require r-mode evaluation
  • Array access in C, Java, etc. 
    
arr[n + 2] = 7;
  • Field access in Java, Scala, etc. 
    
obj.f1 = 7
  • Combinations (including method calls) in Java, etc. 
    
obj.m1 ().f1[n + 2].m2 ().f2 = 7

C / C++