CSC 347 - Concepts of Programming Languages

L-Values

Instructor: Stefan Mitsch

Learning Objectives

When do we read and when write to a memory location?

• Understand the difference between l-values and r-values

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

• Consider

  x = x + 1;
  

• Right-hand x denotes
  • value read from storage location
• Left-hand x denotes
  • the storage location (address)
• Goes back to Strachey in the 1960s
  • Fundamental Concepts in Programming Languages, Christopher Strachey (1967)

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
  

Summary

• L-values: refers to a memory location
• R-value: value of a memory location
• Specialized uses of l-value / r-value