2005/06/03 rvalue, lvalue

These are terms you'll see in programming, and particularly so in C. I find it's easy to remember these in two ways: An lvalue will be found to the left of an assignment, an rvalue to thye right. You can also think of "l" as the location of the value and "r" as the read-out.

In most contexts, it's pretty obvious which is which, which also means that it's easy for a compiler to know when you screw up. To take a silly example, "b=1" is fine, but "1=b" isn't because "1" obviously is not something that has a location or address: it's probably going to be popped right into a register and used directly. I say "probably" because the compiler is free to assign storage for that if it wanted to, or even to just clear b and increment it if that's a faster way to get "1" assigned. Regardless, "1" is not an lvalue.

But things aren't always that obvious.

main () {
enum color { red, green, blue, indigo };
enum color c;
c = green;
// that's fine
// ..
// lots of other code, and we've forgotten what green really is
green=indigo;
}
 

That last line will cause an "invalid lvalue in assignment" from the compiler, because "green" is not an lvalue. There are plenty of other ways a beginning programmer can cause similar objections.

Macworld Mac Security Superguide $9.99

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