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rvalue, lvalue

© June 2005 Tony Lawrence

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 the 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

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.

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More Articles by © Tony Lawrence

Sun Jun 5 05:40:13 2005: 613   Bela

Many years ago, I was once involved in debugging a problem on a CP/M (8080) system, in a program written in Microsoft Fortran.

I don't remember many details, but the punchline was: a floating point constant was being passed to a subroutine that modified that argument. After that moment, references to what (in the source) appeared to be the constant "1.0" (or whatever it was) actually returned the modified value.

The moral: sometimes a compilation system unexpectedly _does_ see constants as lvalues. Much to the detriment of your sanity.



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