CODEWITHSUNDEEP
crvaluelvalue
S.S. Anne
S.S. Anne

Reputation: 15586

What are lvalues and rvalues?

I've heard the terms lvalue and rvalue come up when working with pointers.
However, I don't fully understand their meaning.

What are lvalues and rvalues?

Note: This is a question about C's lvalues and rvalues, not C++'s. It's also about their functionality, not their naming.

Upvotes: 3

Views: 212

Answers (2)

daShier
daShier

Reputation: 2116

According to the C Reference Manual (3rd Edition):

An lvalue is an expression that refers to an object in such a way that the object may be examined or altered. Only an lvalue expression may be used on the left-hand side of an assignment. An expression that is not an lvalue is sometimes called an rvalue because it can only appear on the right-hand side of an assignment. An lvalue can have an incomplete array type, but not void.

Upvotes: 1

Antti Haapala -- Слава Україні
Antti Haapala -- Слава Україні

Reputation: 134038

I've got a longer answer here, but basically C11 draft n1570 6.3.2.1p1:

An lvalue is an expression (with an object type other than void) that potentially designates an object [...]

C11 n1570 Footnote 64:

64) The name lvalue comes originally from the assignment expression E1 = E2, in which the left operand E1 is required to be a (modifiable) lvalue. It is perhaps better considered as representing an object locator value. What is sometimes called rvalue is in this International Standard described as the value of an expression. An obvious example of an lvalue is an identifier of an object. As a further example, if E is a unary expression that is a pointer to an object, *E is an lvalue that designates the object to which E points.

Not all lvalues are modifiable, i.e. can appear on the left side of an assignment. Examples of unmodifiable lvalues are those that

  • have array type,
  • have incomplete type
  • are const-qualified
  • structs or unions that have const-qualified members either directly or recursively

An lvalue can be converted to a value of an expression through lvalue conversion. I.e. in 

int a = 0, b = 1;
a = b;

both a and b are lvalues, as they both potentially - and actually - designate objects, but b undergoes lvalue conversion on the right-hand side of the assignment, and the value of the expression b after lvalue conversion is 1.

"Potentially designating an object" means that given int *p;, *p designates an object of type int iff p points to an object of type int - but *p is an lvalue even if p == NULL or indeterminate.

Upvotes: 4

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