What do "lvalue" and "rvalue" mean

[ShivaniX]

What do "lvalue" and "rvalue" mean?

[mohanl]

lvalue" is an expression that refers to an object (which can have (a) value(s) within it), and "rvalue" is an expression that only has a value and no association with an object.

Basically lvalue is a storage location where data can be stored/ assigned and moreover it is not an expression. rvalue is an expresion which has a value. all lvalues can be used as rvalues. But all rvalues can't be used as lvalues.

lvalue means expression that can be used on the left hand side of an assignment.
All lvalues can be used as rvalues. Reverse isn't always true.

An "L value" is an expression that represents a storage location. The name "L value" comes from the fact that such expressions can go on the left-hand side of an assignment.

In this example:

int p*;
double a[20];
double v;

"p", "v", "(p++)*", "(10+a)*", and "a[12]" are all "L values" because those expressions are all valid on the left-hand side of an assignment (before the "=").

However, "sqrt(v)", "a[12]+5.0", "(10+a)* *10.0", and "(p++)*/v" are not "L values", because, although they are syntactically correct C expressions, they do not represent a storage location and therefore cannot go on the left-hand side of an expression. Such expressions are called "R values", because in an assignment they can only be used on the right hand side.

[retheeshsoman]

Dear Sivani,

When we declare a variable we inform the compiler of two things, the name of the variable and the type of the variable. For example, we declare a variable of type integer with the name k by writing:

int k;

On seeing the "int" part of this statement the compiler sets aside 4 bytes of memory (on a PC) to hold the value of the integer. It also sets up a symbol table. In that table it adds the symbol k and the relative address in memory where those 4 bytes were set aside.

Thus, later if we write:
k = 2;
we expect that, at run time when this statement is executed, the value 2 will be placed in that memory location reserved for the storage of the value of k. In C we refer to a variable such as the integer k as an "object".

In a sense there are two "values" associated with the object k. One is the value of the integer stored there (2 in the above example) and the other the "value" of the memory location, i.e., the address of k. Some texts refer to these two values with the nomenclature rvalue (right value, pronounced "are value") and lvalue (left value, pronounced "el value") respectively.

In some languages, the lvalue is the value permitted on the left side of the assignment operator '=' (i.e. the address where the result of evaluation of the right side ends up). The rvalue is that which is on the right side of the assignment statement, the 2 above. Rvalues cannot be used on the left side of the assignment statement. Thus: 2 = k; is illegal.

Actually, the above definition of "lvalue" is somewhat modified for C. According to K&R II (page 197): [1]

"An object is a named region of storage; an lvalue is an expression referring to an object."

However, at this point, the definition originally cited above is sufficient. As we become more familiar with pointers we will go into more detail on this.

Okay, now consider:

int j, k;

k = 2;
j = 7; <-- line 1
k = j; <-- line 2

In the above, the compiler interprets the j in line 1 as the address of the variable j (its lvalue) and creates code to copy the value 7 to that address. In line 2, however, the j is interpreted as its rvalue (since it is on the right hand side of the assignment operator '='). That is, here the j refers to the value stored at the memory location set aside for j, in this case 7. So, the 7 is copied to the address designated by the lvalue of k.

In all of these examples, we are using 4 byte integers so all copying of rvalues from one storage location to the other is done by copying 4 bytes. Had we been using two byte integers, we would be copying 2 bytes.