Question

In C, is there a way to identify the rvalues and lvalues ?

Some of them are easy to identity say, in an assignment, the left value is lvalue and the value in the right is rvalue.

But other scenarios, identification with such a rule is difficult.

For example : *p++ and i++ (where p is a pointer to integer and i is an integer) – how to identify whether it is an rvalue or lvalue ?
The context is ++*p++ works while ++i++ does not since i++ is an rvalue (as told by serious guys).

How to identify rvalue and lvalue in an expression?

Answer 1

lvalue (from left-hand side (LHS) value) in something that refers to a memory (or register) storage and that you can assign values to. *p++ is an lvalue since it is a dereferenced pointer (i.e. refers to the location in memory that ptr points to while the value of ptr itself is the address of that location) and ++*ptr++ actually means: *ptr = *ptr + 1; ptr = ptr + 1; – it increments the value pointed to by ptr and then increments the pointer value itself. i++ is not an lvalue since it is the value of i incremented by 1 and does not refer to a location in memory. You can think of such values as final – they cannot be further modified and can only be used as values to assign to lvalues. That’s why they are called rvalues (from right-hand side (RHS) value).

LHS and RHS refer to both sides of the assignment expression A = B;. A is the LHS and B is the RHS.