TITLE: Lvalues and rvalues


PROBLEM: sdoki@owl.WPI.EDU (Suresh Doki)

What is an lvalue? What is an rvalue?


RESPONSE: maxtal@physics.su.OZ.AU (John Max Skaller), 6 Feb 95

	An lvalue is an expression which has two meanings: 
the address of an object and the value contained in the object.
An rvalue has only one meaning: a value independent of any object.

	In ISO C, there are only lvalues and non-lvalues.
The ISO C Standard defines which expressions are lvalues and which not.
For example:

	int n; char buf[3];
	buf[n]++;

	n is an lvalue (indeed, a variable)
	buf[n] is also an lvalue
	buf[n]++ isn't an lvalue

Some operator require lvalues:  = (on the LHS), ++, unary &. 

In C++ it is more complicated, rvalues of class type always denote
objects -- temporary objects. This is because calling a member
function requires a "this" pointer, which has to point to an
actual object. 

Note that some expressions ISO C denotes non-lvalues are lvalues in C++. 
And note that the rules for when an lvalue is required by an operator
ONLY apply to built-in operators and NOT compiler generated assignments:

	struct complex { double x; double y; };
	complex sin(complex);
	complex z;
	sin(z)=sin(z); // error in ISO C, OK in C++

Since the "l" in "lvalue" came from the "L" in "Left Hand Side"
of an assignment (and "r" from "Right Hand Side"), you might
think the notion of lvalue in C++ is a bit weird. And you'd be right.