Sunday, May 24, 2015

RVR^LVR Lesson #1

A working definition of lval/rval on http://thbecker.net/articles/rvalue_references/section_01.html says -- An L-value expression is an expression that
1) refers to a memory location (heap/stack but not a literal value), and allows us to take the address of that memory location via the address-of (&) operator.

The other key features are all obscure:
2) (my suggestion) can pass into a reference parameter of a function
3) can be the RHS of a reference variable initializer

If overwhelming, just focus on 1). In short. L-value expression evaluates to a LLLocation in memory

[I] introduces a simple test of Rval vs Lval, introduced by Stephan T. Lavavej -- An Lval expression is anything that has a name. Minor qualifications:
* function returning a reference...

An R-value expression is any expression that is not an L-value expression. C++ standard decrees "A function call is an L-value only if the result type is a reference"

Note both lval and rval refer to "expressions" including variables, but do not refer to objects.

ALL expressions can evaluate to some value, so can appear on the RHS, but only some can appear on the LHS. But this has nothing to do with the lval/rval definitions! 

Something, like a subscript expression, can appear on the RHS but it is strictly a lval expression, not a rval expression. If you (like me) find there are too many categories of lval expression, then just remember an lval expression is typically a variable.

! An expression is either a rval or lval, never both !
! An lval expression can bind only to a lval ref (, never a rval ref ) expression
! [P] a rval expression binds to either:
!! a rval reference variable, or
!! const lval reference variable
!! never to a non-const lval reference variable

* a regular variable can appear on RHS/LHS but is always always a lval, never an rval expression.
** a function call is usually an rval expression, but  sometimes an lval expression
* a literal is always an rval expression, since it is not a "place holder" and has no address.
* [P] arithmetic expressions are rval expressions
* [P] subscript expressions and unwrapped pointers are lval expressions. Most common lval expression is the variable name

[P] rval ref variable must bind to an rval expression, never a lval expression!

[P] rval ref indicates the object referenced will be "relocated" soon. Therefore it should bind to temp objects...

E=[[Eff Modern C++]]
P=[[c++primer]]
I=[[c++for the impatient]]

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