C++: What EXACTLY comprises a LValue?

Aquarius_Girl · 12-22-2011, 01:04 AM

And if I say:

int n;
n = 0;

or

int n = 0;

so, in this case, 'n' is a variable not an expression, right?

3.10

Code:

  Lvalues and rvalues                                              [basic.lval]
  Expressions are categorized according to the taxonomy in Figure 1.
  1
                                                 expression
                                            glvalue      rvalue
                                       lvalue      xvalue      prvalue
                                  Figure 1 — Expression category taxonomy

NevemTeve · 12-22-2011, 02:38 AM

n is an identifier of a variable, and as such it is an expression.

sundialsvcs · 12-22-2011, 04:20 AM

In compiler parlance, an "LValue" is anything that can meaningfully sit on the Left side of an assignment statement.

That is: it not only produces a value, but it can be assigned one. (As opposed to something, such as a constant or a function-call, that only produces a value.)

The term makes perfect sense if you are the sort of person who gets their jollies out of writing language compilers.

Doc CPU · 12-22-2011, 04:57 AM

Hi there,

Quote:

Originally Posted by sundialsvcs

In compiler parlance, an "LValue" is anything that can meaningfully sit on the Left side of an assignment statement.

That is: it not only produces a value, but it can be assigned one. (As opposed to something, such as a constant or a function-call, that only produces a value.)

that's roughly a repetition of what has been established in this thread so far.

[X] Doc CPU

tuxdev · 12-26-2011, 11:57 PM

Quote:

that's roughly a repetition of what has been established in this thread so far.

Not exactly, with the nonsense of "memory locations" and such. The part of the compiler that deals with lvalues doesn't have any notion of "memory locations". The only thing that determines whether something is an lvalue is whether or not the typechecker complains when you put on the left hand side of an assignment. Nothing more.

Code:

const int x = 0;

'x' is *not* an lvalue. You can't do "x = 5;" later in the code. That's it. Like SigTerm said earlier, this is an declaration with an initializer, not an assignment. Initialization follows a different set of rules.

Aquarius_Girl · 12-27-2011, 12:03 AM

Quote:

'x' is *not* an lvalue. You can't do "x = 5;" later in the code.

Not later ofcourse, but "first" time you can assign value to x, therefore
I called it a lvalue.

tuxdev · 12-27-2011, 10:28 AM

Quote:

Not later ofcourse, but "first" time you can assign value to x, therefore
I called it a lvalue.

It's not assignment. It's initialization. Don't confuse the two together, just because they both use the '=' symbol.