When Exactly L-Value to R-Value Conversion Happens In C

Question

If an lvalue appears in a situation in which the compiler expects an rvalue, the compiler converts the lvalue to an rvalue.

An lvalue e of a type T can be converted to an rvalue if T is not a function or array type. The type of e after conversion will be T.

can someone tell when compiler expects Rvalue .or what is exaclty rules such that an expression reducuing to Lvalue is Converted into Rvalue. in C when we try to declare variable size array

  int b=8;
  int a[2*b];     //compiler gives error that constant expression reqd. what is this constant expression (is it rvalue expression)

but when i do

int a[10];
 a[2*b]=89;

it gives no error plz someone elaborate when Lvalue to Rvalue Conversion Happens?? my Confusion is that in array subscript in first case it Lvalue to Rvalue convesion not happens (at declaration time) but in second case it happens

[Variable length arrays](http://en.wikipedia.org/wiki/Variable-length_array) (like in your first example) is actually allowed, maybe you're using an old compiler? — Some programmer dude, Jul 14 '13 at 13:05
Which compiler are you using because gcc supports VLA even in c90 as an extension. — 0decimal0, Jul 14 '13 at 13:36

Shafik Yaghmour · Answer 1 · 2013-07-14T15:04:02.663

It seems like you are possibly using an old compiler or maybe Variable length array(VLA) support needs to be enabled? VLA is supported since c99, if you are using gcc you can see this more clearly, compiling this sample code:

int main()
{
    int b=8;
    int a[2*b];

    return 0 ;
}

with the following options:

gcc -std=c89 -pedantic

you will see this error:

warning: ISO C90 forbids variable length array ‘y’ [-Wvla]

if you use c99 though it is fine:

gcc -std=c99 -pedantic

but it is important to note that as of 2011 C standard VLA support is now optional.

An lvalue is an object that has a memory location while an rvalue is a temporary value that does not persist usually beyond the end of an expression. This article Understanding lvalues and rvalues in C and C++ probably is one of the better detailed explanations.

In this case 2*b is an rvalue since it does not persist beyond the expression.

@Duggs_Dojo Updated with some more details. – Shafik Yaghmour Jul 14 '13 at 15:04 — Shafik Yaghmour, Jul 14 '13 at 15:04

score 0 · Answer 2 · edited May 23 '17 at 11:49

an lvalue is a value has an identity (name) and not movable. and rvalue is a value not a lvalue.

So there are two concepts (is it has a name? is it movable?) that are used to define what an lvalue is and what a rvalue is.

pure rvalue (prvalue) has no name and movable. There are also xvalue and glvalue related to this concept.

The C++11 standard for lvalue and rvalue conversion can be found at chapter 4.1 (page 85 for version 3485). You can also read Does initialization entail lvalue-to-rvalue conversion? Is `int x = x;` UB?

When Exactly L-Value to R-Value Conversion Happens In C

2 Answers2