Potentially evaluated expression.

[L.Suresh]

I'm having problems in understanding 3.2/1.

"An expression is potentially evaluated unless it appears where an
integral constant expression is required (see 5.19)..."

Can someone give me an example of this.

Let's say i define

int y[20 * 5];

Is the expression 20 * 5 not evaluated then?

5.19/1 "In particular, except in sizeof expressions, functions, class
objects, pointers, or references shall not be used, and assignment,
increment, decrement, function-call, or comma operators shall not be
used"

Is the following code valid C++? It does not use a constant expression
in the first place and it also uses the decrement and comma operators.

int i = 8;
int x[--i];
int y[--i, i + 5];

Thanks for your time.

--lsu

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[Victor Bazarov]

L.Suresh wrote:
> I'm having problems in understanding 3.2/1.
>
> "An expression is potentially evaluated unless it appears where an
> integral constant expression is required (see 5.19)..."
>
> Can someone give me an example of this.
>
> Let's say i define
>
> int y[20 * 5];
>
> Is the expression 20 * 5 not evaluated then?

I think "evaluation" refers to run-time. Here the constant expression
is used by the compiler to determine the array size. After that is done,
there is no more "expression" to talk about.

> 5.19/1 "In particular, except in sizeof expressions, functions, class
> objects, pointers, or references shall not be used, and assignment,
> increment, decrement, function-call, or comma operators shall not be
> used"
>
> Is the following code valid C++?

No.

> It does not use a constant expression
> in the first place and it also uses the decrement and comma operators.
>
> int i = 8;
> int x[--i];
> int y[--i, i + 5];

Victor

[Sebastian Redl]

L.Suresh wrote:

> 5.19/1 "In particular, except in sizeof expressions, functions, class
> objects, pointers, or references shall not be used, and assignment,
> increment, decrement, function-call, or comma operators shall not be
> used"
>
> Is the following code valid C++? It does not use a constant expression
> in the first place and it also uses the decrement and comma operators.

C++98 doesn't allow variables to specify stack array size. So the code is
invalid.
Were i a constant, the code would still be invalid because --i would be
attempting to modify it.

--
Sebastian Redl