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Le Chaud Lapin
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 Posted: Mon Sep 29, 2003 6:29 am Post subject: Redefinition Of Variable Permiited In If Statement |
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Hi All,
If the following legal C++?
void foo ()
{
int x = 1;
if (x == 1)
int x = 2;
}
-Chaud Lapin-
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Balog Pal
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| Posted: Mon Sep 29, 2003 9:26 pm Post subject: Re: Redefinition Of Variable Permiited In If Statement |
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"Le Chaud Lapin" <unoriginal_username (AT) yahoo (DOT) com> wrote
| Quote: | If the following legal C++?
void foo ()
{
int x = 1;
if (x == 1)
int x = 2;
}
|
Yep. if() introduces a new scope always. So that is identical to write:
if (x == 1)
{
int x = 2;
}
Paul
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David B. Held
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| Posted: Mon Sep 29, 2003 9:33 pm Post subject: Re: Redefinition Of Variable Permiited In If Statement |
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"Le Chaud Lapin" <unoriginal_username (AT) yahoo (DOT) com> wrote
| Quote: | Hi All,
If the following legal C++?
void foo ()
{
int x = 1;
if (x == 1)
int x = 2;
}
|
Yes, the "inner" x hides the declaration of the outer x in that scope.
This is a well-known visibility rule for many programming languages.
To refer to the outer x, use : . Of course if the scopes are nested
too deeply, it may not be possible to refer to one or more of the
hidden names. Naturally, outside the 'if' scope, the outer x regains
visibility.
Dave
---
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Emlyn Corrin
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| Posted: Mon Sep 29, 2003 9:41 pm Post subject: Re: Redefinition Of Variable Permiited In If Statement |
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Le Chaud Lapin wrote:
| Quote: | Hi All,
If the following legal C++?
void foo ()
{
int x = 1;
if (x == 1)
int x = 2;
}
|
Isn't that exactly the same as:
void foo ()
{
int x = 1;
if (x == 1) {
int x = 2; // hides outer x variable
}
// inner x destroyed, can access outer x again
}
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Anthony Williams
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| Posted: Mon Sep 29, 2003 9:51 pm Post subject: Re: Redefinition Of Variable Permiited In If Statement |
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[email]unoriginal_username (AT) yahoo (DOT) com[/email] (Le Chaud Lapin) writes:
| Quote: | If the following legal C++?
void foo ()
{
int x = 1;
if (x == 1)
int x = 2;
}
|
Yes, the if creates a new scope.
Anthony
--
Anthony Williams
Senior Software Engineer, Beran Instruments Ltd.
Remove NOSPAM when replying, for timely response.
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Attila Feher
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| Posted: Mon Sep 29, 2003 9:53 pm Post subject: Re: Redefinition Of Variable Permiited In If Statement |
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Le Chaud Lapin wrote:
| Quote: | Hi All,
If the following legal C++?
void foo ()
{
int x = 1;
if (x == 1) { // Missing { fixed
int x = 2;
}
|
Yes. But you have to be very very careful about what you say. It is *not*
a redefinition of a variable in an if statement. It is the definition of a
variable inside a block (aka compound statement) and this variable happens
to *hide* the one with the same name from the outer scope.
void bar() {
int x = 1; // refer to is as x1
while (x == 1) {// uses x1
int x = 2; // this is x2
// after the declaration x refers to x2
// inside the declaration? I guess there
// too, but we need a real guru to tell that
}
The above is an endless loop! The x *inside* the compund statement has
*nothing* to do with the x in the conditional of the while loop! The
compound statement is a "separate being", it just happens to be the
statement executed by the while statement! You can imagine it on a way that
every time the loop loops, this second x is created, initialized and then
destroyed.
#include <iostream>
int main() {
int x = 0;
std::cout << x << std::endl;
while (int x = 1) {
std::cout << x << std::endl;
int x = 2;
std::cout << x << std::endl;
break;
}
}
The output of the above program is:
0
1
2
Good compilers warn about hidden names.
--
Attila aka WW
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Early Ehlinger
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| Posted: Mon Sep 29, 2003 10:00 pm Post subject: Re: Redefinition Of Variable Permiited In If Statement |
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"Le Chaud Lapin" <unoriginal_username (AT) yahoo (DOT) com> wrote
| Quote: | Hi All,
If the following legal C++?
void foo ()
{
int x = 1;
if (x == 1)
int x = 2;
}
|
Yes.
The true-branch of the if statement has its own scope. It's as though you
had {} around the statement. Your code is essentially the same as:
void foo( )
{
int x = 1;
if ( x == 1 )
{
int x = 2;
}
}
This may not seem very advantageous, but consider:
void foo( )
{
int x = 1;
if ( x == 1 )
{
std::auto_ptr< bar > p( new bar );
p->stuff( );
p->more_stuff( );
} // p falls out of scope and *p is destructed.
++x;
other_stuff( );
}
Why allow one scope to alias the variables defined in the enclosing scope?
Doing otherwise would be inconsistent with the other scoping rules:
int x = 1; // note: this is :.
void foo( )
{
int x = 43; // local variable X, does not affect :.
}
--
Best Regards,
- Early Ehlinger - CEO - www.ResPower.com - 866-737-7697
- 500+ GHz Self-Service Super-Computer from $0.50/GHz*Hr
- (yes, five hundred gigahertz. point-five terahertz.)
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Thomas Tutone
Guest
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| Posted: Mon Sep 29, 2003 10:02 pm Post subject: Re: Redefinition Of Variable Permiited In If Statement |
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"Le Chaud Lapin" wrote:
| Quote: | If the following legal C++?
void foo ()
{
int x = 1;
if (x == 1)
int x = 2;
}
-Chaud Lapin-
|
Sure, why not? An if statement creates a new inner
scope. The new variable x you've created inside the
scope of the if statement hides the the variable x in
the outer scope. Thus:
#include <iostream>
int main()
{
using std::cout;
int x = 1;
cout << x << 't';
if (x == 1) {
int x = 2;
cout << x << 't';
}
cout << x << std::endl;
}
This should print out "1 2 1".
As I recall, though, VC++ 6 had a bug that prevented
an if statement from creating a new scope in certain
circumstances. If you are using that broken compiler,
these examples may not work properly. Consider
upgrading or switching compilers. I think MSDN has
some macro hackery suggestions for making this
properly in VC++ 6.
Best regards,
Tom
__________________________________
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Dhruv
Guest
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| Posted: Mon Sep 29, 2003 10:04 pm Post subject: Re: Redefinition Of Variable Permiited In If Statement |
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On Mon, 29 Sep 2003 02:29:04 -0400, Le Chaud Lapin wrote:
| Quote: | Hi All,
If the following legal C++?
void foo ()
{
int x = 1;
if (x == 1)
int x = 2;
}
|
Yes, it is legal C++, but it serves no practical purpose, because if you
access x after the if statament, it's value will still be reported as 1.
The x after the if statement is a different variable altogether.
Regards,
-Dhruv.
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Anders J. Munch
Guest
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| Posted: Mon Sep 29, 2003 10:06 pm Post subject: Re: Redefinition Of Variable Permiited In If Statement |
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"Le Chaud Lapin" <unoriginal_username (AT) yahoo (DOT) com> wrote:
| Quote: | Hi All,
If the following legal C++?
void foo ()
{
int x = 1;
if (x == 1)
int x = 2;
}
|
Yes. The statement under the 'if' is a nested scope, so there's no
same-scope redefinition of 'x'.
- Anders
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Ron Natalie
Guest
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| Posted: Mon Sep 29, 2003 10:07 pm Post subject: Re: Redefinition Of Variable Permiited In If Statement |
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"Le Chaud Lapin" <unoriginal_username (AT) yahoo (DOT) com> wrote
| Quote: | Hi All,
If the following legal C++?
void foo ()
{
int x = 1;
if (x == 1)
int x = 2;
|
Yes. The substatement attached to an if statement is legal to be a declaration-statment.
The above creates a very short lived variable called x.
You can even declare a variable in the if condition itself:
int x = 1;
if(int x = 2) {
cout << x; // prints 2
}
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Ron
Guest
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| Posted: Mon Sep 29, 2003 10:54 pm Post subject: Re: Redefinition Of Variable Permiited In If Statement |
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| Quote: | If the following legal C++?
void foo ()
{
int x = 1;
if (x == 1)
int x = 2;
}
|
It's legal. The inner declaration is treated as if it were enclosed
within braces. s.6.4(1): "The substatement in a selection-statement
(both substatements, in the else form of the if state-ment) implicitly
defines a local scope (3.3). If the substatement in a
selection-statement is a single statement and not a
compound-statement, it is as if it was rewritten to be a
compound-statement containing the original substatement."
-Ron
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Attila Feher
Guest
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| Posted: Tue Sep 30, 2003 6:42 pm Post subject: Re: Redefinition Of Variable Permiited In If Statement |
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Attila Feher wrote:
| Quote: | Le Chaud Lapin wrote:
Hi All,
If the following legal C++?
void foo ()
{
int x = 1;
if (x == 1) { // Missing { fixed
int x = 2;
[SNIP] |
if (x == 1) { // Missing { fixed
Khm. Wasn't missing. Style issue. I have my own little rule: the if and
its "body" either goes on the same line or curly braces (compund statement)
must be used. I have managed to fool myself.
--
Attila aka WW
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Brian McKeever
Guest
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| Posted: Wed Oct 01, 2003 8:26 am Post subject: Re: Redefinition Of Variable Permiited In If Statement |
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"Early Ehlinger" <early (AT) respower (DOT) com> wrote
| Quote: | This may not seem very advantageous, but consider:
void foo( )
{
int x = 1;
if ( x == 1 )
{
std::auto_ptr< bar > p( new bar );
p->stuff( );
p->more_stuff( );
} // p falls out of scope and *p is destructed.
++x;
other_stuff( );
}
|
If I understand correctly, here you're using the if to create a
smaller scope for the auto_ptr. If so, I understand the goal, but
wouldn't it be clearer to leave off the if clause? You can introduce
a new scope with {} any time you want, can't you?
I've (occassionally) done it myself, most often with RAII as in your
example. It's unusual enough as it is that I comment it, but even
being familiar with it, it took me a little bit to figure out what you
were doing here.
Brian
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Attila Feher
Guest
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| Posted: Wed Oct 01, 2003 3:55 pm Post subject: Re: Redefinition Of Variable Permiited In If Statement |
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David B. Held wrote:
| Quote: | "Le Chaud Lapin" <unoriginal_username (AT) yahoo (DOT) com> wrote in message
news:fc2e0ade.0309281645.6a37d6dd (AT) posting (DOT) google.com...
Hi All,
If the following legal C++?
void foo ()
{
int x = 1;
if (x == 1)
int x = 2;
}
Yes, the "inner" x hides the declaration of the outer x in that scope.
This is a well-known visibility rule for many programming languages.
To refer to the outer x, use :.
|
Nope. : only accesses globals (namespace scope). It never accesses
variables defined in a scope of a block (compound statement). If you try to
use : in the above code (and of course you do not add any global x) you
will have a compilaton error. The global name x, to which : would refer,
has not been declared.
--
Attila aka WW
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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