const

[Massimiliano Alberti]

Sometimes I've seen something like:

const char* ReturnNull()
{
return "";
}

Is this legal? Why? Is this because the string is a constant?

And... Is this legal:

const int* ReturnZero()
{
const int i = 0;
return &i;
}

[Massimiliano Alberti]

Re: const

> const char* ReturnNull()[color=blue]
> {
> return "";
> }[/color]

Wrong example:

const char* ReturnNull()
{
const char *p = "";
return p;
}

[Ron Natalie]

Re: const


"Massimilia no Alberti" <xanatos@geocit ies.com> wrote in message news:2M5%b.2109 4$gk.899378@new s3.tin.it...[color=blue]
> Sometimes I've seen something like:
>
> const char* ReturnNull()
> {
> return "";
> }[/color]

This is legal because a string literal is persistant. It's a bunch of const chars
at an undetermined location. They are not local to the block.
[color=blue]
> const int* ReturnZero()
> {
> const int i = 0;
> return &i;
> }[/color]

This will compile, but any attempt to use the returned pointer will be problematic
as in this case the value is a pointer to an object that doesn't exist anymore.

[Rolf Magnus]

Re: const

Massimiliano Alberti wrote:
[color=blue]
> Sometimes I've seen something like:
>
> const char* ReturnNull()
> {
> return "";
> }
>
> Is this legal?[/color]

Yes.
[color=blue]
> Why?[/color]

What makes you think it's not?
[color=blue]
> Is this because the string is a constant?[/color]

No, it's because it's a literal, and string literals exist through the
whole life time of the program.
[color=blue]
> And... Is this legal:
>
> const int* ReturnZero()
> {
> const int i = 0;
> return &i;
> }[/color]

No. You're returning a pointer to a local variable that gets destroyed
when the function returns.

[Leor Zolman]

Re: const

On Wed, 25 Feb 2004 13:42:35 -0500, "Ron Natalie" <ron@sensor.com > wrote:[color=blue]
>[color=green]
>> const int* ReturnZero()
>> {
>> const int i = 0;
>> return &i;
>> }[/color]
>
>This will compile, but any attempt to use the returned pointer will be problematic
>as in this case the value is a pointer to an object that doesn't exist anymore.[/color]

It will compile, but every self-respecting compiler will at least issue a
warning about it. If you don't see a warning, consider getting a newer
compiler (or tightening up the warning level options).
-leor


Leor Zolman
BD Software
leor@bdsoft.com
www.bdsoft.com -- On-Site Training in C/C++, Java, Perl & Unix
C++ users: Download BD Software's free STL Error Message
Decryptor at www.bdsoft.com/tools/stlfilt.html

[Nick Hounsome]

Re: const


"Massimilia no Alberti" <xanatos@geocit ies.com> wrote in message
news:8Q5%b.2111 2$gk.900093@new s3.tin.it...[color=blue][color=green]
> > const char* ReturnNull()
> > {
> > return "";
> > }[/color]
>
> Wrong example:
>
> const char* ReturnNull()
> {
> const char *p = "";
> return p;
> }[/color]

No - his example is fine - yours just wastes a bit of stack space to acheive
exactly
the same thing.

Actually according to the principle of calling it what it is it should be

const char* ReturnNull() { return NULL; }
or
const char* ReturnEmptyStri ng() { return ""; }
[color=blue]
>
>[/color]

[Rolf Magnus]

Re: const

Nick Hounsome wrote:
[color=blue]
>
> "Massimilia no Alberti" <xanatos@geocit ies.com> wrote in message
> news:8Q5%b.2111 2$gk.900093@new s3.tin.it...[color=green][color=darkred]
>> > const char* ReturnNull()
>> > {
>> > return "";
>> > }[/color]
>>
>> Wrong example:
>>
>> const char* ReturnNull()
>> {
>> const char *p = "";
>> return p;
>> }[/color]
>
> No - his example is fine - yours just wastes a bit of stack space to
> acheive exactly the same thing.[/color]

The "his" and the "yours" you are talking about are the same person.

[Roger Leigh]

Re: const

"Massimilia no Alberti" <xanatos@geocit ies.com> writes:
[color=blue]
> Sometimes I've seen something like:
>
> const char* ReturnNull()
> {
> return "";
> }
>
> Is this legal? Why? Is this because the string is a constant?[/color]

Yes.

[OT: On a UNIX-like OS (e.g. Linux/ELF), this will probably be put in
the .rodata (read-only data) section of the executable. This part is
mapped into memory with read-only permissions, so if you actually
attempt to modify it, you'll get a SEGV.]
[color=blue]
> And... Is this legal:
>
> const int* ReturnZero()
> {
> const int i = 0;
> return &i;
> }[/color]

It's "legal" in that it will compile. However, it's wrong because i
will be destroyed when it goes out of scope when the function returns.
This will leave you with a pointer to a nonexistent object, and
probably a crash.


--
Roger Leigh

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