Q: operator void* or operator bool?

[Jakob Bieling]

Hi,

I remember reading a document that advised to prefer 'operator void*'
over 'operator bool' or other way round, when I want to provide the ability
to use code like this:

test_class t;
while (t)
{
// do stuff
};

But I cannot remember why and which method was prefered.

Thanks for the help!
--
jb

(replace y with x if you want to reply by e-mail)

[Leor Zolman]

Re: Q: operator void* or operator bool?

On Fri, 5 Mar 2004 15:50:45 +0100, "Jakob Bieling" <netsurf@gmy.ne t> wrote:
[color=blue]
>Hi,
>
> I remember reading a document that advised to prefer 'operator void*'
>over 'operator bool' or other way round, when I want to provide the ability
>to use code like this:
>
>test_class t;
>while (t)
>{
> // do stuff
>};
>
> But I cannot remember why and which method was prefered.
>
>Thanks for the help![/color]

To quote from Eckel/Allison's "Thinking in C++ Volume Two: Practical
Programming" (the footnote on page 167):

"It is customary to use operator void *() in preference to operator bool()
because the implicit conversions from bool to int may cause surprises,
should you incorrectly place a stream in a context where an integer
conversion can be applied. The operator void*() function will only be
called implicitly in the body of a Boolean expression."


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

[Rob Williscroft]

Re: Q: operator void* or operator bool?

Jakob Bieling wrote in news:c2a442$5fn $05$1@news.t-online.com:
[color=blue]
> Hi,
>
> I remember reading a document that advised to prefer 'operator
> void*'
> over 'operator bool' or other way round, when I want to provide the
> ability to use code like this:
>
> test_class t;
> while (t)
> {
> // do stuff
> };
>
> But I cannot remember why and which method was prefered.
>[/color]

Well Leor's answered that, but if you want some extra safety:

#include <iostream>
#include <ios>

struct bool_as_member_ helper
{
int i;
};
typedef int (bool_as_member _helper::*bool_ as_member_ptr);


struct example
{
operator bool_as_member_ ptr ()
{
return condition ? &bool_as_member _helper::i : 0;
}
bool condition;
};

struct bad
{
operator void * () { return 0; }
};

int main()
{
using namespace std;

cerr << boolalpha;

example ex = { false };

cerr << ex << '\n';
cerr << ( ex ? "? True\n" : "? False\n" );

ex.condition = true;

cerr << ex << '\n';
cerr << ( ex ? "? True\n" : "? False\n" );

cerr << "void * (boolalpha):\n" << bad() << '\n';
}

This method also avoid's the unwanted conversion to void *.

Rob.
--

http://www.victim-prime.dsl.pipex.com/