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Jakob Bieling
Guest
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 Posted: Fri Mar 05, 2004 2:50 pm Post subject: Q: operator void* or operator bool? |
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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)
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Leor Zolman
Guest
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| Posted: Fri Mar 05, 2004 3:07 pm Post subject: Re: Q: operator void* or operator bool? |
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On Fri, 5 Mar 2004 15:50:45 +0100, "Jakob Bieling" <netsurf (AT) gmy (DOT) net> wrote:
| Quote: | 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!
|
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
[email]leor (AT) bdsoft (DOT) com[/email]
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
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Rob Williscroft
Guest
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| Posted: Fri Mar 05, 2004 4:27 pm Post subject: Re: Q: operator void* or operator bool? |
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Jakob Bieling wrote in news:c2a442$5fn$05$1 (AT) news (DOT) t-online.com:
| Quote: | 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.
|
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 ? "? Truen" : "? Falsen" );
ex.condition = true;
cerr << ex << 'n';
cerr << ( ex ? "? Truen" : "? Falsen" );
cerr << "void * (boolalpha):n" << bad() << 'n';
}
This method also avoid's the unwanted conversion to void *.
Rob.
--
http://www.victim-prime.dsl.pipex.com/
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