How to use ostream::sentry?

[snnn]

#include <iostream>

using namespace std;
int main(){
const ostream::sentry ok(cout);
if(ok) return 0;
else return -1;
}

The codes above can work well under VC++2003,but not gcc!
When I try to use gcc to compile it ,the error I get
main.cpp: In function `int main()':
main.cpp:37: error: passing `const std::basic_ostream<char,
std::char_traits<char> >::sentry' as `this' argument of `
std::basic_ostream<_CharT, _Traits>::sentry::operator bool() [with _CharT =
char, _Traits = std::char_traits<char>]' discards qualifiers

Who can tell me why?
--
---------------snnn-------------
http://snnn.blogone.net

[Ron Natalie]

snnn wrote:

#include <iostream>

using namespace std;
int main(){
const ostream::sentry ok(cout);
if(ok) return 0;
else return -1;
}

The codes above can work well under VC++2003,but not gcc!
When I try to use gcc to compile it ,the error I get
main.cpp: In function `int main()':
main.cpp:37: error: passing `const std::basic_ostream<char,
std::char_traits<char> >::sentry' as `this' argument of `
std::basic_ostream<_CharT, _Traits>::sentry::operator bool() [with _CharT =
char, _Traits = std::char_traits<char>]' discards qualifiers

Who can tell me why?

What good is a const sentry?

Anyhow, looks like VC++ doesn't declare the operator bool() as const.

[snnn]

??
The codes above can work well under VC++2003,but not gcc!

[snnn]

Oh,i know,i need to remove "const"

[Dietmar Kuehl]

Ron Natalie wrote:

When I try to use gcc to compile it ,the error I get | ^^^ main.cpp: In function `int main()':
main.cpp:37: error: passing `const std::basic_ostream<char,
std::char_traits<char> >::sentry' as `this' argument of `
std::basic_ostream<_CharT, _Traits>::sentry::operator bool() [with _CharT = char, _Traits = std::char_traits<char>]' discards qualifiers

What good is a const sentry?
The 'sentry's operations are done on construction and on destruction.
In
the remainder of its life-time it does not change and can thus be
'const':
nothing wrong with this.
Anyhow, looks like VC++ doesn't declare the operator bool() as const.

No, it is gcc which doesn't declare the 'operator bool()' as 'const',
at
least if the error message is genuine. That's a plain error as the
operator
is supposed to be 'const' according to 27.6.2.3 (lib.ostream::sentry).
--
<mailto:di***********@yahoo.com> <http://www.dietmar-kuehl.de/>
<http://www.contendix.com> - Software Development & Consulting

[Ron Natalie]

Dietmar Kuehl wrote:

The 'sentry's operations are done on construction and on destruction.
In
the remainder of its life-time it does not change and can thus be
'const':
nothing wrong with this.

Perhaps I phrased that wrong. You're right, but there's nothing really
to be gained by making the sentry const since nothing touches it.

Anyhow, looks like VC++ doesn't declare the operator bool() as const.

No, it is gcc which doesn't declare the 'operator bool()' as 'const',
at
least if the error message is genuine. That's a plain error as the
operator
is supposed to be 'const' according to 27.6.2.3 (lib.ostream::sentry).

Yes, whatever compiler that error came from.