Re: Problems with derived IO classes

From:
red floyd <no.spam@here.dude>
Newsgroups:
comp.lang.c++
Date:
Thu, 12 Apr 2007 17:39:33 GMT
Message-ID:
<p9uTh.7948$Kd3.6639@newssvr27.news.prodigy.net>
Ian C wrote:

Hello all

Apologies if this is a dumb question, but this is driving me nuts, and
I'm really an old procedural programmer so not sure if it's me or the
compiler at fault. I can perhaps guess :-)

I have written a derived streambuf class so I can log to a
system-dependent logging interface. I use this in a derived class like so:

class Syslog : public std::ostream
{
public:
    Syslog(eSyslogLevel level)
        : std::ostream(new SyslogBuffer(level))
    {
    }
};

If I use a variable to use this class, it compiles fine:

    Syslog str(SYSLOG_INFO);

    str << "Test Number " << 1 << std::endl;

However, if I use it as a temporary object like this:

    Syslog(SYSLOG_INFO) << "Test Number " << 1 << std::endl;

the compiler complains that it can't find a match for operator<< for the
Syslog class and the character array.


I believe the difference is that in the second case, Syslog(SYSLOG_INFO)
is a temporary, and therefore binds to a const reference, and there is
no operator<< that takes a const ostream& as the LHS.

BTW, you do know you can change the streambuf in a regular ostream, so
you don't have to derive from std::ostream, right?

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