Re: Problems with derived IO classes

From:

"Victor Bazarov" <v.Abazarov@comAcast.net>

Newsgroups:

comp.lang.c++

Date:

Thu, 12 Apr 2007 13:51:08 -0400

Message-ID:

<evlrie$8h2$1@news.datemas.de>

Ian C wrote:

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.

Am I going mad, or just doing something dumb?

The operator that it can find (and uses for the former case, with
the 'str' variable) is probably a non-member, and it has the first
argument a reference to non-const ostream. You cannot bind a non-
const reference to a temporary (in your case 'Syslog(SYSLOG_INFO)'
is the temporary).

There is a trick you can play to get where you want to be:

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

What it does is calling a member function of 'ostream' which then
returns a non-const reference (to the same temporary, but it's OK
since the temporary survives until the end of the full expression),
and then it can re-use the same non-const reference to call the
non-member operator <<.

V
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