Re: Overloaded Streaming Technique

From:

"Igor Tandetnik" <itandetnik@mvps.org>

Newsgroups:

microsoft.public.vc.language

Date:

Thu, 14 Dec 2006 15:15:36 -0500

Message-ID:

<ONx6ey7HHHA.3780@TK2MSFTNGP02.phx.gbl>

Eric Hill <eric@ijack.net> wrote:

I'm trying to come up with a good method to accomplish conditional
streaming. Specifically, I would like to not incur the cost of
computing the stream parameters if the stream itself is disabled.

std::string get_some_data() { ... }

void go() {
std::ostream example = get_foo_stream();
foo::stream_options options;

// get_some_data will be called
options.enable = true;
example << stream_options << get_some_data();

// get_some_data will NOT be called
options.enable = false;
example << stream_options << get_some_data();
}

I don't really care about how this is accomplished, but I'm not even
sure IF it can be accomplished. Any ideas?

Something along these lines:

template <typename T>
class wrapper {
public:
    typedef T (*F)();
    wrapper(F f) : f_(f) {}

    T operator() () const { return f_(); }
private:
    F f_;
};

template <typename T>
wrapper<T> make_wrapper(T (*f)()) { return wrapper<T>(f); }

template <typename T>
ostream& operator<<(ostream& os, const wrapper<T>& w) {
    if (!StreamIsDisabled(os)) {
        os << w();
    }
    return os;
}

example << stream_options << make_wrapper(get_some_data);

For more generic solution, boost::function
(http://boost.org/doc/html/function.html) may be useful.

stream_options can mark the stream as enabled or disabled with xalloc
and iword methods.
--
With best wishes,
    Igor Tandetnik

With sufficient thrust, pigs fly just fine. However, this is not
necessarily a good idea. It is hard to be sure where they are going to
land, and it could be dangerous sitting under them as they fly
overhead. -- RFC 1925