-----Original Message----- From: boost-bounces@lists.boost.org [mailto:boost-bounces@lists.boost.org] On Behalf Of Rob Stewart Sent: Thursday, July 14, 2005 9:53 PM To: boost@lists.boost.org Cc: boost@lists.boost.org Subject: Re: [boost] New Library Proposal: dual_state
From: "Jost, Andrew" <Andrew_Jost@mentor.com>
From: boost-bounces@lists.boost.org [mailto:boost-bounces@lists.boost.org] On Behalf Of Rob Stewart
From: "Jost, Andrew" <Andrew_Jost@mentor.com>
From: boost-bounces@lists.boost.org [mailto:boost-bounces@lists.boost.org] On Behalf Of Fernando Cacciola
Here is a real example. Imagine a researcher with many instruments
Okay. This works, but what about that ugly call to loadPoint? The problem is that the library function is expecting a signal value, not a Boost.Optional! Boost.Optional helped us manage data collection, but does not help us call trend::loadPoint. Let's look at
The library can just as easily expect a Boost.Optional.
But you're assuming that we have control over the library. That's probably the exception rather than the rule in real life.
If you can change the library to use an adaptor for an optional, why can't you change the library to use the optional directly?
But I never gave any indication that we can change the library at all! Not even to use the adapter. The adapter is a function object that returns exactly what the library function expects.
Using adapters, the call to trend::loadPoint in the first example might look like this:
// -- begin // call trend::loadPoint opt_dp::adapter f(-1); for( data_set::iterator p = d.begin(); p != d.end(); ++p ) { tr.loadPoint( f(*p) ); } // -- end
Your version, with Boost.Optional was like this:
for( data_set::iterator p = d.begin(); p != d.end(); ++p ) { tr.loadPoint( *p ? p->get() : -1 ); }
(I think you could change "p->get()" to "**p" if you like, BTW.)
The latter is simpler and seems far more direct and, therefore, readable. I don't see that your adapter has added any value.
I tried to make the examples as syntactically plain as possile; they are optimized for clarity. There is a key advantage in the example with adapters because the adapter is a function object. In that respect, the door has been flung open to many templated constructs that are more difficlt with the ?: operator. For example, a boost::transform_iterator could be created with the adapter to automatically access the underlying objects in a sequence.
Hmmm. This reminds me of using the STL algorithms before having Boost.Lambda, Boost.Bind, etc. Without a library of predefined function objects that perform useful, reusable adaptations, or a lambda approach to creating the adapters, one winds up separating the value extraction logic from the context in which it is used.
The Boost.Optional::adapter syntax is its own predefined library and, though lambda is a fantastic approach, it won't compile using Microsoft so we can't say it's the new paradigm.
I don't really see the value as presented thus far. There is a tiny amount of syntactic sugar, but is that warranted?
I'm not sure.
It was your idea, and now you're questioning it? I take from that the responses you've gotten have caused you to rethink not just the approach, but the idea?
Not at all. I would distrust anyone who has total faith in the usefulness of a new idea such as this one. The need for another opinion is why forums such as this one even exist.
Your requirements are flawed. The adapter should hold an instance of type T as provided by the constructor. Then, if the Boost.Optional has no value, the adapter can return it's default instance. IOW, the adapter shouldn't impose its needs on the optional type. Boost.Optional would then work fine, though I still question the value of your adapter.
I would also like to see the adapter avoid "imposing its needs" on the optional object, but conversely, I don't think it would be right to to return whole T objects (instead of references).
There's no need. As I pointed out, the adapter can hold its own T instance which represents the default. Then, when asked for a value (returned by reference), the adapter queries the optional. If the optional has a value, the adapter returns the optional's value. If not, the adapter returns the default value it holds.
I considered that approach. It could be used to return only const references, which may or may not be enough. -Andy