[boost] Re: Property

From: Reece Dunn (msclrhd_at_[hidden])
Date: 2005-01-07 17:17:47

Jason Hise wrote:
> Reece Dunn wrote:
> 
>> I have worked on a property implementation that is available under the 
>> boost sandbox (if you want a zip distribution I can create one):
>>    boost-sandbox/boost/property
>>    boost-sandbox/libs/property
>>
>> My version supports various property implementations (aliased, 
>> set/get, value) and supports scalar (single-value) and rank1 (1D 
>> array) properties.
> 
> Your various types of properties look interesting, and I definitely 
> think that indexable properties are definitely a neat idea.  However, 
> there are two issues that I think could be improved.  First, it would 
> not appear that you support write-only properties.

This is easy to support by providing your own property implementation 
that provides:

    template< typename T >
    struct writeonly_property
    {
       void set( const T & v );
    };

or, for rank1 (indexed) properties:

    template< typename T, typename Index >
    struct writeonly_property
    {
       void set( Index i, const T & v );
    };

The design is such that you can supply your own class that supports the 
get and/or set operations and pass that to the scalar_property template 
(for single-valued properties) or rank1_property (for 1D array properties).

In the larger picture, I would like to support a generic property 
template, similar to the likes of Boost.Function and Boost.Signal that 
would deduce the rank from the form that get/set take.

> Second, it appears 
> that your set type is fixed, meaning that if it is possible to assign an 
> A to a B, it may not be possible to assign an A to a B property.

This is true.

> Mine 
> avoids this problem by templating the set method to take any type, 
> allowing compilation to fail if the type client code attempts to use is 
> invalid.

True. This would mean templatizing scalar_property and rank1_property 
that implement the various operators based on get/set operations as well.

> I could be incorrect in my assessment of your code, as I have 
> not compiled it myself, so please correct me if I am wrong.

The assessment is correct. There is also an issue w.r.t. using default 
generated copy assignment/constructor for classes that use properties 
and I don't have an ideal (library-based) solution for this.

Regards,
Reece