Subject: Re: [boost] [property] interest in C# like properties for C++?
From: Edward Diener (eldiener_at_[hidden])
Date: 2009-10-22 23:20:50
Stefan Strasser wrote:
> Am Friday 23 October 2009 01:41:57 schrieb Sid Sacek:
>>>> [ Edward Diener wrote: ]
>>>> Manipulating a reference property would be done purely through
>>>> accessing the property. Accessing a reference property returns a T & or
>>>> a T const & depending on whether the property is const or not. In the
>>>> case where a T & is returned the reference property is read/write if T
>>>> itself is not const, since you can manipulate T through the reference,
>>>> whereas in the case where T const & is returned ( or where T & is
>>>> returns but T is itself const), the reference property is read only.
>> Who says that's how properties work? That just happens to be your version
>> of the implementation details.
>
> I don't understand what a "reference property" is supposed to be. a property
> getter can return a reference (if it chooses to do so, not in general, since
> there may not be an underlying data member), but what is a reference
> property?
In my implementation a "reference property" is a property whose getter
returns a reference to the type of the property. A reference property
does not have a setter. The actual backing data for a "reference
property" can be a value type itself or a reference. The type T of a
reference property does not need to be copy constructible.
This is as opposed to what most people, and other language
implementations, posit as a "property", which is generally a value type,
meaning that a value is returned by the getter and a value is passed to
the setter, of the particular type. The type T of a value type property
does need to be copy constructible in order to pass the value back and
forth.
My own reasons for bifurcating the notion of a "property" between the
more common value type property and the reference type property is
because I do no think a clean implementation of the "property" concept
can be done in C++ which allows either a reference or a value to be
returned from the same C++ property implementation.
I am sorry I have not posted my own "property" implementation anywhere
yet, but I am still working on some other "property" issues which are
important to me ( besides trying like everyone else to make a living as
a software designer/programmer ).
I originally added my comments about a "reference" type property
because a poster in this thread felt that one could not implement this
without language support and I do not believe that is the case, and
wanted to explain in general how one could at least theoretically
implement a "reference property".
In a way I am arguing for the notion that a single implementation of the
"property" concept, in other words a single template class which
implements a C++ "property", while desirable from the point of view of
usability, may not be practical from the point of view of flexibility
and an end user's needs.