property syntax problems

[Alex Burton]

Robert Jacques Wrote:

> On Thu, 05 Feb 2009 06:55:46 -0500, Alex Burton <alexibu at mac.com> wrote:
> 
> > Hi,
> >
> > I just found a bug that comes out of the property syntax.
> >
> > The property syntax is great in that it allows a smooth transition from  
> > simple code dealing with public member variables to the use of  
> > interfaces without needing to update the client code.
> > i.e. A.bob = 1 can stay as A.bob = 1 when bob changes from being an int  
> > to being void A::bob(int i)
> > instead of changing to A.bob(1).
> >
> > But this can introduce the bug I show below.
> >
> > Proposal :
> > If the temporary returned by the property syntax getter function is  
> > modified, then the corresponding setter function needs to be called with  
> > the temporary as argument.
> >
> > struct A
> > {
> >       int i;
> >       int j;
> > };
> >
> > class B
> > {
> >   A mA;
> > public:
> > 	A a() { return mA; }
> > 	void a(A a) { mA = a; }
> > };
> >
> >
> > int main()
> > {
> >      B b;
> >      b.a.j = 10;     // error b.a is a temporary.
> > }
> 
> This isn't a bug, it's a feature. What you wanted to use were ref returns  
> (see http://www.digitalmars.com/d/2.0/function.html )
> ref A a() { return mA; }


Using
ref A a() { return mA; }
requires us to have a member variable mA ( which is actually there in the example).
One of the reasons for using the setter and getter functions instead of the raw member variable is that there is not actually a member variable. For example:
A a()
{
     Dataset ds = mDatabase.Execute("SELECT A,B FROM TABLE");
     return A(ds[0][0],ds[0][1]);
}

void a(A a)
{
     mDatabase.Execute(format("INSERT %d,%d INTO TABLE;",a.i,a.j));
}

The only way I can see to handle this correctly is to use my proposal above.

This could be a really valuable feature in D if correctly implemented.
Being able to transparently change from a member variable in a struct to getters and setters on a struct to getters and setters on a class and in reverse order is really powerful and allows code to evolve in a much more fluid way.

Alex