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February 06, 2009 Re: property syntax problems | ||||
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Robert Jacques Wrote: > On Thu, 05 Feb 2009 06:55:46 -0500, Alex Burton <alexibu@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 |
February 06, 2009 Re: property syntax problems | ||||
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Posted in reply to Alex Burton | On Thu, 05 Feb 2009 20:26:03 -0500, Alex Burton <alexibu@mac.com> wrote:
> Robert Jacques Wrote:
>
>> On Thu, 05 Feb 2009 06:55:46 -0500, Alex Burton <alexibu@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
Alex, this looks like you want to use proxy structs/objects. Remember each of the following are equivalent:
b.a.j = 10;
<=>
(b.a).j = 10;
<=>
auto c = b.a;
c.j = 10;
And that last case is a kinda tricky.
Also, the x.y.z = 10 not doing anything when y is a struct from your other post is a well known issue is all languages that have POD struct (As far as I know). The solution is to move x.y to a ref return property. Moving from POD members to functions in order to support more complex logic is the primary motivation of properties.
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February 06, 2009 Re: property syntax problems | ||||
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Posted in reply to Robert Jacques | Robert Jacques wrote:
> Also, the x.y.z = 10 not doing anything when y is a struct from your other post is a well known issue is all languages that have POD struct (As far as I know). The solution is to move x.y to a ref return property. Moving from POD members to functions in order to support more complex logic is the primary motivation of properties.
This has given me some trouble in C#. As a result, anything complicated in my company's product (anything that is difficult to initialize with all the proper fields) is a class, and anything remaining that is a struct has only read-only properties.
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