Is there any (hacky) way of accessing a private field from outside a data type? (The equivalent of reflection in managed languages.)

I'm trying to write a piece of marshaling code that needs access to a data type's fields, but can't access them because it's not allowed to. :(

On Tue, 17 May 2011 19:47:29 -0400, Mehrdad <wfunction@hotmail.com> wrote:

> Is there any (hacky) way of accessing a private field from outside a data type? (The equivalent of reflection in managed languages.)
>
> I'm trying to write a piece of marshaling code that needs access to a data type's fields, but can't access them because it's not allowed to. :(

The short answer:
T.tupleof works for anything that you have the source to, but doesn't support polymorphism out of the box.

The long answer (from my json library):
    static Value from(T)(T value) {
        static if(is(T == class) ) if( value is null ) return Value(Null.init);
	//...
        static if(is(T == struct) || is(T == class)) {
            Value[string] result;
            foreach(i,v;value.tupleof) {
                result[value.tupleof[i].stringof["value.".length..$]]  = from(v);
            }
            return Value( result );
        }
        //..
    }

P.S. Are you marshaling to/from an open format or something proprietary?

Check out the __traits(allMembers, TYPE) system:

http://dpldocs.info/traits

On 5/17/2011 5:13 PM, Robert Jacques wrote:
> On Tue, 17 May 2011 19:47:29 -0400, Mehrdad <wfunction@hotmail.com> wrote:
>
>> Is there any (hacky) way of accessing a private field from outside a data type? (The equivalent of reflection in managed languages.)
>>
>> I'm trying to write a piece of marshaling code that needs access to a data type's fields, but can't access them because it's not allowed to. :(
>
> The short answer:
> T.tupleof works for anything that you have the source to, but doesn't support polymorphism out of the box.
>
> The long answer (from my json library):
>     static Value from(T)(T value) {
>         static if(is(T == class) ) if( value is null ) return Value(Null.init);
>     //...
>         static if(is(T == struct) || is(T == class)) {
>             Value[string] result;
>             foreach(i,v;value.tupleof) {
>                 result[value.tupleof[i].stringof["value.".length..$]]  = from(v);
>             }
>             return Value( result );
>         }
>         //..
>     }
>
> P.S. Are you marshaling to/from an open format or something proprietary?
T.tupleof has a problem though: It doesn't seem to let me actually access the value; it just give me a tuple I can't do anything with. Your example only gets the name of the field, but you never actually seem to access it.

(P.S.: I'm trying to marshal from/to Windows data structures, so I guess it's kinda both?)

On 5/17/2011 5:18 PM, Adam D. Ruppe wrote:
> Check out the __traits(allMembers, TYPE) system:
>
> http://dpldocs.info/traits
Thanks, but traits doesn't really let me read or write to the variable though. :(

On Tue, 17 May 2011 20:44:24 -0400, Mehrdad <wfunction@hotmail.com> wrote:

> On 5/17/2011 5:13 PM, Robert Jacques wrote:
>> On Tue, 17 May 2011 19:47:29 -0400, Mehrdad <wfunction@hotmail.com> wrote:
>>
>>> Is there any (hacky) way of accessing a private field from outside a data type? (The equivalent of reflection in managed languages.)
>>>
>>> I'm trying to write a piece of marshaling code that needs access to a data type's fields, but can't access them because it's not allowed to. :(
>>
>> The short answer:
>> T.tupleof works for anything that you have the source to, but doesn't support polymorphism out of the box.
>>
>> The long answer (from my json library):
>>     static Value from(T)(T value) {
>>         static if(is(T == class) ) if( value is null ) return Value(Null.init);
>>     //...
>>         static if(is(T == struct) || is(T == class)) {
>>             Value[string] result;
>>             foreach(i,v;value.tupleof) {
>>                 result[value.tupleof[i].stringof["value.".length..$]]  = from(v);
>>             }
>>             return Value( result );
>>         }
>>         //..
>>     }
>>
>> P.S. Are you marshaling to/from an open format or something proprietary?
> T.tupleof has a problem though: It doesn't seem to let me actually access the value; it just give me a tuple I can't do anything with. Your example only gets the name of the field, but you never actually seem to access it.
>
> (P.S.: I'm trying to marshal from/to Windows data structures, so I guess it's kinda both?)

I actually do both (name and value) in the above example.
Name: value.tupleof[i].stringof["value.".length..$]
Value: v
You can also use value.tupleof[i] if all you want is to set a field.

> Thanks, but traits doesn't really let me read or write to the variable though. :(

Use getMember there.

foreach(member; __traits(allMembers, TYPE))
   __traits(getMember, instance_of_type, member) = something;

tupleof is probably better for this though, since it only includes actual data members; it excludes methods.

You can write to tupleof by using an index.

On Tue, 17 May 2011 20:52:02 -0400, Robert Jacques <sandford@jhu.edu> wrote:

> On Tue, 17 May 2011 20:44:24 -0400, Mehrdad <wfunction@hotmail.com> wrote:
>
>> On 5/17/2011 5:13 PM, Robert Jacques wrote:
>>> On Tue, 17 May 2011 19:47:29 -0400, Mehrdad <wfunction@hotmail.com> wrote:
>>>
>>>> Is there any (hacky) way of accessing a private field from outside a data type? (The equivalent of reflection in managed languages.)
>>>>
>>>> I'm trying to write a piece of marshaling code that needs access to a data type's fields, but can't access them because it's not allowed to. :(
>>>
>>> The short answer:
>>> T.tupleof works for anything that you have the source to, but doesn't support polymorphism out of the box.
>>>
>>> The long answer (from my json library):
>>>     static Value from(T)(T value) {
>>>         static if(is(T == class) ) if( value is null ) return Value(Null.init);
>>>     //...
>>>         static if(is(T == struct) || is(T == class)) {
>>>             Value[string] result;
>>>             foreach(i,v;value.tupleof) {
>>>                 result[value.tupleof[i].stringof["value.".length..$]]  = from(v);
>>>             }
>>>             return Value( result );
>>>         }
>>>         //..
>>>     }
>>>
>>> P.S. Are you marshaling to/from an open format or something proprietary?
>> T.tupleof has a problem though: It doesn't seem to let me actually access the value; it just give me a tuple I can't do anything with. Your example only gets the name of the field, but you never actually seem to access it.
>>
>> (P.S.: I'm trying to marshal from/to Windows data structures, so I guess it's kinda both?)
>
> I actually do both (name and value) in the above example.
> Name: value.tupleof[i].stringof["value.".length..$]
> Value: v
> You can also use value.tupleof[i] if all you want is to set a field.
P.S. In fact, since foreach(i,ref v;value.tupleof) won't compile, you have to use value.tupleof[i] to assign to a field.

On 5/17/2011 5:52 PM, Robert Jacques wrote:
> On Tue, 17 May 2011 20:44:24 -0400, Mehrdad <wfunction@hotmail.com> wrote:
>
>> On 5/17/2011 5:13 PM, Robert Jacques wrote:
>> T.tupleof has a problem though: It doesn't seem to let me actually access the value; it just give me a tuple I can't do anything with. Your example only gets the name of the field, but you never actually seem to access it.
>>
>> (P.S.: I'm trying to marshal from/to Windows data structures, so I guess it's kinda both?)
>
> I actually do both (name and value) in the above example.
> Name: value.tupleof[i].stringof["value.".length..$]
> Value: v
> You can also use value.tupleof[i] if all you want is to set a field.

Hm, my bad. Seems like the newest version of DMD allows this, though I could swear I couldn't do that before (it would say the member is inaccessible). :-)
(And yeah, I already noticed that thing about foreach(ref), thanks.)

On 5/17/2011 5:56 PM, Adam D. Ruppe wrote:
>> Thanks, but traits doesn't really let me read or write to the
>> variable though. :(
> Use getMember there.
>
> foreach(member; __traits(allMembers, TYPE))
>     __traits(getMember, instance_of_type, member) = something;
>
> tupleof is probably better for this though, since it only includes
> actual data members; it excludes methods.
>
> You can write to tupleof by using an index.

Nope, getMember doesn't work, it gives me an inaccessible field error.

On 2011-05-18 01:47, Mehrdad wrote:
> Is there any (hacky) way of accessing a private field from outside a
> data type? (The equivalent of reflection in managed languages.)
>
> I'm trying to write a piece of marshaling code that needs access to a
> data type's fields, but can't access them because it's not allowed to. :(

You can have a look at my serialization library, Orange: http://www.dsource.org/projects/orange

I don't think it works with the latest compilers but you have have a look at the source: http://www.dsource.org/projects/orange/browser/orange/util/Reflection.d#L277

-- 
/Jacob Carlborg