Calling method by name.

Jonathan M Davis jmdavisProg at gmx.com
Thu Feb 3 20:07:55 PST 2011


On Thursday 03 February 2011 19:29:15 Robert Jacques wrote:
> On Thu, 03 Feb 2011 08:49:54 -0500, Jacob Carlborg <doob at me.com> wrote:
> > On 2011-02-03 05:52, Robert Jacques wrote:
> >> On Wed, 02 Feb 2011 12:55:37 -0500, %u <fghf at jhgjhb.com> wrote:
> >>> I know is possible to create an object from its name. It's possible to
> >>> call a method from that object if the name is only known at runtime?
> >>> 
> >>> Would something like the following be possible?
> >>> 
> >>> string classname, methodname;
> >>> // Ask the user for class and method.
> >>> auto obj = Object.factory(classname);
> >>> invoke(methodname, obj, param1, param2);
> >>> 
> >>> Thanks
> >> 
> >> I've been working on an update to std.variant, which includes a
> >> compile-time reflection to runtime-reflection system. (See
> >> https://jshare.johnshopkins.edu/rjacque2/public_html/) From the docs:
> >> 
> >> Manually registers a class with Variant's runtime-reflection system.
> >> Note that Variant automatically registers any types it is exposed. Note
> >> how in the example below, only Student is manually registered; Grade is
> >> automatically registered by Variant via compile-time reflection of
> >> Student.
> >> 
> >> module example;
> >> class Grade { real mark; }
> >> class Student { Grade grade; }
> >> void main(string[] args) {
> >> Variant.__register!Student;
> >> Variant grade = Object.factory("example.Grade");
> >> grade.mark(96.6);
> >> assert(grade.mark == 96.6);
> >> }
> >> 
> >> And dynamic method/field calls are handled via the __reflect(string
> >> name, Variant[] args...) method like so:
> >> 
> >> grade.__reflect("mark",Variant(96.6));
> >> assert(grade.__reflect("mark") == 96.6);
> > 
> > Why would you need to pass in Variants in __reflect? Why not just make
> > it a variadic method and automatically convert to Variant?
> 
> Well, opDispatch does exactly that. __reflect, on the other hand, was
> designed as a quasi-backend function primarily for a) internal use (hence
> the double underscore), b) scripting language interfacing/implementing and
> c) user-extension. So efficiency was of key importance. And the reflection
> system is extensible, as Variant knows to call __reflect on user defined
> types. This makes things like prototype style objects possible. (There's
> even a beta implementation of a prototype object in the library) But this
> requires that the use __reflect methods not be templated.
> 
> I'm not well versed in dynamic reflection and its use cases, so when I
> considered the combination of a runtime method name and compile-time
> argument type information, I classed it as 'rare in practice'. But if
> that's not the case, I'd like to know and would greatly appreciate a use
> case/unit test.

Most of the good examples of runtime reflection that I'm aware of require user-
defined attributes. But there are libraries in Java (and presumably C#) that do 
stuff like allow you to mark your classes with certain attributes indicating what 
type of XML elements that they should be, and then another library which knows 
_nothing_ about your classes is able to serialize them to and from XML. Another 
example would be Hibernate, which does the same sort of stuff only it deals with 
talking to databases. Full-on runtime reflection combined with user-defined 
attributes can do some powerful stuff. However, I do think that runtime reflection 
without user-defined attributes doesn't tend to be anywhere near as useful. To 
really get that sort of stuff working, we'd need D to properly support both user-
defined attributes and runtime reflection. Both are future possibilities but 
obviously aren't happening any time soon.

- Jonathan M Davis


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