[cfe-dev] Reflection

[Russell Harmon]

I was talking about Java-style reflection, which is apparently really
introspection.

I didn't really think it mattered since C++ is (mostly) just a
superset of C, but I had actually intended to do this for C, not C++.

--
Russell Harmon



On Tue, Dec 14, 2010 at 09:52, David Chisnall <csdavec at swan.ac.uk> wrote:
> On 13 Dec 2010, at 18:37, Stuart Carnie wrote:
>
>> I imagine some of the things you would need are:
>>       • A runtime, similar to Objective-C's, which needs some of the following capabilities
>>               • collection of metadata classes which describe methods, types, etc
>>               • APIs to read over data sections to extract metadata and generate aforementioned classes
>>               • Public APIs to query for this metadata, create instances of classes via name, etc
>>       • Changes to compiler
>>               • Generation of metadata into special data section
>>               • Generation of entry points / APIs to compiled libraries, executables to allow you to do things like create instances of classes / types by name, etc
>>
>
> It's worth noting that C++ already has some of this via RTTI, although not all of it is public.  The classes declared in <typeinfo> give you some information about an object via public interfaces (e.g. what its superclasses are and so on).  They also include a name field which contains an encoding of the field types.
>
> Unlike Objective-C, this encoding is defined by the ABI, rather than by the language, although you can use the typeid operator to get a type info object for a specific type and then use dynamic_cast<> to see which subclass of std::type_info it is.  The (BSD-licensed) libelf-tc library in the elftoolchain project has code for parsing these encodings for the old-GNU, new-GNU (Itanium), and ARM ABIs, so it wouldn't be too much effort to create something that gives you more useful introspection metadata.
>
> This information does not, as far as I am aware, give you type encodings for methods, so that would need to be added, which would modify the ABI somewhat (although not necessarily in an incompatible way).
>
> One question though: When you say reflection, do you actually mean reflection, or do you just mean introspection?  For example, true reflection would allow you to add methods to a C++ class, which would require modifying the vtable.  If you're only replacing methods, this is quite simple (although, again, ABI-dependent), but adding methods would be much harder.  Creating new types at run time would also be relatively difficult.  Simply extending C++ RTTI to provide useful introspection would be a lot easier.
>
> David
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