Re: [boost] Clang: Open-source C/C++ front end under development

From: Chris Lattner (clattner_at_[hidden])
Date: 2007-09-01 16:09:38

>> David's characterization is somewhat correct, but is also a bit
>> simplistic.  The LLVM project does certainly use templates (including
>> partial specialization etc), but we prefer to keep this as an
>> implementation detail in a library.  Exposing "complex" template code
>> through the public interface of a library generally make the library
>> "scary" to those developers who don't consider themselves to be C++
>> gurus.  This design point also reduces build time for the LLVM code
>> itself.
>
> If you're building this framework as a bunch of libraries, that means
> expressivity at all kinds of boundaries that would be strictly
> "internal" in a monolithic compiler will be limited by your idea of
> what may scare people.  Are you sure this constraint is a good idea?

It's much easier to give concrete examples than abstract arguments.   
As I mentioned, LLVM does use templates heavily in some libraries.   
Also, note that David is talking more about the LLVM optimizer and  
code generator libraries than the clang front-end.

> Seems likely there are good reasons to templatize a lexer and lots of
> other components in a C++ compiler.

Can you give me a specific example of where making the entire lexer a  
template would be more useful than adding a couple of virtual method  
calls?  It isn't meaningful to lex anything other than a sequence of  
char's for example, and efficient buffer management (at least in our  
context) means that the input to the lexer isn't useful as an  
iterator interface.

The point of most of the LLVM design decisions is not to hide from  
templates: it is to use the right C++ design features in the places  
where they make the most sense.  C++ is a very rich language and  
offers a wide design space to choose from.  Templates are one  
important piece of this design space, but C++ also offers many other  
great things.

-Chris