Re: [boost] dramatically improved template error messages

Subject: Re: [boost] dramatically improved template error messages
From: Eric Niebler (eric_at_[hidden])
Date: 2012-08-14 17:28:52

On 8/14/2012 1:12 PM, Andrew Sutton wrote:
>> I'd also like to know *why* this works, and indeed if it's guaranteed
>> to. In particular, is it guaranteed that the virtual what() member of
>> the sfinae_error class is instantiated at the outermost call site and
>> not elsewhere (thereby eliminating the deep template back-trace)? I
>> really just stumbled on this technique by accident, and I don't know how
>> thin the ice is here. Both clang-trunk and gcc-4.7 think it's ok.
> 
> I'll wager an explanation... maybe somebody will be able to refine it.
> 
> Because what() is virtual, it needs to build the vtable for
> sfinae_error. However, because what() isn't actually used in the
> program, it is instantiated at the very end of translation. Basically,
> the point of instantiation is at the top-level, at the end of the
> translation unit. There's no template instantiation stack to unwind.
> 
> Comment out the "virtual" keyword in front of what(). The program
> should actually compile. The try_call doesn't actually force a
> compilation error, it suppresses it. This only works because
> sfinae_error happens to be polymorphic.
> 
> Using unused virtual functions to defer instantiation traps is
> incredibly clever... If my understanding is correct :)

The technique also works if the what() function is replaced with a
non-virtual destructor. So I don't think "virtual" is the magic
ingredient here.

>> IMO, this is now approaching something of real value. My personal
>> experience using this with Proto-11 is that it dramatically improves
>> error messages. I can also easily imagine how this can be used to
>> implement a very nice concept-checking library for C++11 that
>> approximates auto-concepts. Andrew, any thoughts on this? You've done
>> far more work in this space than I have.
> 
> I want to say yes, but I'm not sure. What's really needed is a
> combination of this and static_assert. I want compilation to stop
> *and* I want brief errors. Hmm... Let me get back to you on this.

Couldn't you avoid generating the error in sfinae_error and instead
static_assert that a given function call doesn't have a sfinae_error
return type?

-- 
Eric Niebler
BoostPro Computing
http://www.boostpro.com