[llvm-dev] llvm:cl::parser subclasses final in 3.7?

Matthew O'Connor via llvm-dev llvm-dev at lists.llvm.org
Mon Oct 12 14:54:57 PDT 2015

On Thu, Oct 8, 2015 at 5:01 PM, David Blaikie <dblaikie at gmail.com> wrote:

> On Thu, Oct 8, 2015 at 3:36 PM, Matthew O'Connor via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>> All,
>> I'm upgrading some code that uses LLVM from 3.6 to 3.7. It looks like the
>> llvm::cl::parser subclasses are now final?
>> We had been doing:
>> struct MaxThreadsParser : public llvm::cl::parser<unsigned> {
>>   bool parse(llvm::cl::Option &O, llvm::StringRef ArgName,
>> llvm::StringRef Arg,
>>              unsigned &Val);
>> };
>> But that's now causing:
>> In file included from
>> /home/lak/my_svn/llvm-carte/llvm-3.7.0/tools/carte++/tools/ir2v/ThreadSupport.cpp:1:
>> /home/lak/my_svn/llvm-carte/llvm-3.7.0/tools/carte++/tools/ir2v/ThreadSupport.h:12:34:
>> error:
>>       base 'parser' is marked 'final'
>> struct MaxThreadsParser : public llvm::cl::parser<unsigned> {
>> What's the new way to do this now? It looks like the documentation at
>> http://llvm.org/releases/3.7.0/docs/CommandLine.html#custom-parsers
>> describes a way that doesn't work anymore.
> Yeah, someone filed https://llvm.org/bugs/show_bug.cgi?id=25104 for this
> recently.
> I was the person who made this API change - hadn't seen the documentation
> we had suggesting/encouraging people to do this, but I'm sort of inclined
> to say that was a mistake - but open to discussion.
> The reason for the change was to devirtualize parser<T>'s dtor - since it
> wasn't ever owned/destroyed polymorphically, so there was no need for it.
> But to keep the type safe, this meant making the dtor protected in the base
> class and making the derived classes final (so there couldn't be further
> derived classes and then polymorphic ownership of the intermediate class
> type).
> Not sure how other people feel about it - whether this is a particularly
> compelling extension point for us to support.
> It looks like you could just write your class as a subclass of
> basic_parser<unsigned> and go from there instead of deriving from parser<T>
> directly. You might have to implement one or two more functions that way.
> - David

It ended up being fairly simple to subclass basic_parser<unsigned>, though
it did require a non-obvious (to me), explicit template specialization of
llvm::cl::printOptionDiff. I liked the simplicity of the being able to
subclass parser<T> and define a single method, but it really doesn't matter
that much.

Thanks for the information and for pointing me in the right direction.
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