Re: [boost] [variant] Breaking change introduced by variadic support

On 18/01/2014 02:32 p.m., Antony Polukhin wrote:

2014/1/18 Agustín K-ballo Bergé

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The recent changes to introduce variadic support to `variant` introduce breaking changes in the documented macros. When variadic support is detected, `BOOST_VARIANT_LIMIT_TYPES` is not defined and `BOOST_VARIANT_ENUM_[SHIFTED_]PARAMS` uses variadic templates instead of enumerating parameters.

This is a breaking change in the documented interface, and one that requires all but the simplest use cases to be rewritten. For instance, this breaks _Boost.Spirit_ support of `variant` (https://svn.boost.org/trac/ boost/ticket/9238), requiring two different implementations to be provided.

Is this the preferred path to take? I would suggest instead to keep the macros as documented at all times, and let the user handle the variadic case manually. This would allow users to support both variadic and non-variadic `variant`s up to a certain limit with a single code case (without breaking the documented interface), and result in cleaner code for those cases that only target compilers with variadic support. There's no point in using a macro if it won't work for both cases.

Unfortunately this will make previous code just "look like" it works. Variadic template version of variant (`variant`) is not same as variant generated by BOOST_VARIANT_LIMIT_TYPES and preprocessor (`variant`). Leaving BOOST_VARIANT_LIMIT_TYPES defined will result in many-many hard detectable template errors.

Nod, this is unfortunate but at least the current approach results in hard to diagnose *compilation* errors, which is better IMO.

I've put a lot of effort to make transition as smooth as possible: * users that use BOOST_VARIANT_ENUM_[SHIFTED_]PARAMS shall notice no change: // BOOST_VARIANT_ENUM_PARAMS(class Something) => class Something0, class... SomethingN // BOOST_VARIANT_ENUM_PARAMS(Something) => Something0, SomethingN...

This is not the case, see the compilation failure in Spirit. The enum macros are documented to define param##N arguments. Additionally, dealing with all template arguments at the same time gives better compile-time performance than recursive approaches. This is not prevented by variadic templates, but having them separated in head and tail do make it slightly more complicated. I would still suggest that the variadic case we handled manually by the user, while the macros would be there only to support the simplest existing use cases.

* users are free to define BOOST_VARIANT_DO_NOT_USE_VARIADIC_TEMPLATES and do not use variadic templates version at all.

The end user may be free to define it, libraries cannot.

* users that use BOOST_VARIANT_LIMIT_TYPES won't get silent templates errors and will be able to take care about the issues.

Any ideas about how to make transition smoother are welcomed!

I will continue thinking about this. I find unfortunate that users are forced to provide different codes for variadic and non-variadic cases. Worse, the compilation errors do not point to `variant` at all, and one would not usually suspect that a macro named `ENUM` does not enumerate anymore. Regards, -- Agustín K-ballo Bergé.- http://talesofcpp.fusionfenix.com