[boost] Boost.Proto: specializing context::eval classes

Subject: [boost] Boost.Proto: specializing context::eval classes
From: Brook Milligan (brook_at_[hidden])
Date: 2016-05-06 13:07:39

I am trying to use Boost.Proto with some user-defined function terminals.  Following the documentation, this requires specialization of context::eval classes to handle new terminals.  (Using the callable_context<> mechanism is too simplistic because that seems to require a uniform result_type.)  However, doing as suggested (see the "Defining an Evaluation Context" section) does not compile when function terminals are involved.  My solution is a new traits class that deduces the type of a terminal.  This does allow successful specialization of context::eval on the type of terminal, even with functions.

To illustrate this, I have attached a set of three files, one implementing an example proto function, one implementing the relevant metafunction, and one demonstrating both use cases: the one corresponding to the Boost.Proto documentation and the one corresponding to my alternative.  I would appreciate comments on the following:

- Is this a valid problem and a valid solution or have I missed a standard method using Boost.Proto that works to solve this pattern?

- Is this the standard way of creating function terminals in Boost.Proto or have I done something there to trigger the problem?

- If the answer to these questions indicates a missing feature within Boost.Proto, what is the process for fixing that?

Thanks for any help you can offer.

Cheers,
Brook