[boost] Visual Studio 2015 Update 3 has removed std::unary_function and std::binary_function

Subject: [boost] Visual Studio 2015 Update 3 has removed std::unary_function and std::binary_function
From: Marshall Clow (mclow.lists_at_[hidden])
Date: 2016-11-05 15:21:36

... if you build with /std:c++latest.

See https://svn.boost.org/trac/boost/ticket/12586 for an example.

Other vendors will be removing these types in C++17 mode in the future (or
at least requiring a -D RETAIN_REMOVED_STUFF=1 or something like that)

A quick grep for these terms found 153 instances of "std::unary_function"
and 118 of "std::binary_function" across several libraries, including
accumulators, algorithm, config, container, function, gil, graph, icl, mpi,
msm, polygon, ptr_container, serialization, smart_ptr, tr1, unordered,
utility - and probably others.

I think that we ought to have a boost-wide solution for this (i.e, everyone
should solve this the same way).

I can think of three possibilities:
* Deny, deny, deny: Boost doesn't work with C++17.  Not my recommendation.
* Since std::unary|binary_function is an empty struct with a set of two (or
three) typedefs, just define the typedefs everywhere.  Note that the
typedefs (result_type, first_argument_type, second_argument_type) were
deemed to not be useful in C++17 due to the existence of advanced type
traits, etc.
* Define a boost:unary_function and boost::binary_function struct that
mimics the one that was in namespace std, and change over to use that.
* Something else.

Comments?

-- Marshall