tree 391092dba7a1fd2c420181fb516a2207392ebd39
parent fd7517037b0e108d8acef539ce4d33ae3bde02fc
author Jason Rhinelander <jason@imaginary.ca> 1484891966 -0500
committer Wenzel Jakob <wenzel.jakob@epfl.ch> 1487974790 +0100

Add an ability to avoid forcing rvp::move

Eigen::Ref objects, when returned, are almost always returned as
rvalues; what's important is the data they reference, not the outer
shell, and so we want to be able to use `::copy`,
`::reference_internal`, etc. to refer to the data the Eigen::Ref
references (in the following commits), rather than the Eigen::Ref
instance itself.

This moves the policy override into a struct so that code that wants to
avoid it (or wants to provide some other Return-type-conditional
override) can create a specialization of
return_value_policy_override<Return> in order to override the override.

This lets an Eigen::Ref-returning function be bound with `rvp::copy`,
for example, to specify that the data should be copied into a new numpy
array rather than referenced, or `rvp::reference_internal` to indicate
that it should be referenced, but a keep-alive used (actually, we used
the array's `base` rather than a py::keep_alive in such a case, but it
accomplishes the same thing).