Currently pybind11 always translates values returned by Python functions
invoked from C++ code by copying, even when moving is feasible--and,
more importantly, even when moving is required.
The first, and relatively minor, concern is that moving may be
considerably more efficient for some types. The second problem,
however, is more serious: there's currently no way python code can
return a non-copyable type to C++ code.
I ran into this while trying to add a PYBIND11_OVERLOAD of a virtual
method that returns just such a type: it simply fails to compile because
this:
overload = ...
overload(args).template cast<ret_type>();
involves a copy: overload(args) returns an object instance, and the
invoked object::cast() loads the returned value, then returns a copy of
the loaded value.
We can, however, safely move that returned value *if* the object has the
only reference to it (i.e. if ref_count() == 1) and the object is
itself temporary (i.e. if it's an rvalue).
This commit does that by adding an rvalue-qualified object::cast()
method that allows the returned value to be move-constructed out of the
stored instance when feasible.
This basically comes down to three cases:
- For objects that are movable but not copyable, we always try the move,
with a runtime exception raised if this would involve moving a value
with multiple references.
- When the type is both movable and non-trivially copyable, the move
happens only if the invoked object has a ref_count of 1, otherwise the
object is copied. (Trivially copyable types are excluded from this
case because they are typically just collections of primitive types,
which can be copied just as easily as they can be moved.)
- Non-movable and trivially copy constructible objects are simply
copied.
This also adds examples to example-virtual-functions that shows both a
non-copyable object and a movable/copyable object in action: the former
raises an exception if returned while holding a reference, the latter
invokes a move constructor if unreferenced, or a copy constructor if
referenced.
Basically this allows code such as:
class MyClass(Pybind11Class):
def somemethod(self, whatever):
mt = MovableType(whatever)
# ...
return mt
which allows the MovableType instance to be returned to the C++ code
via its move constructor.
Of course if you attempt to violate this by doing something like:
self.value = MovableType(whatever)
return self.value
you get an exception--but right now, the pybind11-side of that code
won't compile at all.
It was already pretty badly intrusive, but it also appears to make MSVC
segfault. Rather than investigating and fixing it, it's easier to just
remove it.
As discussed in #320.
The adds a documentation block that mentions that the trampoline classes
must provide overrides for both the classes' own virtual methods *and*
any inherited virtual methods. It also provides a templated solution to
avoiding method duplication.
The example includes a third method (only mentioned in the "see also"
section of the documentation addition), using multiple inheritance.
While this approach works, and avoids code generation in deep
hierarchies, it is intrusive by requiring that the wrapped classes use
virtual inheritance, which itself is more instrusive if any of the
virtual base classes need anything other than default constructors. As
per the discussion in #320, it is kept as an example, but not suggested
in the documentation.
This renames example files from `exampleN` to `example-description`.
Specifically, the following renaming is applied:
example1 -> example-methods-and-attributes
example2 -> example-python-types
example3 -> example-operator-overloading
example4 -> example-constants-and-functions
example5 -> example-callbacks (*)
example6 -> example-sequence-and-iterators
example7 -> example-buffers
example8 -> example-custom-ref-counting
example9 -> example-modules
example10 -> example-numpy-vectorize
example11 -> example-arg-keywords-and-defaults
example12 -> example-virtual-functions
example13 -> example-keep-alive
example14 -> example-opaque-types
example15 -> example-pickling
example16 -> example-inheritance
example17 -> example-stl-binders
example18 -> example-eval
example19 -> example-custom-exceptions
* the inheritance parts of example5 are moved into example-inheritance
(previously example16), and the remainder is left as example-callbacks.
This commit also renames the internal variables ("Example1",
"Example2", "Example4", etc.) into non-numeric names ("ExampleMandA",
"ExamplePythonTypes", "ExampleWithEnum", etc.) to correspond to the
file renaming.
The order of tests is preserved, but this can easily be changed if
there is some more natural ordering by updating the list in
examples/CMakeLists.txt.
