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Support multiple inheritance from python

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This commit allows multiple inheritance of pybind11 classes from
Python, e.g.

    class MyType(Base1, Base2):
        def __init__(self):
            Base1.__init__(self)
            Base2.__init__(self)

where Base1 and Base2 are pybind11-exported classes.

This requires collapsing the various builtin base objects
(pybind11_object_56, ...) introduced in 2.1 into a single
pybind11_object of a fixed size; this fixed size object allocates enough
space to contain either a simple object (one base class & small* holder
instance), or a pointer to a new allocation that can contain an
arbitrary number of base classes and holders, with holder size
unrestricted.

* "small" here means having a sizeof() of at most 2 pointers, which is
enough to fit unique_ptr (sizeof is 1 ptr) and shared_ptr (sizeof is 2
ptrs).

To minimize the performance impact, this repurposes
`internals::registered_types_py` to store a vector of pybind-registered
base types.  For direct-use pybind types (e.g. the `PyA` for a C++ `A`)
this is simply storing the same thing as before, but now in a vector;
for Python-side inherited types, the map lets us avoid having to do a
base class traversal as long as we've seen the class before.  The
change to vector is needed for multiple inheritance: Python types
inheriting from multiple registered bases have one entry per base.
This commit is contained in:
Jason Rhinelander
2017-02-22 21:36:09 -05:00
parent caedf74a89
commit e45c211497
11 changed files with 830 additions and 343 deletions
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30
docs/advanced/classes.rst
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@@ -619,27 +619,19 @@ interspersed with alias types and holder types (discussed earlier in this
document)---pybind11 will automatically find out which is which. The only
requirement is that the first template argument is the type to be declared.
There are two caveats regarding the implementation of this feature:
It is also permitted to inherit multiply from exported C++ classes in Python,
as well as inheriting from multiple Python and/or pybind-exported classes.
1. When only one base type is specified for a C++ type that actually has
multiple bases, pybind11 will assume that it does not participate in
multiple inheritance, which can lead to undefined behavior. In such cases,
add the tag ``multiple_inheritance``:
There is one caveat regarding the implementation of this feature:
.. code-block:: cpp
When only one base type is specified for a C++ type that actually has multiple
bases, pybind11 will assume that it does not participate in multiple
inheritance, which can lead to undefined behavior. In such cases, add the tag
``multiple_inheritance`` to the class constructor:
py::class_<MyType, BaseType2>(m, "MyType", py::multiple_inheritance());
.. code-block:: cpp
The tag is redundant and does not need to be specified when multiple base
types are listed.
py::class_<MyType, BaseType2>(m, "MyType", py::multiple_inheritance());
2. As was previously discussed in the section on :ref:`overriding_virtuals`, it
is easy to create Python types that derive from C++ classes. It is even
possible to make use of multiple inheritance to declare a Python class which
has e.g. a C++ and a Python class as bases. However, any attempt to create a
type that has *two or more* C++ classes in its hierarchy of base types will
fail with a fatal error message: ``TypeError: multiple bases have instance
lay-out conflict``. Core Python types that are implemented in C (e.g.
``dict``, ``list``, ``Exception``, etc.) also fall under this combination
and cannot be combined with C++ types bound using pybind11 via multiple
inheritance.
The tag is redundant and does not need to be specified when multiple base types
are listed.