Classes

Both Python and C++ support object-oriented code through classes and thus it is logical to expose C++ classes as Python ones, including the full inheritance hierarchy.

The C++ code used for the examples below can be found here, and it is assumed that that code is loaded at the start of any session. Download it, save it under the name features.h, and load it:

>>> import cppyy
>>> cppyy.include('features.h')
>>>

Basics

All bound C++ code starts off from the global C++ namespace, represented in Python by gbl. This namespace, as any other namespace, is treated as a module after it has been loaded. Thus, we can import C++ classes that live underneath it:

>>> from cppyy.gbl import Concrete
>>> Concrete
<class cppyy.gbl.Concrete at 0x2058e30>
>>>

Placing classes in the same structure as imposed by C++ guarantees identity, even if multiple Python modules bind the same class. There is, however, no necessity to expose that structure to end-users: when developing a Python package that exposes C++ classes through cppyy, consider cppyy.gbl an “internal” module, and expose the classes in any structure you see fit. The C++ names will continue to follow the C++ structure, however, as is needed for e.g. pickling:

>>> from cppyy.gbl import Namespace
>>> Concrete == Namespace.Concrete
False
>>> n = Namespace.Concrete.NestedClass()
>>> type(n)
<class cppyy.gbl.Namespace.Concrete.NestedClass at 0x22114c0>
>>> type(n).__name__
NestedClass
>>> type(n).__module__
cppyy.gbl.Namespace.Concrete
>>> type(n).__cppname__
Namespace::Concrete::NestedClass
>>>

Inheritance

The output of help shows the inheritance hierarchy, constructors, public methods, and public data. For example, Concrete inherits from Abstract and it has a constructor that takes an int argument, with a default value of 42. Consider:

>>> from cppyy.gbl import Abstract
>>> issubclass(Concrete, Abstract)
True
>>> a = Abstract()
Traceback (most recent call last):
  File "<console>", line 1, in <module>
TypeError: cannot instantiate abstract class 'Abstract'
>>> c = Concrete()
>>> isinstance(c, Concrete)
True
>>> isinstance(c, Abstract)
True
>>> d = Concrete(13)
>>>

Just like in C++, interface classes that define pure virtual methods, such as Abstract does, can not be instantiated, but their concrete implementations can. As the output of help showed, the Concrete constructor takes an integer argument, that by default is 42.

Typedefs

Typedefs are simple python references to the actual classes to which they refer.

>>> from cppyy.gbl import Concrete_t
>>> Concrete is Concrete_t
True
>>>

Data members

The Concrete instances have a public data member m_int that is treated as a Python property, albeit a typed one:

>>> c.m_int, d.m_int
(42, 13)
>>> c.m_int = 3.14   # a float does not fit in an int
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: int/long conversion expects an integer object
>>> c.m_int = int(3.14)
>>> c.m_int, d.m_int
(3, 13)
>>>

Note that private and protected data members are not accessible and C++ const-ness is respected:

>>> c.m_const_int = 71    # declared 'const int' in class definition
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: assignment to const data not allowed
>>>

Static C++ data members act like Python class-level data members. They are also represented by property objects and both read and write access behave as expected:

>>> Concrete.s_int       # access through class
321
>>> c.s_int = 123        # access through instance
>>> Concrete.s_int
123

Methods

C++ methods are represented as Python ones: these are first-class objects and can be bound to an instance. If a method is virtual in C++, the proper concrete method is called, whether or not the concrete class is bound. Similarly, if all classes are bound, the normal Python rules apply:

>>> c.abstract_method()
called Concrete::abstract_method
>>> c.concrete_method()
called Concrete::concrete_method
>>> m = c.abstract_method
>>> m()
called Concrete::abstract_method
>>>

Templates

Templated classes are instantiated using square brackets. (For backwards compatibility reasons, parentheses work as well.) The instantiation of a templated class yields a class, which can then be used to create instances.

Templated classes need not pre-exist in the bound code, just their declaration needs to be available. This is true for e.g. all of STL:

>>> cppyy.gbl.std.vector                # template metatype
<cppyy.Template 'std::vector' object at 0x7fffed2674d0>
>>> cppyy.gbl.std.vector(int)           # instantiates template -> class
<class cppyy.gbl.std.vector<int> at 0x1532190>
cppyy.gbl.std.vector[int]()             # instantiates class -> object
<cppyy.gbl.std.vector<int> object at 0x2341ec0>
>>>

The template arguments may be actual types or their names as a string, whichever is more convenient. Thus, the following are equivalent:

>>> from cppyy.gbl.std import vector
>>> type1 = vector[Concrete]
>>> type2 = vector['Concrete']
>>> type1 == type2
True
>>>