This page aims to get you quickly up to speed so you can wrap C++ interfaces
with a minimum of pain and 'surprises'.
Cython v0.13 introduces native support for most of the C++ language. This means that the previous tricks that were used to wrap C++ classes (as described in http://wiki.cython.org/WrappingCPlusPlus_ForCython012AndLower) are no longer useful.
In the past, Pyrex only supported wrapping of C APIs, and not C++. To wrap
C++, one had to write a pure-C shim, containing functions for
constructors/destructors and method invocations. Object pointers were passed
around as opaque void pointers, and cast to/from object pointers as needed.
This approach did work, but it got awfully messy and error-prone when trying
to wrap APIs with large class hierarchies and lots of inheritance.
Wrapping C++ classes with Cython is now much more straightforward. This document describe in details the new way of wrapping C++ code.
These days, though, Pyrex offers an adequate bare minimum of C++ support,
which Cython has inherited. The approach described in this document will help
you wrap a lot of C++ code with only moderate effort. There are some
limitations, which we will discuss at the end of the document.
For users of previous Cython versions, here is a brief overview of the main new features of Cython v0.13 regarding C++ support:
* C++ objects can now be dynamically allocated with ``new`` and ``del`` keywords.
* C++ objects can now be stack-allocated.
* C++ classes can be declared with the new keyword ``cppclass``.
* Templated classes are supported.
* Overloaded functions are supported.
* Overloading of C++ operators (such as operator+, operator[],...) is supported.
Procedure Overview
====================
-------------------
The general procedure for wrapping a C++ file can now be described as follow:
* Specify C++ language in :file:`setup.py` script
* Create ``cdef extern from`` blocks and declare classes as
``ctypedef struct`` blocks
* Create constructors and destructors
* Add class methods as function pointers
* Create Cython wrapper class
* Create ``cdef extern from`` blocks with the optional namespace (if exists) and the namespace name as string
* Declare classes as ``cdef cppclass`` blocks
* Declare public attributes (variables, methods and constructors)
A simple Tutorial
==================
An example C++ API
===================
-------------------
Here is a tiny C++ API which we will use as an example throughout this
document. Let's assume it will be in a header file called
...
...
@@ -43,6 +46,7 @@ document. Let's assume it will be in a header file called
.. sourcecode:: c++
namespace shapes {
class Rectangle {
public:
int x0, y0, x1, y1;
...
...
@@ -53,6 +57,7 @@ document. Let's assume it will be in a header file called
int getArea();
void move(int dx, int dy);
};
}
and the implementation in the file called :file:`Rectangle.cpp`:
...
...
@@ -98,11 +103,11 @@ and the implementation in the file called :file:`Rectangle.cpp`:
This is pretty dumb, but should suffice to demonstrate the steps involved.
Specify C++ language in setup.py
=================================
---------------------------------
In Cython :file:`setup.py` scripts, one normally instantiates an Extension
object. To make Cython generate and compile a C++ source, you just need
to add a keyword to your Extension construction statement, as in::
to add the keyword ``language="c++"`` to your Extension construction statement, as in::
ext = Extension(
"rectangle", # name of extension
...
...
@@ -114,103 +119,85 @@ to add a keyword to your Extension construction statement, as in::
cmdclass = {'build_ext': build_ext}
)
and Cython will generate and compile the :file:`rectangle.cpp` file (from the
Cython will generate and compile the :file:`rectangle.cpp` file (from the
:file:`rectangle.pyx`), then it will compile :file:`Rectangle.cpp`
(implementation of the ``Rectangle`` class) and link both objects files
together into :file:`rectangle.so`, which you can then import in Python using
``import rectangle`` (if you forget to link the :file:`Rectangle.o`, you will
get missing symbols while importing the library in Python).
With the language="c++" keyword, Cython distutils will generate a C++ file.
Create cdef extern from block
==============================
Alternatively, one can also use the ``cython`` command-line utility to generate a C++ ``.cpp`` file, and then compile it into a python extension. C++ mode for the ``cython`` command is turned on with the ``--cplus`` option.
Declaring a C++ class interface
--------------------------------
The procedure for wrapping a C++ class is quite similar to that for wrapping
normal C structs, with a couple of additions. Let's start here by creating the
basic ``cdef extern from`` block::
cdef extern from "Rectangle.h":
cdef extern from "Rectangle.h" namespace "shapes":
This will make the C++ class def for Rectangle available.
This will make the C++ class def for Rectangle available. Note the namespace declaration.
Declare class as a ctypedef struct
-----------------------------------
Declare class with cdef cppclass
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Now, let's add the Rectangle class to this extern from block -- just copy the
class def from :file:`Rectangle.h` and adjust for Cython syntax, so now it
becomes::
Now, let's add the Rectangle class to this extern from block - just copy the class name from Rectangle.h and adjust for Cython syntax, so now it becomes::
cdef extern from "Rectangle.h":
# known in Cython namespace as 'c_Rectangle' but in C++ as 'Rectangle'
ctypedef struct c_Rectangle "Rectangle":
int x0, y0, x1, y1
cdef extern from "Rectangle.h" namespace "shapes":
cdef cppclass Rectangle:
We don't have any way of accessing the constructor/destructor or methods, but
we'll cover this now.
Add public attributes
^^^^^^^^^^^^^^^^^^^^^^
Add constructors and destructors
----------------------------------
We now need to declare the attributes for use on Cython::
We now need to expose a constructor and destructor into the Cython
namespace. Again, we'll be using C name specifications::
cdef extern from "Rectangle.h":
ctypedef struct c_Rectangle "Rectangle":
cdef extern from "Rectangle.h" namespace "shapes":
cdef cppclass Rectangle:
Rectangle(int, int, int, int)
int x0, y0, x1, y1
c_Rectangle *new_Rectangle "new Rectangle" (int x0, int y0, int x1, int y1)
void del_Rectangle "delete" (c_Rectangle *rect)
Add class methods
-------------------
int getLength()
int getHeight()
int getArea()
void move(int, int)
Now, let's add the class methods. You can circumvent Cython syntax
limitations by declaring these as function pointers. Recall that in the C++
class we have:
Declare a var with the wrapped C++ class
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
.. sourcecode:: c++
Now, we use cdef to declare a var of the class with the know C++ ``new`` statement::
int getLength();
int getHeight();
int getArea();
void move(int dx, int dy);
cdef Rectangle *rec = new Rectangle(1, 2, 3, 4)
cdef int recLength = rec.getLength()
...
del rec #delete heap allocated object
So if we convert each of these to function pointers and stick them in our
extern block, we now get::
It's also possible to declare a stack allocated object, but it's necessary to have a "default" constructor::
cdef extern from "Rectangle.h":
ctypedef struct c_Rectangle "Rectangle":
int x0, y0, x1, y1
int getLength()
int getHeight()
int getArea()
void move(int dx, int dy)
c_Rectangle *new_Rectangle "new Rectangle" (int x0, int y0, int x1, int y1)
void del_Rectangle "delete" (c_Rectangle *rect)
cdef extern from "Foo.h":
cdef cppclass Foo:
Foo()
This will fool Cython into generating C++ method calls even though
Cython is mostly oblivious to C++.
cdef Foo foo
In Pyrex you must explicitly declare these as function pointers, i.e.
``(int *getArea)()``.
Note that, like C++, if the class has only one constructor and it is a default one, it's not necessary to declare it.
Create Cython wrapper class
=============================
----------------------------
At this point, we have exposed into our pyx file's namespace a struct which
gives us access to the interface of a C++ Rectangle type. Now, we need to make
At this point, we have exposed into our pyx file's namespace the interface of the C++ Rectangle type. Now, we need to make
this accessible from external Python code (which is our whole point).
Common programming practice is to create a Cython extension type which
holds a C++ instance pointer as an attribute ``thisptr``, and create a bunch of
forwarding methods. So we can implement the Python extension type as::
cdef class Rectangle:
cdef c_Rectangle *thisptr # hold a C++ instance which we're wrapping
cdef class PyRectangle:
cdef Rectangle *thisptr # hold a C++ instance which we're wrapping
def __cinit__(self, int x0, int y0, int x1, int y1):
self.thisptr = new_Rectangle(x0, y0, x1, y1)
self.thisptr = newRectangle(x0, y0, x1, y1)
def __dealloc__(self):
del_Rectangle(self.thisptr)
del self.thisptr
def getLength(self):
return self.thisptr.getLength()
def getHeight(self):
...
...
@@ -229,62 +216,141 @@ attribute access, you could just implement some properties::
def __set__(self, x0): self.thisptr.x0 = x0
...
Caveats and Limitations
========================
In this document, we have discussed a relatively straightforward way of
wrapping C++ classes with Cython. However, there are some limitations in
this approach, some of which could be overcome with clever workarounds (anyone
here want to share some?), but some of which will require new features in
Cython.
Advanced C++ features
======================
The major limitations I'm most immediately aware of (and there will be many
more) include:
We describe here all the C++ features that were not discussed in the above tutorial.
Overloading
------------
Presently, it's not easy to overload methods or constructors, but there may be
a workaround if you try some creative C name specifications
Overloading is very simple. Just declare the method with different parameters and use any of them::
Access to C-only functions
---------------------------
cdef extern from "Foo.h":
cdef cppclass Foo:
Foo(int)
Foo(bool)
Foo(int, bool)
Foo(int, int)
Whenever generating C++ code, Cython generates declarations of and calls
to functions assuming these functions are C++ (ie, not declared as extern "C"
{...} . This is ok if the C functions have C++ entry points, but if they're C
only, you will hit a roadblock. If you have a C++ Cython module needing
to make calls to pure-C functions, you will need to write a small C++ shim
module which:
Overloading operators
----------------------
* includes the needed C headers in an extern "C" block
* contains minimal forwarding functions in C++, each of which calls the
respective pure-C function
Cython uses C++ for overloading operators::
Inherited C++ methods
----------------------
cdef extern from "foo.h":
cdef cppclass Foo:
Foo()
Foo* operator+(Foo*)
Foo* operator-(Foo)
int operator*(Foo*)
int operator/(int)
cdef Foo* foo = new Foo()
cdef int x
cdef Foo* foo2 = foo[0] + foo
foo2 = foo[0] - foo[0]
x = foo[0] * foo2
x = foo[0] / 1
cdef Foo f
foo = f + &f
foo2 = f - f
del foo, foo2
Nested class declarations
--------------------------
C++ allows nested class declaration. Class declarations can also be nested in Cython::
cdef extern from "<vector>" namespace "std":
cdef cppclass vector[T]:
cppclass iterator:
T operator*()
iterator operator++()
bint operator==(iterator)
bint operator!=(iterator)
vector()
void push_back(T&)
T& operator[](int)
T& at(int)
iterator begin()
iterator end()
cdef vector[int].iterator iter #iter is declared as being of type vector<int>::iterator
Note that the nested class is declared with a ``cppclass`` but without a ``cdef``.
C++ operators not compatible with Python syntax
------------------------------------------------
Cython try to keep a syntax as close as possible to standard Python. Because of this, certain C++ operators, like the preincrement ``++foo`` or the dereferencing operator ``*foo`` cannot be used with the same syntax as C++. Cython provides functions replacing these operators in a special module ``cython.operator``. The functions provided are:
* ``cython.operator.dereference`` for dereferencing. ``dereference(foo)`` will produce the C++ code ``*foo``
* ``cython.operator.preincrement`` for pre-incrementation. ``preincrement(foo)`` will produce the C++ code ``++foo``
* ...
These functions need to be cimported. Of course, one can use a ``from ... cimport ... as`` to have shorter and more readable functions. For example: ``from cython.operator cimport dereference as deref``.
Templates
----------
Cython uses a bracket syntax for templating. A simple example for wrapping C++ vector::
from cython.operator cimport dereference as deref, preincrement as inc #dereference and increment operators
cdef extern from "<vector>" namespace "std":
cdef cppclass vector[T]:
cppclass iterator:
T operator*()
iterator operator++()
bint operator==(iterator)
bint operator!=(iterator)
vector()
void push_back(T&)
T& operator[](int)
T& at(int)
iterator begin()
iterator end()
cdef vector[int] *v = new vector[int]()
cdef int i
for i in range(10):
v.push_back(i)
cdef vector[int].iterator it = v.begin()
while it != v.end():
print deref(it)
inc(it)
del v
Multiple template parameters can be defined as a list, such as [T, U, V] or [int, bool, char].
Standard library
-----------------
Most of the containers of the C++ Standard Library have been declared in pxd files located in ``/Cython/Includes/libcpp``. These containers are: deque, list, map, pair, queue, set, stack, vector.
If you have a class ``Foo`` with a child class ``Bar``, and ``Foo`` has a
method :meth:`fred`, then you'll have to cast to access this method from
``Bar`` objects.
For example::
class MyClass:
Bar *b
...
def myfunc(self):
...
b.fred() # wrong, won't work
(<Foo *>(self.b)).fred() # should work, Cython now thinks it's a 'Foo'
from libcpp.vector cimport vector
It might take some experimenting by others (you?) to find the most elegant
ways of handling this issue.
cdef vector[int] vect
cdef int i
for i in range(10):
vect.push_back(i)
for i in range(10):
print vect[i]
Advanced C++ features
----------------------
The pxd files in ``/Cython/Includes/libcpp`` also work as good examples on how to declare C++ classes.
Exceptions
^^^^^^^^^^^
-----------
Cython cannot throw C++ exceptions, or catch them with a try-except statement,
but it is possible to declare a function as potentially raising an C++
...
...
@@ -310,57 +376,51 @@ called, which allows one to do custom C++ to Python error "translations." If
raise_py_error does not actually raise an exception a RuntimeError will be
raised.
Templates
^^^^^^^^^^
Cython does not natively understand C++ templates but we can put them to use
in some way. As an example consider an STL vector of C ints::
cdef extern from "some .h file which includes <vector>":
To support function overloading simply add a different alias to each
signature, so if you have e.g.
Caveats and Limitations
========================
.. sourcecode:: c++
Access to C-only functions
---------------------------
int foo(int a);
int foo(int a, int b);
Whenever generating C++ code, Cython generates declarations of and calls
to functions assuming these functions are C++ (ie, not declared as extern "C"
{...} . This is ok if the C functions have C++ entry points, but if they're C
only, you will hit a roadblock. If you have a C++ Cython module needing
to make calls to pure-C functions, you will need to write a small C++ shim
module which:
in your C++ header then interface it like this in your ::
* includes the needed C headers in an extern "C" block
* contains minimal forwarding functions in C++, each of which calls the
respective pure-C function
int fooi "foo"(int)
int fooii "foo"(int, int)
Inherited C++ methods
----------------------
Operators
^^^^^^^^^^
If you have a class ``Foo`` with a child class ``Bar``, and ``Foo`` has a
method :meth:`fred`, then you'll have to cast to access this method from
``Bar`` objects.
For example::
Some operators (e.g. +,-,...) can be accessed from Cython like this::
cdef class MyClass:
Bar *b
...
def myfunc(self):
...
b.fred() # wrong, won't work
(<Foo *>(self.b)).fred() # should work, Cython now thinks it's a 'Foo'
ctypedef struct c_Rectangle "Rectangle":
c_Rectangle add "operator+"(c_Rectangle right)
It might take some experimenting by others (you?) to find the most elegant
ways of handling this issue.
Declaring/Using References
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
---------------------------
Question: How do you declare and call a function that takes a reference as an argument?
Conclusion
============
C++ left-values
----------------
C++ allows functions returning a reference to be left-values. This is currently not supported in Cython. ``cython.operator.dereference(foo)`` is also not considered a left-value.
A great many existing C++ classes can be wrapped using these techniques, in a
way much easier than writing a large messy C shim module. There's a bit of
manual work involved, and an annoying maintenance burden if the C++ library
you're wrapping is frequently changing, but this recipe should hopefully keep
