cppyy: C++ bindings for PyPy

The cppyy module delivers dynamic Python-C++ bindings. It is designed for automation, high performance, scale, interactivity, and handling all of modern C++ (11, 14, etc.). It is based on Cling which, through LLVM/clang, provides C++ reflection and interactivity. Reflection information is extracted from C++ header files. Cppyy itself is built into PyPy (an alternative exists for CPython), but it requires a backend, installable through pip, to interface with Cling.


This assumes PyPy2.7 v5.7 or later; earlier versions use a Reflex-based cppyy module, which is no longer supported. Both the tooling and user-facing Python codes are very backwards compatible, however. Further dependencies are cmake (for general build), Python2.7 (for LLVM), and a modern C++ compiler (one that supports at least C++11).

Assuming you have a recent enough version of PyPy installed, use pip to complete the installation of cppyy:

$ MAKE_NPROCS=4 pypy-c -m pip install --verbose PyPy-cppyy-backend

Set the number of parallel builds (‘4’ in this example, through the MAKE_NPROCS environment variable) to a number appropriate for your machine. The building process may take quite some time as it includes a customized version of LLVM as part of Cling, which is why –verbose is recommended so that you can see the build progress.

The default installation will be under $PYTHONHOME/site-packages/cppyy_backend/lib, which needs to be added to your dynamic loader path (LD_LIBRARY_PATH). If you need the dictionary and class map generation tools (used in the examples below), you need to add $PYTHONHOME/site-packages/cppyy_backend/bin to your executable path (PATH).

Basic bindings example

These examples assume that cppyy_backend is pointed to by the environment variable CPPYYHOME, and that CPPYYHOME/lib is added to LD_LIBRARY_PATH and CPPYYHOME/bin to PATH.

Let’s first test with a trivial example whether all packages are properly installed and functional. Create a C++ header file with some class in it (all functions are made inline for convenience; if you have out-of-line code, link with it as appropriate):

$ cat MyClass.h
class MyClass {
    MyClass(int i = -99) : m_myint(i) {}

    int GetMyInt() { return m_myint; }
    void SetMyInt(int i) { m_myint = i; }

    int m_myint;

Then, generate the bindings using genreflex (installed under cppyy_backend/bin in site_packages), and compile the code:

$ genreflex MyClass.h
$ g++ -std=c++11 -fPIC -rdynamic -O2 -shared -I$CPPYYHOME/include MyClass_rflx.cpp -o libMyClassDict.so -L$CPPYYHOME/lib -lCling

Next, make sure that the library can be found through the dynamic lookup path (the LD_LIBRARY_PATH environment variable on Linux, PATH on Windows), for example by adding ”.”. Now you’re ready to use the bindings. Since the bindings are designed to look pythonistic, it should be straightforward:

$ pypy-c
>>>> import cppyy
>>>> cppyy.load_reflection_info("libMyClassDict.so")
<CPPLibrary object at 0xb6fd7c4c>
>>>> myinst = cppyy.gbl.MyClass(42)
>>>> print myinst.GetMyInt()
>>>> myinst.SetMyInt(33)
>>>> print myinst.m_myint
>>>> myinst.m_myint = 77
>>>> print myinst.GetMyInt()
>>>> help(cppyy.gbl.MyClass)   # shows that normal python introspection works

That’s all there is to it!

Automatic class loader

There is one big problem in the code above, that prevents its use in a (large scale) production setting: the explicit loading of the reflection library. Clearly, if explicit load statements such as these show up in code downstream from the MyClass package, then that prevents the MyClass author from repackaging or even simply renaming the dictionary library.

The solution is to make use of an automatic class loader, so that downstream code never has to call load_reflection_info() directly. The class loader makes use of so-called rootmap files, which genreflex can produce. These files contain the list of available C++ classes and specify the library that needs to be loaded for their use (as an aside, this listing allows for a cross-check to see whether reflection info is generated for all classes that you expect). By convention, the rootmap files should be located next to the reflection info libraries, so that they can be found through the normal shared library search path. They can be concatenated together, or consist of a single rootmap file per library. For example:

$ genreflex MyClass.h --rootmap=libMyClassDict.rootmap --rootmap-lib=libMyClassDict.so
$ g++ -std=c++11 -fPIC -rdynamic -O2 -shared -I$CPPYYHOME/include MyClass_rflx.cpp -o libMyClassDict.so -L$CPPYYHOME/lib -lCling

where the first option (--rootmap) specifies the output file name, and the second option (--rootmap-lib) the name of the reflection library where MyClass will live. It is necessary to provide that name explicitly, since it is only in the separate linking step where this name is fixed. If the second option is not given, the library is assumed to be libMyClass.so, a name that is derived from the name of the header file.

With the rootmap file in place, the above example can be rerun without explicit loading of the reflection info library:

$ pypy-c
>>>> import cppyy
>>>> myinst = cppyy.gbl.MyClass(42)
>>>> print myinst.GetMyInt()
>>>> # etc. ...

As a caveat, note that the class loader is currently limited to classes only.

Advanced example

The following snippet of C++ is very contrived, to allow showing that such pathological code can be handled and to show how certain features play out in practice:

$ cat MyAdvanced.h
#include <string>

class Base1 {
    Base1(int i) : m_i(i) {}
    virtual ~Base1() {}
    int m_i;

class Base2 {
    Base2(double d) : m_d(d) {}
    virtual ~Base2() {}
    double m_d;

class C;

class Derived : public virtual Base1, public virtual Base2 {
    Derived(const std::string& name, int i, double d) : Base1(i), Base2(d), m_name(name) {}
    virtual C* gimeC() { return (C*)0; }
    std::string m_name;

Base2* BaseFactory(const std::string& name, int i, double d) {
    return new Derived(name, i, d);

This code is still only in a header file, with all functions inline, for convenience of the example. If the implementations live in a separate source file or shared library, the only change needed is to link those in when building the reflection library.

If you were to run genreflex like above in the basic example, you will find that not all classes of interest will be reflected, nor will be the global factory function. In particular, std::string will be missing, since it is not defined in this header file, but in a header file that is included. In practical terms, general classes such as std::string should live in a core reflection set, but for the moment assume we want to have it in the reflection library that we are building for this example.

The genreflex script can be steered using a so-called selection file (see “Generating Reflex Dictionaries”) which is a simple XML file specifying, either explicitly or by using a pattern, which classes, variables, namespaces, etc. to select from the given header file. With the aid of a selection file, a large project can be easily managed: simply #include all relevant headers into a single header file that is handed to genreflex. In fact, if you hand multiple header files to genreflex, then a selection file is almost obligatory: without it, only classes from the last header will be selected. Then, apply a selection file to pick up all the relevant classes. For our purposes, the following rather straightforward selection will do (the name lcgdict for the root is historical, but required):

$ cat MyAdvanced.xml
    <class pattern="Base?" />
    <class name="Derived" />
    <class name="std::string" />
    <function name="BaseFactory" />

Now the reflection info can be generated and compiled:

$ genreflex MyAdvanced.h --selection=MyAdvanced.xml
$ g++ -std=c++11 -fPIC -rdynamic -O2 -shared -I$CPPYYHOME/include MyAdvanced_rflx.cpp -o libAdvExDict.so -L$CPPYYHOME/lib -lCling

and subsequently be used from PyPy:

>>>> import cppyy
>>>> cppyy.load_reflection_info("libAdvExDict.so")
<CPPLibrary object at 0x00007fdb48fc8120>
>>>> d = cppyy.gbl.BaseFactory("name", 42, 3.14)
>>>> type(d)
<class '__main__.Derived'>
>>>> isinstance(d, cppyy.gbl.Base1)
>>>> isinstance(d, cppyy.gbl.Base2)
>>>> d.m_i, d.m_d
(42, 3.14)
>>>> d.m_name == "name"

Again, that’s all there is to it!

A couple of things to note, though. If you look back at the C++ definition of the BaseFactory function, you will see that it declares the return type to be a Base2, yet the bindings return an object of the actual type Derived? This choice is made for a couple of reasons. First, it makes method dispatching easier: if bound objects are always their most derived type, then it is easy to calculate any offsets, if necessary. Second, it makes memory management easier: the combination of the type and the memory address uniquely identifies an object. That way, it can be recycled and object identity can be maintained if it is entered as a function argument into C++ and comes back to PyPy as a return value. Last, but not least, casting is decidedly unpythonistic. By always providing the most derived type known, casting becomes unnecessary. For example, the data member of Base2 is simply directly available. Note also that the unreflected gimeC method of Derived does not preclude its use. It is only the gimeC method that is unusable as long as class C is unknown to the system.


The following is not meant to be an exhaustive list, since cppyy is still under active development. Furthermore, the intention is that every feature is as natural as possible on the python side, so if you find something missing in the list below, simply try it out. It is not always possible to provide exact mapping between python and C++ (active memory management is one such case), but by and large, if the use of a feature does not strike you as obvious, it is more likely to simply be a bug. That is a strong statement to make, but also a worthy goal. For the C++ side of the examples, refer to this example code, which was bound using:

$ genreflex example.h --deep --rootmap=libexampleDict.rootmap --rootmap-lib=libexampleDict.so
$ g++ -std=c++11 -fPIC -rdynamic -O2 -shared -I$CPPYYHOME/include example_rflx.cpp -o libexampleDict.so -L$CPPYYHOME/lib -lCling
  • abstract classes: Are represented as python classes, since they are needed to complete the inheritance hierarchies, but will raise an exception if an attempt is made to instantiate from them. Example:

    >>>> from cppyy.gbl import AbstractClass, ConcreteClass
    >>>> a = AbstractClass()
    Traceback (most recent call last):
      File "<console>", line 1, in <module>
    TypeError: cannot instantiate abstract class 'AbstractClass'
    >>>> issubclass(ConcreteClass, AbstractClass)
    >>>> c = ConcreteClass()
    >>>> isinstance(c, AbstractClass)
  • arrays: Supported for builtin data types only, as used from module array. Out-of-bounds checking is limited to those cases where the size is known at compile time (and hence part of the reflection info). Example:

    >>>> from cppyy.gbl import ConcreteClass
    >>>> from array import array
    >>>> c = ConcreteClass()
    >>>> c.array_method(array('d', [1., 2., 3., 4.]), 4)
    1 2 3 4
  • builtin data types: Map onto the expected equivalent python types, with the caveat that there may be size differences, and thus it is possible that exceptions are raised if an overflow is detected.

  • casting: Is supposed to be unnecessary. Object pointer returns from functions provide the most derived class known in the hierarchy of the object being returned. This is important to preserve object identity as well as to make casting, a pure C++ feature after all, superfluous. Example:

    >>>> from cppyy.gbl import AbstractClass, ConcreteClass
    >>>> c = ConcreteClass()
    >>>> ConcreteClass.show_autocast.__doc__
    'AbstractClass* ConcreteClass::show_autocast()'
    >>>> d = c.show_autocast()
    >>>> type(d)
    <class '__main__.ConcreteClass'>

    However, if need be, you can perform C++-style reinterpret_casts (i.e. without taking offsets into account), by taking and rebinding the address of an object:

    >>>> from cppyy import addressof, bind_object
    >>>> e = bind_object(addressof(d), AbstractClass)
    >>>> type(e)
    <class '__main__.AbstractClass'>
  • classes and structs: Get mapped onto python classes, where they can be instantiated as expected. If classes are inner classes or live in a namespace, their naming and location will reflect that. Example:

    >>>> from cppyy.gbl import ConcreteClass, Namespace
    >>>> ConcreteClass == Namespace.ConcreteClass
    >>>> n = Namespace.ConcreteClass.NestedClass()
    >>>> type(n)
    <class '__main__.Namespace::ConcreteClass::NestedClass'>
  • data members: Public data members are represented as python properties and provide read and write access on instances as expected. Private and protected data members are not accessible. Example:

    >>>> from cppyy.gbl import ConcreteClass
    >>>> c = ConcreteClass()
    >>>> c.m_int
  • default arguments: C++ default arguments work as expected, but python keywords are not supported. It is technically possible to support keywords, but for the C++ interface, the formal argument names have no meaning and are not considered part of the API, hence it is not a good idea to use keywords. Example:

    >>>> from cppyy.gbl import ConcreteClass
    >>>> c = ConcreteClass()       # uses default argument
    >>>> c.m_int
    >>>> c = ConcreteClass(13)
    >>>> c.m_int
  • doc strings: The doc string of a method or function contains the C++ arguments and return types of all overloads of that name, as applicable. Example:

    >>>> from cppyy.gbl import ConcreteClass
    >>>> print ConcreteClass.array_method.__doc__
    void ConcreteClass::array_method(int*, int)
    void ConcreteClass::array_method(double*, int)
  • enums: Are translated as ints with no further checking.

  • functions: Work as expected and live in their appropriate namespace (which can be the global one, cppyy.gbl).

  • inheritance: All combinations of inheritance on the C++ (single, multiple, virtual) are supported in the binding. However, new python classes can only use single inheritance from a bound C++ class. Multiple inheritance would introduce two “this” pointers in the binding. This is a current, not a fundamental, limitation. The C++ side will not see any overridden methods on the python side, as cross-inheritance is planned but not yet supported. Example:

    >>>> from cppyy.gbl import ConcreteClass
    >>>> help(ConcreteClass)
    Help on class ConcreteClass in module __main__:
    class ConcreteClass(AbstractClass)
     |  Method resolution order:
     |      ConcreteClass
     |      AbstractClass
     |      cppyy.CPPObject
     |      __builtin__.CPPInstance
     |      __builtin__.object
     |  Methods defined here:
     |  ConcreteClass(self, *args)
     |      ConcreteClass::ConcreteClass(const ConcreteClass&)
     |      ConcreteClass::ConcreteClass(int)
     |      ConcreteClass::ConcreteClass()
     etc. ....
  • memory: C++ instances created by calling their constructor from python are owned by python. You can check/change the ownership with the _python_owns flag that every bound instance carries. Example:

    >>>> from cppyy.gbl import ConcreteClass
    >>>> c = ConcreteClass()
    >>>> c._python_owns            # True: object created in Python
  • methods: Are represented as python methods and work as expected. They are first class objects and can be bound to an instance. Virtual C++ methods work as expected. To select a specific virtual method, do like with normal python classes that override methods: select it from the class that you need, rather than calling the method on the instance. To select a specific overload, use the __dispatch__ special function, which takes the name of the desired method and its signature (which can be obtained from the doc string) as arguments.

  • namespaces: Are represented as python classes. Namespaces are more open-ended than classes, so sometimes initial access may result in updates as data and functions are looked up and constructed lazily. Thus the result of dir() on a namespace shows the classes available, even if they may not have been created yet. It does not show classes that could potentially be loaded by the class loader. Once created, namespaces are registered as modules, to allow importing from them. Namespace currently do not work with the class loader. Fixing these bootstrap problems is on the TODO list. The global namespace is cppyy.gbl.

  • NULL: Is represented as cppyy.gbl.nullptr. In C++11, the keyword nullptr is used to represent NULL. For clarity of intent, it is recommended to use this instead of None (or the integer 0, which can serve in some cases), as None is better understood as void in C++.

  • operator conversions: If defined in the C++ class and a python equivalent exists (i.e. all builtin integer and floating point types, as well as bool), it will map onto that python conversion. Note that char* is mapped onto __str__. Example:

    >>>> from cppyy.gbl import ConcreteClass
    >>>> print ConcreteClass()
    Hello operator const char*!
  • operator overloads: If defined in the C++ class and if a python equivalent is available (not always the case, think e.g. of operator||), then they work as expected. Special care needs to be taken for global operator overloads in C++: first, make sure that they are actually reflected, especially for the global overloads for operator== and operator!= of STL vector iterators in the case of gcc (note that they are not needed to iterate over a vector). Second, make sure that reflection info is loaded in the proper order. I.e. that these global overloads are available before use.

  • pointers: For builtin data types, see arrays. For objects, a pointer to an object and an object looks the same, unless the pointer is a data member. In that case, assigning to the data member will cause a copy of the pointer and care should be taken about the object’s life time. If a pointer is a global variable, the C++ side can replace the underlying object and the python side will immediately reflect that.

  • PyObject*: Arguments and return types of PyObject* can be used, and passed on to CPython API calls. Since these CPython-like objects need to be created and tracked (this all happens through cpyext) this interface is not particularly fast.

  • static data members: Are represented as python property objects on the class and the meta-class. Both read and write access is as expected.

  • static methods: Are represented as python’s staticmethod objects and can be called both from the class as well as from instances.

  • strings: The std::string class is considered a builtin C++ type and mixes quite well with python’s str. Python’s str can be passed where a const char* is expected, and an str will be returned if the return type is const char*.

  • templated classes: Are represented in a meta-class style in python. This may look a little bit confusing, but conceptually is rather natural. For example, given the class std::vector<int>, the meta-class part would be std.vector. Then, to get the instantiation on int, do std.vector(int) and to create an instance of that class, do std.vector(int)():

    >>>> import cppyy
    >>>> cppyy.load_reflection_info('libexampleDict.so')
    >>>> cppyy.gbl.std.vector                # template metatype
    <cppyy.CppyyTemplateType object at 0x00007fcdd330f1a0>
    >>>> cppyy.gbl.std.vector(int)           # instantiates template -> class
    <class '__main__.std::vector<int>'>
    >>>> cppyy.gbl.std.vector(int)()         # instantiates class -> object
    <__main__.std::vector<int> object at 0x00007fe480ba4bc0>

    Note that templates can be build up by handing actual types to the class instantiation (as done in this vector example), or by passing in the list of template arguments as a string. The former is a lot easier to work with if you have template instantiations using classes that themselves are templates in the arguments (think e.g a vector of vectors). All template classes must already exist in the loaded reflection info, they do not work (yet) with the class loader.

    For compatibility with other bindings generators, use of square brackets instead of parenthesis to instantiate templates is supported as well.

  • templated functions: Automatically participate in overloading and are used in the same way as other global functions.

  • templated methods: For now, require an explicit selection of the template parameters. This will be changed to allow them to participate in overloads as expected.

  • typedefs: Are simple python references to the actual classes to which they refer.

  • unary operators: Are supported if a python equivalent exists, and if the operator is defined in the C++ class.

You can always find more detailed examples and see the full of supported features by looking at the tests in pypy/module/cppyy/test.

If a feature or reflection info is missing, this is supposed to be handled gracefully. In fact, there are unit tests explicitly for this purpose (even as their use becomes less interesting over time, as the number of missing features decreases). Only when a missing feature is used, should there be an exception. For example, if no reflection info is available for a return type, then a class that has a method with that return type can still be used. Only that one specific method can not be used.


Templates can be automatically instantiated, assuming the appropriate header files have been loaded or are accessible to the class loader. This is the case for example for all of STL. For example:

$ cat MyTemplate.h
#include <vector>

class MyClass {
    MyClass(int i = -99) : m_i(i) {}
    MyClass(const MyClass& s) : m_i(s.m_i) {}
    MyClass& operator=(const MyClass& s) { m_i = s.m_i; return *this; }
    ~MyClass() {}
    int m_i;

Run the normal genreflex and compilation steps:

$ genreflex MyTemplate.h --selection=MyTemplate.xml
$ g++ -std=c++11 -fPIC -rdynamic -O2 -shared -I$CPPYYHOME/include MyTemplate_rflx.cpp -o libTemplateDict.so -L$CPPYYHOME/lib -lCling

Subsequent use should be as expected. Note the meta-class style of “instantiating” the template:

>>>> import cppyy
>>>> cppyy.load_reflection_info("libTemplateDict.so")
>>>> std = cppyy.gbl.std
>>>> MyClass = cppyy.gbl.MyClass
>>>> v = std.vector(MyClass)()
>>>> v += [MyClass(1), MyClass(2), MyClass(3)]
>>>> for m in v:
....     print m.m_i,
1 2 3

The arguments to the template instantiation can either be a string with the full list of arguments, or the explicit classes. The latter makes for easier code writing if the classes passed to the instantiation are themselves templates.

The fast lane

By default, cppyy will use direct function pointers through CFFI whenever possible. If this causes problems for you, you can disable it by setting the CPPYY_DISABLE_FASTPATH environment variable.


Most of the ideas in cppyy come originally from the PyROOT project, which contains a CPython-based cppyy.py module (with similar dependencies as the one that comes with PyPy). A standalone pip-installable version is planned, but for now you can install ROOT through your favorite distribution installer (available in the science section).

There are a couple of minor differences between the two versions of cppyy (the CPython version has a few more features). Work is on-going to integrate the nightly tests of both to make sure their feature sets are equalized.


The CPython version of cppyy supports Python3, assuming your packager has build the backend for it. The cppyy module has not been tested with the Py3k version of PyPy. Note that the generated reflection information (from genreflex) is fully independent of Python, and does not need to be rebuild when switching versions or interpreters.