Boost C++ Libraries

...one of the most highly regarded and expertly designed C++ library projects in the world. Herb Sutter and Andrei Alexandrescu, C++ Coding Standards

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Exposing Classes

Constructors
Class Data Members
Class Properties
Inheritance
Class Virtual Functions
Virtual Functions with Default Implementations
Class Operators/Special Functions

Now let's expose a C++ class to Python.

Consider a C++ class/struct that we want to expose to Python:

struct World
{
    void set(std::string msg) { this->msg = msg; }
    std::string greet() { return msg; }
    std::string msg;
};

We can expose this to Python by writing a corresponding Boost.Python C++ Wrapper:

#include <boost/python.hpp>
using namespace boost::python;

BOOST_PYTHON_MODULE(hello)
{
    class_<World>("World")
        .def("greet", &World::greet)
        .def("set", &World::set)
    ;
}

Here, we wrote a C++ class wrapper that exposes the member functions greet and set. Now, after building our module as a shared library, we may use our class World in Python. Here's a sample Python session:

>>> import hello
>>> planet = hello.World()
>>> planet.set('howdy')
>>> planet.greet()
'howdy'

Constructors

Our previous example didn't have any explicit constructors. Since World is declared as a plain struct, it has an implicit default constructor. Boost.Python exposes the default constructor by default, which is why we were able to write

>>> planet = hello.World()

We may wish to wrap a class with a non-default constructor. Let us build on our previous example:

struct World
{
    World(std::string msg): msg(msg) {} // added constructor
    void set(std::string msg) { this->msg = msg; }
    std::string greet() { return msg; }
    std::string msg;
};

This time World has no default constructor; our previous wrapping code would fail to compile when the library tried to expose it. We have to tell class_<World> about the constructor we want to expose instead.

#include <boost/python.hpp>
using namespace boost::python;

BOOST_PYTHON_MODULE(hello)
{
    class_<World>("World", init<std::string>())
        .def("greet", &World::greet)
        .def("set", &World::set)
    ;
}

init<std::string>() exposes the constructor taking in a std::string (in Python, constructors are spelled ""__init__"").

We can expose additional constructors by passing more init<...>s to the def() member function. Say for example we have another World constructor taking in two doubles:

class_<World>("World", init<std::string>())
    .def(init<double, double>())
    .def("greet", &World::greet)
    .def("set", &World::set)
;

On the other hand, if we do not wish to expose any constructors at all, we may use no_init instead:

class_<Abstract>("Abstract", no_init)

This actually adds an __init__ method which always raises a Python RuntimeError exception.

Class Data Members

Data members may also be exposed to Python so that they can be accessed as attributes of the corresponding Python class. Each data member that we wish to be exposed may be regarded as read-only or read-write. Consider this class Var:

struct Var
{
    Var(std::string name) : name(name), value() {}
    std::string const name;
    float value;
};

Our C++ Var class and its data members can be exposed to Python:

class_<Var>("Var", init<std::string>())
    .def_readonly("name", &Var::name)
    .def_readwrite("value", &Var::value);

Then, in Python, assuming we have placed our Var class inside the namespace hello as we did before:

>>> x = hello.Var('pi')
>>> x.value = 3.14
>>> print x.name, 'is around', x.value
pi is around 3.14

Note that name is exposed as read-only while value is exposed as read-write.

>>> x.name = 'e' # can't change name
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
AttributeError: can't set attribute

Class Properties

In C++, classes with public data members are usually frowned upon. Well designed classes that take advantage of encapsulation hide the class' data members. The only way to access the class' data is through access (getter/setter) functions. Access functions expose class properties. Here's an example:

struct Num
{
    Num();
    float get() const;
    void set(float value);
    ...
};

However, in Python attribute access is fine; it doesn't neccessarily break encapsulation to let users handle attributes directly, because the attributes can just be a different syntax for a method call. Wrapping our Num class using Boost.Python:

class_<Num>("Num")
    .add_property("rovalue", &Num::get)
    .add_property("value", &Num::get, &Num::set);

And at last, in Python:

>>> x = Num()
>>> x.value = 3.14
>>> x.value, x.rovalue
(3.14, 3.14)
>>> x.rovalue = 2.17 # error!

Take note that the class property rovalue is exposed as read-only since the rovalue setter member function is not passed in:

.add_property("rovalue", &Num::get)

Inheritance

In the previous examples, we dealt with classes that are not polymorphic. This is not often the case. Much of the time, we will be wrapping polymorphic classes and class hierarchies related by inheritance. We will often have to write Boost.Python wrappers for classes that are derived from abstract base classes.

Consider this trivial inheritance structure:

struct Base { virtual ~Base(); };
struct Derived : Base {};

And a set of C++ functions operating on Base and Derived object instances:

void b(Base*);
void d(Derived*);
Base* factory() { return new Derived; }

We've seen how we can wrap the base class Base:

class_<Base>("Base")
    /*...*/
    ;

Now we can inform Boost.Python of the inheritance relationship between Derived and its base class Base. Thus:

class_<Derived, bases<Base> >("Derived")
    /*...*/
    ;

Doing so, we get some things for free:

  1. Derived automatically inherits all of Base's Python methods (wrapped C++ member functions)
  2. If Base is polymorphic, Derived objects which have been passed to Python via a pointer or reference to Base can be passed where a pointer or reference to Derived is expected.

Now, we will expose the C++ free functions b and d and factory:

def("b", b);
def("d", d);
def("factory", factory);

Note that free function factory is being used to generate new instances of class Derived. In such cases, we use return_value_policy<manage_new_object> to instruct Python to adopt the pointer to Base and hold the instance in a new Python Base object until the the Python object is destroyed. We will see more of Boost.Python call policies later.

// Tell Python to take ownership of factory's result
def("factory", factory,
    return_value_policy<manage_new_object>());

Class Virtual Functions

In this section, we will learn how to make functions behave polymorphically through virtual functions. Continuing our example, let us add a virtual function to our Base class:

struct Base
{
    virtual ~Base() {}
    virtual int f() = 0;
};

One of the goals of Boost.Python is to be minimally intrusive on an existing C++ design. In principle, it should be possible to expose the interface for a 3rd party library without changing it. It is not ideal to add anything to our class Base. Yet, when you have a virtual function that's going to be overridden in Python and called polymorphically from C++, we'll need to add some scaffoldings to make things work properly. What we'll do is write a class wrapper that derives from Base that will unintrusively hook into the virtual functions so that a Python override may be called:

struct BaseWrap : Base, wrapper<Base>
{
    int f()
    {
        return this->get_override("f")();
    }
};

Notice too that in addition to inheriting from Base, we also multiply- inherited wrapper<Base> (See Wrapper). The wrapper template makes the job of wrapping classes that are meant to overridden in Python, easier.

BaseWrap's overridden virtual member function f in effect calls the corresponding method of the Python object through get_override.

Finally, exposing Base:

class_<BaseWrap, boost::noncopyable>("Base")
    .def("f", pure_virtual(&Base::f))
    ;

pure_virtual signals Boost.Python that the function f is a pure virtual function.

[Note] Note

member function and methods

Python, like many object oriented languages uses the term methods. Methods correspond roughly to C++'s member functions

Virtual Functions with Default Implementations

We've seen in the previous section how classes with pure virtual functions are wrapped using Boost.Python's class wrapper facilities. If we wish to wrap non-pure-virtual functions instead, the mechanism is a bit different.

Recall that in the previous section, we wrapped a class with a pure virtual function that we then implemented in C++, or Python classes derived from it. Our base class:

struct Base
{
    virtual int f() = 0;
};

had a pure virtual function f. If, however, its member function f was not declared as pure virtual:

struct Base
{
    virtual ~Base() {}
    virtual int f() { return 0; }
};

We wrap it this way:

struct BaseWrap : Base, wrapper<Base>
{
    int f()
    {
        if (override f = this->get_override("f"))
            return f(); // *note*
        return Base::f();
    }

    int default_f() { return this->Base::f(); }
};

Notice how we implemented BaseWrap::f. Now, we have to check if there is an override for f. If none, then we call Base::f().

Finally, exposing:

class_<BaseWrap, boost::noncopyable>("Base")
    .def("f", &Base::f, &BaseWrap::default_f)
    ;

Take note that we expose both &Base::f and &BaseWrap::default_f. Boost.Python needs to keep track of 1) the dispatch function f and 2) the forwarding function to its default implementation default_f. There's a special def function for this purpose.

In Python, the results would be as expected:

>>> base = Base()
>>> class Derived(Base):
...     def f(self):
...         return 42
...
>>> derived = Derived()

Calling base.f():

>>> base.f()
0

Calling derived.f():

>>> derived.f()
42

Class Operators/Special Functions

Python Operators

C is well known for the abundance of operators. C++ extends this to the extremes by allowing operator overloading. Boost.Python takes advantage of this and makes it easy to wrap C++ operator-powered classes.

Consider a file position class FilePos and a set of operators that take on FilePos instances:

class FilePos { /*...*/ };

FilePos     operator+(FilePos, int);
FilePos     operator+(int, FilePos);
int         operator-(FilePos, FilePos);
FilePos     operator-(FilePos, int);
FilePos&    operator+=(FilePos&, int);
FilePos&    operator-=(FilePos&, int);
bool        operator<(FilePos, FilePos);

The class and the various operators can be mapped to Python rather easily and intuitively:

class_<FilePos>("FilePos")
    .def(self + int())          // __add__
    .def(int() + self)          // __radd__
    .def(self - self)           // __sub__
    .def(self - int())          // __sub__
    .def(self += int())         // __iadd__
    .def(self -= other<int>())
    .def(self < self);          // __lt__

The code snippet above is very clear and needs almost no explanation at all. It is virtually the same as the operators' signatures. Just take note that self refers to FilePos object. Also, not every class T that you might need to interact with in an operator expression is (cheaply) default-constructible. You can use other<T>() in place of an actual T instance when writing "self expressions".

Special Methods

Python has a few more Special Methods. Boost.Python supports all of the standard special method names supported by real Python class instances. A similar set of intuitive interfaces can also be used to wrap C++ functions that correspond to these Python special functions. Example:

class Rational
{ public: operator double() const; };

Rational pow(Rational, Rational);
Rational abs(Rational);
ostream& operator<<(ostream&,Rational);

class_<Rational>("Rational")
    .def(float_(self))                  // __float__
    .def(pow(self, other<Rational>))    // __pow__
    .def(abs(self))                     // __abs__
    .def(str(self))                     // __str__
    ;

Need we say more?

[Note] Note

What is the business of operator<<? Well, the method str requires the operator<< to do its work (i.e. operator<< is used by the method defined by def(str(self)).


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