Add Member Functions To A Boost Python Class After The Fact?

In Boost Python, you can expose a C++ class to Python like this: object obj = class_('MyClass') .def('SomeFunc', &MyClass::SomeFunc) ; Let's say MyClass

Solution 1:

obj doesn't have a member function named def, but class_ does. class_ inherits from object, which is why that assignment works, but you don't have to do that. The following works fine:

auto obj = class_<MyClass>("MyClass")
    .def("SomeFunc", &MyClass::SomeFunc)
;

// now obj is an instance of py::class_<MyClass> instead of py::object
obj.def("SomeOtherFunc", &MyClass::SomeOtherFunc);

The fact that you never name the class_ instance may distract from the fact that this is a simple constructor. You could have also written the above as:

class_<MyClass> obj("MyClass");
obj.def("SomeFunc", &MyClass::SomeFunc);
obj.def("SomeOtherFunc", &MyClass::SomeOtherFunc);

You don't have to define everything in one go.

Bonus points if someone can explain exactly what def is doing and why I don't need commas in between defs.

def is a member function on the class template class_. It has a bunch of overloads, all of which returns a reference to self:

template <classInit>         class_& def(Init );
template <classF>            class_& def(charconst*, Fn );
template <classFn, classA1> class_& def(charconst*, Fn, A1 );
// etc.

That's why you can chain them. Each call independently sets up the internal machinery to expose that function to python and just returns *this. I don't know what you would need commas for.