...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
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.
Copyright © 2002-2003 David Abrahams
Copyright © 2002-2003 Joel de Guzman
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