Faq example "wrap a function which takes a function pointer as an argument" not work

[hzhangxyz]

This example code not work on my pc. https://www.boost.org/doc/libs/1_72_0/libs/python/doc/html/faq.html

E.cpp

#include <boost/python.hpp>
#include <boost/function.hpp>
#include <string>

using namespace boost::python;

typedef boost::function<void (std::string s) > funcptr;

void foo(funcptr fp)
{
  fp("hello,world!");
}

BOOST_PYTHON_MODULE(E)
{
  def("foo",foo);
}

compile wth g++ E.cpp -I/usr/include/python3.8 -lpython3.8 -shared -fPIC -o Epython3-config --extension-suffix-std=c++17 -I../include -lboost_python38

and run python -c 'import E; E.foo(print)'

it say

Traceback (most recent call last):
  File "<string>", line 1, in <module>
Boost.Python.ArgumentError: Python argument types in
    E.foo(builtin_function_or_method)
did not match C++ signature:
    foo(boost::function<void (std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >)>)

environment: archlinux, boost 1.72.0, gcc 9.3.0 python 3.8.2

[jordanbriere]

This example code not work on my pc. https://www.boost.org/doc/libs/1_72_0/libs/python/doc/html/faq.html

You probably figured it out by now, but if you read that page carefully, it actually shows you an example of what "you can't" do. 😄

The short answer is: "you can't". This is not a Boost.Python limitation so much as a limitation of C++. The problem is that a Python function is actually data, and the only way of associating data with a C++ function pointer is to store it in a static variable of the function. The problem with that is that you can only associate one piece of data with every C++ function, and we have no way of compiling a new C++ function on-the-fly for every Python function you decide to pass to foo. In other words, this could work if the C++ function is always going to invoke the same Python function, but you probably don't want that.

If you have the luxury of changing the C++ code you're wrapping, pass it an object instead and call that; the overloaded function call operator will invoke the Python function you pass it behind the object.

[hzhangxyz]

hhhhhhhhhhhhhhhhhhhhhh, so stupid I am