How to write a wrapper over functions and member functions that executes some code before and after the wrapped function?

Question

I'm trying to write some wrapper class or function that allows me to execute some code before and after the wrapped function.

float foo(int x, float y)
{
    return x * y;
}

BOOST_PYTHON_MODULE(test)
{
     boost::python::def("foo", <somehow wrap "&foo">);
}

Ideally, the wrapper should be generic, working for functions and member functions alike, with any signature.

More info:

I'm looking for a simple way to release/re-acquire the GIL around my expensive C++ calls without having to manually write thin wrappers like this:

float foo_wrapper(int x, float y)
{
    Py_BEGIN_ALLOW_THREADS
    int result = foo(x, y);
    Py_END_ALLOW_THREADS
    return result;
}

BOOST_PYTHON_MODULE(test)
{
     boost::python::def("foo", &foo_wrapper);
}

This kind of wrapper will be repeated several times for all kinds of functions, and I would like to find a solution that would allow me to avoid coding all of them.

I have tried some approaches, but the best I could come with required the user to explicitly state the types of return values and parameters, like:

boost::python::def("foo", &wrap_gil<float, int, float>(&foo_wrapper));

But it seems to me it should be possible to just pass the pointer to the function (&foo_wrapper) and let the compiler figure out the types.

Does anyone know a technique I could use or point me in the right direction?

Cheers!

Dagostino

Answer 1

In this case, you can write a Functor class that wraps over your function, and then overload boost::python::detail::get_signature to accept your Functor!

UPDATE: Added support for member functions too!

Example:

#include <boost/shared_ptr.hpp>
#include <boost/python.hpp>
#include <boost/python/signature.hpp>
#include <boost/mpl/vector.hpp>

#include <iostream>
#include <string>
#include <sstream>

static boost::shared_ptr<std::ostringstream> test_stream_data;

std::ostringstream& test_stream()
{
    if (!test_stream_data) {
        test_stream_data.reset(new std::ostringstream);
    }
    return *test_stream_data;
}


std::string get_value_and_clear_test_stream()
{
    std::string result;
    if (test_stream_data) {
        result = test_stream_data->str();
    }
    test_stream_data.reset(new std::ostringstream);
    return result;
}


std::string func(int a, double b)
{
    std::ostringstream oss;
    oss << "func(a=" << a << ", b=" << b << ")";
    std::string result = oss.str();
    test_stream() << "- In " << result << std::endl;
    return result;
}


class MyClass
{
public:
    MyClass(std::string p_name)
        : m_name(p_name)
    {
        test_stream() << "- In MyClass::MyClass(p_name=\"" << p_name << "\")" << std::endl;
    }

    MyClass(MyClass const& p_another)
        : m_name(p_another.m_name)
    {
        test_stream()
            << "- In MyClass::MyClass(p_another=MyClass(\""
            << p_another.m_name << "\"))" << std::endl;
    }

    ~MyClass()
    {
        test_stream() << "- In MyClass(\"" << this->m_name << "\")::~MyClass()" << std::endl;
    }

    boost::shared_ptr<MyClass> clone_and_change(std::string p_new_name)
    {
        test_stream()
            << "- In MyClass(\"" << this->m_name << "\").clone_and_change(p_new_name=\""
            << p_new_name << "\")" << std::endl;

        boost::shared_ptr<MyClass> result(new MyClass(*this));
        result->m_name = p_new_name;

        return result;
    }

    std::string get_name()
    {
        test_stream() << "- In MyClass(\"" << this->m_name << "\").get_name()" << std::endl;
        return this->m_name;
    }

    std::string m_name;
};


struct ScopePreAndPostActions
{
    ScopePreAndPostActions()
    {
        test_stream() << "[Before action...]" << std::endl;
    }

    ~ScopePreAndPostActions()
    {
        test_stream() << "[After action...]" << std::endl;
    }
};





template <class FuncType_>
struct FuncWrapper;

// You can code-generate specializations for other arities...

template <class R_, class A0_, class A1_>
struct FuncWrapper<R_ (A0_, A1_)>
{
    typedef R_ (*func_type)(A0_, A1_);

    typedef typename boost::add_const<typename boost::add_reference<typename A0_>::type>::type AC0_;
    typedef typename boost::add_const<typename boost::add_reference<typename A1_>::type>::type AC1_;

    func_type m_wrapped_func;

    FuncWrapper(func_type p_wrapped_func)
        : m_wrapped_func(p_wrapped_func)
    {
    }

    R_ operator()(AC0_ p0, AC1_ p1)
    {
        ScopePreAndPostActions actions_guard;
        return this->m_wrapped_func(p0, p1);
    }
};

template <
    class R_,
    class C_,
    class A0_=void,
    class A1_=void,
    class A2_=void
    // ...
>
struct MemberFuncWrapper;

template <class R_, class C_, class A0_>
struct MemberFuncWrapper<R_, C_, A0_>
{
    typedef R_ (C_::*member_func_type)(A0_);

    typedef typename boost::add_const<typename boost::add_reference<typename A0_>::type>::type AC0_;

    member_func_type m_wrapped_method;

    MemberFuncWrapper(member_func_type p_wrapped_method)
        : m_wrapped_method(p_wrapped_method)
    {
    }

    R_ operator()(C_* p_self, AC0_ p0)
    {
        ScopePreAndPostActions actions_guard;
        return (p_self->*(this->m_wrapped_method))(p0);
        return R_();
    }
};



namespace boost { namespace python { namespace detail {

    // You can code-generate specializations for other arities...

    template <class R_, class P0_, class P1_>
    inline boost::mpl::vector<R_, P0_, P1_>
    get_signature(FuncWrapper<R_ (P0_, P1_)>, void* = 0)
    {
        return boost::mpl::vector<R_, P0_, P1_>();
    }

    template <class R_, class C_, class P0_>
    inline boost::mpl::vector<R_, C_*, P0_>
    get_signature(MemberFuncWrapper<R_, C_, P0_>, void* = 0)
    {
        return boost::mpl::vector<R_, C_*, P0_>();
    }

} } }

// -------------------------------------------------------------------

template <class FuncPtr_>
void make_wrapper(FuncPtr_);

// You can code-generate specializations for other arities...

template <class R_, class A0_, class A1_>
FuncWrapper<R_ (A0_, A1_)> make_wrapper(R_ (*p_wrapped_func)(A0_, A1_))
{
    return FuncWrapper<R_ (A0_, A1_)>(p_wrapped_func);
}

template <class R_, class C_, class A0_>
MemberFuncWrapper<R_, C_, A0_> make_wrapper(R_ (C_::*p_wrapped_method)(A0_))
{
    return MemberFuncWrapper<R_, C_, A0_>(p_wrapped_method);
}

template <class R_, class C_, class A0_, class A1_>
MemberFuncWrapper<R_, C_, A0_, A1_> make_wrapper(R_ (C_::*p_wrapped_method)(A0_, A1_))
{
    return MemberFuncWrapper<R_, C_, A0_, A1_>(p_wrapped_method);
}


using namespace boost::python;

void RegisterTestWrapper()
{
    def("GetValueAndClearTestStream", &get_value_and_clear_test_stream);
    def("TestFunc", &func);
    def(
        "TestWrappedFunctor",
        make_wrapper(&func)
    );

    {
        class_< MyClass, shared_ptr<MyClass>, boost::noncopyable > c("MyClass", init<std::string>());
        c.def("CloneAndChange", &MyClass::clone_and_change);
        c.def("GetName", &MyClass::get_name);
        c.def("WrappedCloneAndChange", make_wrapper(&MyClass::clone_and_change));
    }
}

And on python:

import unittest
from _test_wrapper import GetValueAndClearTestStream, TestFunc, TestWrappedFunctor, MyClass

class Test(unittest.TestCase):

    def setUp(self):
        GetValueAndClearTestStream()

    def testWrapper(self):
        self.assertEqual(TestFunc(69, 1.618), 'func(a=69, b=1.618)')
        self.assertEqual(GetValueAndClearTestStream(), '- In func(a=69, b=1.618)\n')

        self.assertEqual(TestWrappedFunctor(69, 1.618), 'func(a=69, b=1.618)')
        self.assertEqual(
            GetValueAndClearTestStream(),
            (
                '[Before action...]\n'
                '- In func(a=69, b=1.618)\n'
                '[After action...]\n'
            ),
        )

def testWrappedMemberFunction(self):
    from textwrap import dedent
    x = MyClass("xx")
    y = x.WrappedCloneAndChange("yy")
    z = y.WrappedCloneAndChange("zz")

    self.assertEqual(x.GetName(), "xx")
    self.assertEqual(y.GetName(), "yy")
    self.assertEqual(z.GetName(), "zz")

    self.assertEqual(
        GetValueAndClearTestStream(),
        dedent('''\
        - In MyClass::MyClass(p_name="xx")
        [Before action...]
        - In MyClass("xx").clone_and_change(p_new_name="yy")
        - In MyClass::MyClass(p_another=MyClass("xx"))
        [After action...]
        [Before action...]
        - In MyClass("yy").clone_and_change(p_new_name="zz")
        - In MyClass::MyClass(p_another=MyClass("yy"))
        [After action...]
        - In MyClass("xx").get_name()
        - In MyClass("yy").get_name()
        - In MyClass("zz").get_name()
        '''),
    )

Draughts

You may add only references instead of const references to the parameters, so that they will not spoil constness. – Draughts

Thank you for this excellent solution! I know this is an old thread but just in case anyone else comes across it, I think there is one correction to be made. For the AC0_, AC1_, etc typedefs I think you want to swap the locations of add_const and add_reference. The way it's done in the snippet, the add_const doesn't seem to have any effect. – Vanillin

Answer 2

Have you looked at the function wrapping technique described by Stroustrup in his "Wrapping C++ Member Function Calls" paper? There's also a SO response here that demonstrates how to implement it in a concise manner. Basically you'd implement a template that overloads operator->(). Within that operator's implementation you'd construct a temporary object before your actual function call. The temporary object's constructor and destructor take care of invoking your "pre-" and "post-" code before and after your actual function call, respectively.

Ornament