Can't bind functions taking char& as an argument

[lyskov]

Dear Pybind11 developers,
Looks like both latest master and stable seems to produce an error when trying to bind function taking char &. Could you please looks this up?

Thanks,

#include <pybind11/pybind11.h>

void foo_char(char &) {}
void foo_bool(bool &) {}
void foo_int(int &) {}
void foo_long(long &) {}
void foo_long_long(long long &) {}
void foo_float(float &) {}
void foo_double(double &) {}

PYBIND11_PLUGIN(example) {
    pybind11::module m("example", "pybind11 example plugin");

	m.def("foo_char",      &foo_char);
	m.def("foo_bool",      &foo_bool);
	m.def("foo_int",       &foo_int);
	m.def("foo_long",      &foo_long);
	m.def("foo_long_long", &foo_long_long);
	m.def("foo_float",     &foo_float);
	m.def("foo_double",    &foo_double);

    return m.ptr();
}

Error:

In file included from /home/benchmark/src/pybind11/include/pybind11/pybind11.h:43:
In file included from /home/benchmark/src/pybind11/include/pybind11/attr.h:13:
/home/benchmark/src/pybind11/include/pybind11/cast.h:1858:38: error: non-const lvalue reference to type 'char' cannot bind to a temporary of type 'typename make_caster<char &>::cast_op_type<typename std::add_rvalue_reference<char &>::type>' (aka 'char')
        return std::forward<Func>(f)(cast_op<Args>(std::move(std::get<Is>(argcasters)))...);
                                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/benchmark/src/pybind11/include/pybind11/cast.h:1840:35: note: in instantiation of function template specialization 'pybind11::detail::argument_loader<char &>::call_impl<void, void (*&)(char &), 0, pybind11::detail::void_type>' requested here
        std::move(*this).template call_impl<Return>(std::forward<Func>(f), indices{}, Guard{});
                                  ^
/home/benchmark/src/pybind11/include/pybind11/pybind11.h:154:52: note: in instantiation of function template specialization 'pybind11::detail::argument_loader<char &>::call<void, pybind11::detail::void_type, void (*&)(char &)>' requested here
                std::move(args_converter).template call<Return, Guard>(cap->f), policy, call.parent);
                                                   ^
/home/benchmark/src/pybind11/include/pybind11/pybind11.h:58:9: note: in instantiation of function template specialization 'pybind11::cpp_function::initialize<void (*&)(char &), void, char &, pybind11::name, pybind11::scope, pybind11::sibling>' requested here
        initialize(f, f, extra...);
        ^
/home/benchmark/src/pybind11/include/pybind11/pybind11.h:806:22: note: in instantiation of function template specialization 'pybind11::cpp_function::cpp_function<void, char &, pybind11::name, pybind11::scope, pybind11::sibling>' requested here
        cpp_function func(std::forward<Func>(f), name(name_), scope(*this),
                     ^
ret-reference.cpp:14:4: note: in instantiation of function template specialization 'pybind11::module::def<void (*)(char &)>' requested here
        m.def("foo_char",      &foo_char);
          ^
1 warning and 1 error generated.

[jagerman]

I'm away for a couple days and won't be able to look into this in detail until I get back, but one quick question: did it work in 2.1?

[lyskov]

It certainly worked before i merged last Pybind11 upstream... but it am not sure if it was after 2.1 or before it. However i can tell you exact commit where it was still working: it is Dec 16, 2016 RosettaCommons@1d1f81b

Thank you!

[lyskov]

Small update: it looks like returning char & is still ok, - no compilation errors. Hope this helps!

[lyskov]

@jagerman are there any unexpected complication with this? Should i plan to implement work around for this for now?

[jagerman]

See #1128; that should fix it.

[lyskov]

Great, thank you @jagerman !

[lyskov]

Just letting you know that i tested this and it works fine, - thanks again!

[trelau]

In case this is related, in a C++ library I am trying to wrap I get a compilation error with some character inputs. This reproduces the error on MSVC with both the pybind11 master and the branch for this fix:

#include <pybind11\pybind11.h>

namespace py = pybind11;

typedef const char* TheString;

void TheMethod(TheString& theString) {};

PYBIND11_MODULE(pymod2, mod) {

	mod.def("TheMethod", (void (*) (TheString &)) &TheMethod);
	
}

results in

'void (TheString &)': cannot convert argument 1 from 'char *' to 'TheString &'

[jagerman]

@trelau - I've just merged #1128; what you have there looks like a different issue (though in a related part of the code); can you file a new issue for it?

[trelau]

@jagerman Done. Thanks for the support.