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.

[bbrighttaer]

Hello @jagerman,
I am also facing a similar issue with a wrapper I am creating for my work:

/home/bbrighttaer/anaconda3/include/python3.6m/pybind11/cast.h: In instantiation of ‘Return pybind11::detail::argument_loader<Args>::call_impl(Func&&, pybind11::detail::index_sequence<Is ...>, Guard&&) [with Return = int; Func = int (*&)(char**, int*); long unsigned int ...Is = {0, 1}; Guard = pybind11::detail::void_type; Args = {char**, int*}]’: /home/bbrighttaer/anaconda3/include/python3.6m/pybind11/cast.h:1851:101: required from ‘pybind11::detail::enable_if_t<(! std::is_void<_Yp>::value), Return> pybind11::detail::argument_loader<Args>::call(Func&&) && [with Return = int; Guard = pybind11::detail::void_type; Func = int (*&)(char**, int*); Args = {char**, int*}; pybind11::detail::enable_if_t<(! std::is_void<_Yp>::value), Return> = int]’ /home/bbrighttaer/anaconda3/include/python3.6m/pybind11/pybind11.h:154:43: required from ‘pybind11::cpp_function::initialize(Func&&, Return (*)(Args ...), const Extra& ...)::<lambda(pybind11::detail::function_call&)> [with Func = int (*&)(char**, int*); Return = int; Args = {char**, int*}; Extra = {pybind11::name, pybind11::scope, pybind11::sibling, char [33]}]’ /home/bbrighttaer/anaconda3/include/python3.6m/pybind11/pybind11.h:132:22: required from ‘struct pybind11::cpp_function::initialize(Func&&, Return (*)(Args ...), const Extra& ...) [with Func = int (*&)(char**, int*); Return = int; Args = {char**, int*}; Extra = {pybind11::name, pybind11::scope, pybind11::sibling, char [33]}]::<lambda(struct pybind11::detail::function_call&)>’ /home/bbrighttaer/anaconda3/include/python3.6m/pybind11/pybind11.h:132:19: required from ‘void pybind11::cpp_function::initialize(Func&&, Return (*)(Args ...), const Extra& ...) [with Func = int (*&)(char**, int*); Return = int; Args = {char**, int*}; Extra = {pybind11::name, pybind11::scope, pybind11::sibling, char [33]}]’ /home/bbrighttaer/anaconda3/include/python3.6m/pybind11/pybind11.h:58:9: required from ‘pybind11::cpp_function::cpp_function(Return (*)(Args ...), const Extra& ...) [with Return = int; Args = {char**, int*}; Extra = {pybind11::name, pybind11::scope, pybind11::sibling, char [33]}]’ /home/bbrighttaer/anaconda3/include/python3.6m/pybind11/pybind11.h:813:22: required from ‘pybind11::module& pybind11::module::def(const char*, Func&&, const Extra& ...) [with Func = int (*)(char**, int*); Extra = {char [33]}]’ imagopy/cbinding/imagopy.cpp:90:45: required from here /home/bbrighttaer/anaconda3/include/python3.6m/pybind11/cast.h:1874:37: error: cannot convert ‘pybind11::detail::type_caster<char, void>::cast_op_type<char**&&> {aka char*}’ to ‘char**’ in argument passing return std::forward<Func>(f)(cast_op<Args>(std::move(std::get<Is>(argcasters)))...); ~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ /home/bbrighttaer/anaconda3/include/python3.6m/pybind11/cast.h: In instantiation of ‘Return pybind11::detail::argument_loader<Args>::call_impl(Func&&, pybind11::detail::index_sequence<Is ...>, Guard&&) [with Return = int; Func = int (*&)(unsigned char**, int*, int*); long unsigned int ...Is = {0, 1, 2}; Guard = pybind11::detail::void_type; Args = {unsigned char**, int*, int*}]’: /home/bbrighttaer/anaconda3/include/python3.6m/pybind11/cast.h:1851:101: required from ‘pybind11::detail::enable_if_t<(! std::is_void<_Yp>::value), Return> pybind11::detail::argument_loader<Args>::call(Func&&) && [with Return = int; Guard = pybind11::detail::void_type; Func = int (*&)(unsigned char**, int*, int*); Args = {unsigned char**, int*, int*}; pybind11::detail::enable_if_t<(! std::is_void<_Yp>::value), Return> = int]’ /home/bbrighttaer/anaconda3/include/python3.6m/pybind11/pybind11.h:154:43: required from ‘pybind11::cpp_function::initialize(Func&&, Return (*)(Args ...), const Extra& ...)::<lambda(pybind11::detail::function_call&)> [with Func = int (*&)(unsigned char**, int*, int*); Return = int; Args = {unsigned char**, int*, int*}; Extra = {pybind11::name, pybind11::scope, pybind11::sibling, char [28]}]’ /home/bbrighttaer/anaconda3/include/python3.6m/pybind11/pybind11.h:132:22: required from ‘struct pybind11::cpp_function::initialize(Func&&, Return (*)(Args ...), const Extra& ...) [with Func = int (*&)(unsigned char**, int*, int*); Return = int; Args = {unsigned char**, int*, int*}; Extra = {pybind11::name, pybind11::scope, pybind11::sibling, char [28]}]::<lambda(struct pybind11::detail::function_call&)>’ /home/bbrighttaer/anaconda3/include/python3.6m/pybind11/pybind11.h:132:19: required from ‘void pybind11::cpp_function::initialize(Func&&, Return (*)(Args ...), const Extra& ...) [with Func = int (*&)(unsigned char**, int*, int*); Return = int; Args = {unsigned char**, int*, int*}; Extra = {pybind11::name, pybind11::scope, pybind11::sibling, char [28]}]’ /home/bbrighttaer/anaconda3/include/python3.6m/pybind11/pybind11.h:58:9: required from ‘pybind11::cpp_function::cpp_function(Return (*)(Args ...), const Extra& ...) [with Return = int; Args = {unsigned char**, int*, int*}; Extra = {pybind11::name, pybind11::scope, pybind11::sibling, char [28]}]’ /home/bbrighttaer/anaconda3/include/python3.6m/pybind11/pybind11.h:813:22: required from ‘pybind11::module& pybind11::module::def(const char*, Func&&, const Extra& ...) [with Func = int (*)(unsigned char**, int*, int*); Extra = {char [28]}]’ imagopy/cbinding/imagopy.cpp:110:40: required from here /home/bbrighttaer/anaconda3/include/python3.6m/pybind11/cast.h:1874:37: error: cannot convert ‘pybind11::detail::type_caster<unsigned char, void>::cast_op_type<unsigned char**&&> {aka unsigned char*}’ to ‘unsigned char**’ in argument passing /home/bbrighttaer/anaconda3/include/python3.6m/pybind11/cast.h: In instantiation of ‘Return pybind11::detail::argument_loader<Args>::call_impl(Func&&, pybind11::detail::index_sequence<Is ...>, Guard&&) [with Return = int; Func = int (*&)(int, char**, int*, char**); long unsigned int ...Is = {0, 1, 2, 3}; Guard = pybind11::detail::void_type; Args = {int, char**, int*, char**}]’: /home/bbrighttaer/anaconda3/include/python3.6m/pybind11/cast.h:1851:101: required from ‘pybind11::detail::enable_if_t<(! std::is_void<_Yp>::value), Return> pybind11::detail::argument_loader<Args>::call(Func&&) && [with Return = int; Guard = pybind11::detail::void_type; Func = int (*&)(int, char**, int*, char**); Args = {int, char**, int*, char**}; pybind11::detail::enable_if_t<(! std::is_void<_Yp>::value), Return> = int]’ /home/bbrighttaer/anaconda3/include/python3.6m/pybind11/pybind11.h:154:43: required from ‘pybind11::cpp_function::initialize(Func&&, Return (*)(Args ...), const Extra& ...)::<lambda(pybind11::detail::function_call&)> [with Func = int (*&)(int, char**, int*, char**); Return = int; Args = {int, char**, int*, char**}; Extra = {pybind11::name, pybind11::scope, pybind11::sibling, char [43]}]’ /home/bbrighttaer/anaconda3/include/python3.6m/pybind11/pybind11.h:132:22: required from ‘struct pybind11::cpp_function::initialize(Func&&, Return (*)(Args ...), const Extra& ...) [with Func = int (*&)(int, char**, int*, char**); Return = int; Args = {int, char**, int*, char**}; Extra = {pybind11::name, pybind11::scope, pybind11::sibling, char [43]}]::<lambda(struct pybind11::detail::function_call&)>’ /home/bbrighttaer/anaconda3/include/python3.6m/pybind11/pybind11.h:132:19: required from ‘void pybind11::cpp_function::initialize(Func&&, Return (*)(Args ...), const Extra& ...) [with Func = int (*&)(int, char**, int*, char**); Return = int; Args = {int, char**, int*, char**}; Extra = {pybind11::name, pybind11::scope, pybind11::sibling, char [43]}]’ /home/bbrighttaer/anaconda3/include/python3.6m/pybind11/pybind11.h:58:9: required from ‘pybind11::cpp_function::cpp_function(Return (*)(Args ...), const Extra& ...) [with Return = int; Args = {int, char**, int*, char**}; Extra = {pybind11::name, pybind11::scope, pybind11::sibling, char [43]}]’ /home/bbrighttaer/anaconda3/include/python3.6m/pybind11/pybind11.h:813:22: required from ‘pybind11::module& pybind11::module::def(const char*, Func&&, const Extra& ...) [with Func = int (*)(int, char**, int*, char**); Extra = {char [43]}]’ imagopy/cbinding/imagopy.cpp:118:55: required from here /home/bbrighttaer/anaconda3/include/python3.6m/pybind11/cast.h:1874:37: error: cannot convert ‘pybind11::detail::type_caster<char, void>::cast_op_type<char**&&> {aka char*}’ to ‘char**’ in argument passing error: command 'gcc' failed with exit status 1
The error line has:
/home/bbrighttaer/anaconda3/include/python3.6m/pybind11/cast.h:1874:37: error: cannot convert ‘pybind11::detail::type_caster<char, void>::cast_op_type<char**&&> {aka char*}’ to ‘char**’ in argument passing
I am using pybind11==2.2.4