variant_caster cant handle c++ type nullptr_t #839

RedSkotina · 2017-05-08T10:22:59Z

I have compile errors while compile this code (gcc 7.1 and vs 2017 checked)

    py::module m("xpytest", "pybind testing plugin");
    m.def("load_variant", [](std::variant<int, std::nullptr_t> v) { 
        return std::visit(visitor(), v);
    });

gcc output:

F:/devel/mingw-w64/i686-7.1.0-posix-dwarf-rt_v5-rev0/mingw32/bin/g++.exe  -c  "E:/code/zog/ptest3/xpytest.cpp" -O0 -g -std=c++17 -D_hypot=hypot -D_DEBUG  -o ./Debug/up_xpytest.cpp.o -I. -I. -If:\lang\Python35-32\include
In file included from f:\lang\Python35-32\include/pybind11/attr.h:13:0,
                 from f:\lang\Python35-32\include/pybind11/pybind11.h:36,
                 from E:/code/zog/ptest3/xpytest.cpp:3:
f:\lang\Python35-32\include/pybind11/cast.h: In instantiation of 'typename pybind11::detail::make_caster<T>::cast_op_type<T> pybind11::detail::cast_op(pybind11::detail::make_caster<T>&) [with T = std::nullptr_t; typename pybind11::detail::make_caster<T>::cast_op_type<T> = std::nullptr_t&; pybind11::detail::make_caster<T> = pybind11::detail::type_caster<std::nullptr_t>; typename pybind11::detail::intrinsic_type<T>::type = std::nullptr_t]':
f:\lang\Python35-32\include/pybind11/stl.h:305:31:   required from 'bool pybind11::detail::variant_caster<V<Ts ...> >::load_alternative(pybind11::handle, bool, pybind11::detail::type_list<U, Us ...>) [with U = std::nullptr_t; Us = {}; V = std::variant; Ts = {int, std::nullptr_t}]'
f:\lang\Python35-32\include/pybind11/stl.h:308:32:   required from 'bool pybind11::detail::variant_caster<V<Ts ...> >::load_alternative(pybind11::handle, bool, pybind11::detail::type_list<U, Us ...>) [with U = int; Us = {std::nullptr_t}; V = std::variant; Ts = {int, std::nullptr_t}]'
f:\lang\Python35-32\include/pybind11/stl.h:314:32:   required from 'bool pybind11::detail::variant_caster<V<Ts ...> >::load(pybind11::handle, bool) [with V = std::variant; Ts = {int, std::nullptr_t}]'
f:\lang\Python35-32\include/pybind11/cast.h:1469:9:   required from 'bool pybind11::detail::argument_loader<Args>::load_impl_sequence(pybind11::detail::function_call&, std::index_sequence<Indices1 ...>) [with unsigned int ...Is = {0}; Args = {std::variant<int, std::nullptr_t>}; std::index_sequence<Indices1 ...> = std::integer_sequence<unsigned int, 0>]'
f:\lang\Python35-32\include/pybind11/cast.h:1449:34:   required from 'bool pybind11::detail::argument_loader<Args>::load_args(pybind11::detail::function_call&) [with Args = {std::variant<int, std::nullptr_t>}]'
f:\lang\Python35-32\include/pybind11/pybind11.h:132:17:   required from 'pybind11::cpp_function::initialize(Func&&, Return (*)(Args ...), const Extra& ...)::<lambda(pybind11::detail::function_call&)> [with Func = pybind11_init()::<lambda(std::variant<int, std::nullptr_t>)>; Return = const char*; Args = {std::variant<int, std::nullptr_t>}; Extra = {pybind11::name, pybind11::scope, pybind11::sibling}]'
f:\lang\Python35-32\include/pybind11/pybind11.h:128:22:   required from 'struct pybind11::cpp_function::initialize(Func&&, Return (*)(Args ...), const Extra& ...) [with Func = pybind11_init()::<lambda(std::variant<int, std::nullptr_t>)>; Return = const char*; Args = {std::variant<int, std::nullptr_t>}; Extra = {pybind11::name, pybind11::scope, pybind11::sibling}]::<lambda(struct pybind11::detail::function_call&)>'
f:\lang\Python35-32\include/pybind11/pybind11.h:128:19:   required from 'void pybind11::cpp_function::initialize(Func&&, Return (*)(Args ...), const Extra& ...) [with Func = pybind11_init()::<lambda(std::variant<int, std::nullptr_t>)>; Return = const char*; Args = {std::variant<int, std::nullptr_t>}; Extra = {pybind11::name, pybind11::scope, pybind11::sibling}]'
f:\lang\Python35-32\include/pybind11/pybind11.h:62:9:   required from 'pybind11::cpp_function::cpp_function(Func&&, const Extra& ...) [with Func = pybind11_init()::<lambda(std::variant<int, std::nullptr_t>)>; Extra = {pybind11::name, pybind11::scope, pybind11::sibling}; <template-parameter-1-3> = void]'
f:\lang\Python35-32\include/pybind11/pybind11.h:743:22:   required from 'pybind11::module& pybind11::module::def(const char*, Func&&, const Extra& ...) [with Func = pybind11_init()::<lambda(std::variant<int, std::nullptr_t>)>; Extra = {}]'

msvc 2017 output:

1>------ Build started: Project: xpytest, Configuration: Debug Win32 ------
1>C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\Tools\MSVC\14.10.25017\bin\HostX86\x86\CL.exe /c /I"f:\lang\Python35-32\include" /I"C:\Program Files (x86)\Visual Leak Detector\include" /Zi /nologo /W3 /WX- /diagnostics:classic /Od /Ob0 /Oy- /D WIN32 /D _WINDOWS /D PYBIND11_CPP17 /D _DEBUG /D "CMAKE_INTDIR=\"Debug\"" /D xpytest_EXPORTS /D _WINDLL /D _MBCS /Gm- /EHsc /RTC1 /MDd /GS /fp:precise /Zc:wchar_t /Zc:forScope /Zc:inline /GR /std:c++latest /Fo"xpytest.dir\Debug\\" /Fd"xpytest.dir\Debug\vc141.pdb" /Gd /TP /analyze- /errorReport:prompt E:\code\zog\pytest2\xpytest.cpp
1>xpytest.cpp
1>f:\lang\python35-32\include\pybind11\common.h(30): warning C4005: 'PYBIND11_CPP17': macro redefinition
1>f:\lang\python35-32\include\pybind11\common.h(30): note: command-line arguments:  see previous definition of 'PYBIND11_CPP17'
1>f:\lang\Python35-32\include\pybind11/pybind11.h(148): warning C4101: 'dd': unreferenced local variable
1>f:\lang\Python35-32\include\pybind11/pybind11.h(63): note: see reference to function template instantiation 'void pybind11::cpp_function::initialize<_Ty,R,pybind11::handle,>(Func &&,Return (__cdecl *)(pybind11::handle))' being compiled
1>        with
1>        [
1>            _Ty=pybind11::detail::keep_alive_impl::<lambda_fcb9b197e266298ec55444104890fed0>,
1>            R=void,
1>            Func=pybind11::detail::keep_alive_impl::<lambda_fcb9b197e266298ec55444104890fed0>,
1>            Return=void
1>        ]
1>f:\lang\Python35-32\include\pybind11/pybind11.h(1310): note: see reference to function template instantiation 'pybind11::cpp_function::cpp_function<pybind11::detail::keep_alive_impl::<lambda_fcb9b197e266298ec55444104890fed0>,,void>(Func &&)' being compiled
1>        with
1>        [
1>            Func=pybind11::detail::keep_alive_impl::<lambda_fcb9b197e266298ec55444104890fed0>
1>        ]
1>f:\lang\python35-32\include\pybind11\cast.h(490): error C2738: 'operator std::nullptr_t &': is ambiguous or is not a member of 'pybind11::detail::type_caster<std::nullptr_t,void>'
1>f:\lang\python35-32\include\pybind11\cast.h(656): note: see declaration of 'pybind11::detail::type_caster<std::nullptr_t,void>'
1>f:\lang\Python35-32\include\pybind11/stl.h(305): note: see reference to function template instantiation '_Ty &pybind11::detail::cast_op<U>(pybind11::detail::type_caster<std::nullptr_t,void> &)' being compiled
1>        with
1>        [
1>            _Ty=std::nullptr_t,
1>            U=std::nullptr_t
1>        ]
1>f:\lang\Python35-32\include\pybind11/stl.h(308): note: see reference to function template instantiation 'bool pybind11::detail::variant_caster<std::variant<int,std::nullptr_t>>::load_alternative<std::nullptr_t,>(pybind11::handle,bool,pybind11::detail::type_list<std::nullptr_t>)' being compiled
1>f:\lang\Python35-32\include\pybind11/stl.h(308): note: see reference to function template instantiation 'bool pybind11::detail::variant_caster<std::variant<int,std::nullptr_t>>::load_alternative<std::nullptr_t,>(pybind11::handle,bool,pybind11::detail::type_list<std::nullptr_t>)' being compiled
1>f:\lang\Python35-32\include\pybind11/stl.h(314): note: see reference to function template instantiation 'bool pybind11::detail::variant_caster<std::variant<int,std::nullptr_t>>::load_alternative<int,std::nullptr_t>(pybind11::handle,bool,pybind11::detail::type_list<int,std::nullptr_t>)' being compiled
1>f:\lang\Python35-32\include\pybind11/stl.h(314): note: see reference to function template instantiation 'bool pybind11::detail::variant_caster<std::variant<int,std::nullptr_t>>::load_alternative<int,std::nullptr_t>(pybind11::handle,bool,pybind11::detail::type_list<int,std::nullptr_t>)' being compiled
1>f:\lang\Python35-32\include\pybind11/stl.h(313): note: while compiling class template member function 'bool pybind11::detail::variant_caster<std::variant<int,std::nullptr_t>>::load(pybind11::handle,bool)'
1>f:\lang\python35-32\include\pybind11\cast.h(1469): note: see reference to function template instantiation 'bool pybind11::detail::variant_caster<std::variant<int,std::nullptr_t>>::load(pybind11::handle,bool)' being compiled
1>f:\lang\Python35-32\include\pybind11/stl.h(329): note: see reference to class template instantiation 'pybind11::detail::variant_caster<std::variant<int,std::nullptr_t>>' being compiled
1>C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\Tools\MSVC\14.10.25017\include\tuple(199): note: see reference to class template instantiation 'pybind11::detail::type_caster<std::variant<int,std::nullptr_t>,void>' being compiled
1>C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\Tools\MSVC\14.10.25017\include\tuple(689): note: see reference to class template instantiation 'std::_Tuple_val<_This>' being compiled
1>        with
1>        [
1>            _This=pybind11::detail::type_caster<std::variant<int,std::nullptr_t>,void>
1>        ]
1>f:\lang\python35-32\include\pybind11\cast.h(1480): note: see reference to class template instantiation 'std::tuple<pybind11::detail::type_caster<std::variant<int,std::nullptr_t>,void>>' being compiled
1>f:\lang\Python35-32\include\pybind11/pybind11.h(124): note: see reference to class template instantiation 'pybind11::detail::argument_loader<std::variant<int,std::nullptr_t>>' being compiled
1>f:\lang\Python35-32\include\pybind11/pybind11.h(63): note: see reference to function template instantiation 'void pybind11::cpp_function::initialize<_Ty,R,std::variant<int,std::nullptr_t>,pybind11::name,pybind11::scope,pybind11::sibling>(Func &&,Return (__cdecl *)(std::variant<int,std::nullptr_t>),const pybind11::name &,const pybind11::scope &,const pybind11::sibling &)' being compiled
1>        with
1>        [
1>            _Ty=pybind11_init::<lambda_60b635692d52d37640d68fd3d08bd061>,
1>            R=const char *,
1>            Func=pybind11_init::<lambda_60b635692d52d37640d68fd3d08bd061>,
1>            Return=const char *
1>        ]
1>f:\lang\Python35-32\include\pybind11/pybind11.h(744): note: see reference to function template instantiation 'pybind11::cpp_function::cpp_function<_Ty,pybind11::name,pybind11::scope,pybind11::sibling,void>(Func &&,const pybind11::name &,const pybind11::scope &,const pybind11::sibling &)' being compiled
1>        with
1>        [
1>            _Ty=pybind11_init::<lambda_60b635692d52d37640d68fd3d08bd061>,
1>            Func=pybind11_init::<lambda_60b635692d52d37640d68fd3d08bd061>
1>        ]
1>E:\code\zog\pytest2\xpytest.cpp(26): note: see reference to function template instantiation 'pybind11::module &pybind11::module::def<pybind11_init::<lambda_60b635692d52d37640d68fd3d08bd061>,>(const char *,Func &&)' being compiled
1>        with
1>        [
1>            Func=pybind11_init::<lambda_60b635692d52d37640d68fd3d08bd061>
1>        ]
1>Done building project "xpytest.vcxproj" -- FAILED.
========== Build: 0 succeeded, 1 failed, 1 up-to-date, 0 skipped ==========

The text was updated successfully, but these errors were encountered:

Fixes pybind#839.

RedSkotina · 2017-05-08T11:48:17Z

i only start learn template magic, but i fix this error by specializing template for nullptr_t via SFINAE(it is correct name for this trick?) :

    template <typename U, typename... Us>
	detail::enable_if_t<!std::is_null_pointer<U>::value, bool>
	 load_alternative(handle src, bool convert, type_list<U, Us...>) {
		auto caster = make_caster<U>();
                if (caster.load(src, convert)) {
                        value = cast_op<U>(caster);
                        return true;
               }
               return load_alternative(src, convert, type_list<Us...>{});
    }

    template <typename U, typename... Us>
	detail::enable_if_t<std::is_null_pointer<U>::value, bool>
	load_alternative(handle src, bool convert, type_list<U, Us...>) {
		if (src.is_none()) {
			value = nullptr;
			return true;
		}
		return load_alternative(src, convert, type_list<Us...>{});
	}

Test:

c++

struct visitor {
        const char *operator()(int) { return "int"; }
        const char *operator()(std::string) { return "std::string"; }
        const char *operator()(double) { return "double"; }
	const char *operator()(nullptr_t) { return "nullptr_t"; }
    };

....

    m.def("load_variant", [](std::variant<int, std::nullptr_t> v) { 
        return std::visit(visitor(), v);
    });

python

t1 = xpytest.load_variant(1)
print(t1)
t2 = xpytest.load_variant(None)
print(t2)

Test result:

int
nullptr_t

dean0x7d · 2017-05-08T11:56:26Z

Should be fixed in #840. The underlying issue is in void_caster, not specific to variant. And your new example here answers my question from the PR: the None -> std::nullptr_t should be enabled to support this. However, that would make std::variant<double*, std::nullptr_t> an ambiguous conversion from None. (Might also need to look into supporting std::monostate.)

RedSkotina · 2017-05-08T12:05:48Z

I do not understand completely, but do not the types differ? (double* and nullptr_t).
Can we use std::is_null_pointer and std::is_pointer ?

dean0x7d · 2017-05-08T12:18:17Z

They are different types in C++ at compile time, but when converting at run time from Python's None type, both double* and std::nullptr_t are equally good.

RedSkotina · 2017-05-08T13:06:01Z

Then can you give preference to std::nullptr_t because we can convert nullptr_t to double* later ?

dean0x7d · 2017-05-08T13:28:19Z

I was looking into it, but it ends up being a bit complicated because we only have two priority levels and those are already divided into char* vs. everything else. Ideally, None -> std::nullptr_t should be the highest priority overload for None, but this would break the current behavior since char* would have the same priority as all other T* overloads. @jagerman What do you think? Any way around this?

jagerman · 2017-05-08T15:26:32Z

I think I'm missing something--how does char * enter into this issue (or is that more of a hypothetical question)?

jagerman · 2017-05-08T17:25:11Z

Perhaps we could solve this by treating char * as an intrinsic type rather than throwing away the pointer? I think in practice, char * is almost used as a C-style string, not as a pointer to a char. (And, in fact, we don't actually support binding something that is a pointer to a single char--pybind always treats it as a c-style string).

I did try severing this in make_caster back in #624, but that was indeed the wrong place. It might make more sense providing intrinsic_t specializations for char *, though.

The other solution—and actually, I think this makes logical sense even with the above—is to change the various type casters to make loading None an operation requiring convert, so that instead of:

if (src.is_none()) {
    value = nullptr;
    return true;
}

we do:

if (src.is_none()) {
    if (!convert) return false;
    value = nullptr;
    return true;
}

Then the nullptr_t caster gets first-pass priority for a None. That makes a lot of sense anyway; it would mean you could explicitly overload with a py::none argument and get the expected behaviour.

dean0x7d · 2017-05-08T18:40:23Z

Yeah, that's what I had in mind for giving void_caster higher priority with regard to None (and the issue with char* which already has lower priority compared to everything else). But I now realize that this wouldn't actually solve anything with regard to variant<double*, std::nullptr_t> because overload resolution is only done for functions but not within variant_caster -- it just loads the first compatible alternative. variant_caster would need to grow a two-phase loader like the function dispatcher, but that feels like an overcomplication.

* Fix compilation error with std::nullptr_t * Enable conversion from None to std::nullptr_t and std::nullopt_t Fixes #839.

dean0x7d · 2017-05-10T09:24:18Z

I overestimated the complication to variant_caster::load(). It ended up being just 2 lines of code: #845. Together with #843, this should resolve everything here.

dean0x7d added a commit to dean0x7d/pybind11 that referenced this issue May 8, 2017

Fix compilation error with std::nullptr_t caster

0d60b5e

Fixes pybind#839.

dean0x7d mentioned this issue May 8, 2017

Fix compilation error with std::nullptr_t caster #840

Merged

dean0x7d closed this as completed in #840 May 9, 2017

dean0x7d added a commit that referenced this issue May 9, 2017

Fix std::nullptr_t caster (#840)

78f1dcf

* Fix compilation error with std::nullptr_t * Enable conversion from None to std::nullptr_t and std::nullopt_t Fixes #839.

dean0x7d mentioned this issue May 10, 2017

Improve constructor resolution in variant_caster #845

Merged

jagerman mentioned this issue May 10, 2017

Defer None loading to second pass #843

Merged

rwgk mentioned this issue Feb 9, 2023

FWD pybind11 google/pybind11clif#839

Closed

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variant_caster cant handle c++ type nullptr_t #839

variant_caster cant handle c++ type nullptr_t #839

RedSkotina commented May 8, 2017

RedSkotina commented May 8, 2017 •

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dean0x7d commented May 10, 2017

variant_caster cant handle c++ type nullptr_t #839

variant_caster cant handle c++ type nullptr_t #839

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RedSkotina commented May 8, 2017

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