About a simple cast which works in pybind11 but not in nanobind

[shakfu]

I'm wrapping a cpp api using pybind11 primarily but also using nanobind after the pybind11 demonstrably works for comparison. In one of the cases which wraps a method, I have a simple cast which works without issues in pybind11 and I am at a loss to make work in nanobind.

This is the working pybind11 version:

    m.def("create_interpreter_dsp_factory_from_file", [](const std::string& filename, py::args& args) -> interpreter_dsp_factory* {
        std::vector<std::string> params;
        std::string error_msg;
        std::vector<const char *> argv;
        for (const auto &arg : args) {
            params.push_back(arg.cast<std::string>());
        }
        argv.reserve(params.size());
        for (auto &i : params)
            argv.push_back(const_cast<char *>(i.c_str()));
        interpreter_dsp_factory* factory = (interpreter_dsp_factory*)createInterpreterDSPFactoryFromFile(filename, argv.size(), argv.data(), error_msg);
        if (!factory) {
            std::cerr << "Cannot create factory : " << error_msg;
            return NULL;
        }
        return factory;
    }, py::arg("filename"), "Create a Faust DSP factory from a DSP source code as a file.", py::return_value_policy::reference);

and in nanobind:

m.def("create_interpreter_dsp_factory_from_file", [](const std::string& filename, nb::args& args) -> interpreter_dsp_factory* {
        std::vector<std::string> params;
        std::string error_msg;
        std::vector<const char *> argv;
        for (const auto &arg : args) {
            params.push_back(arg.cast<std::string>()); // <- THIS DOESN'T WORK
        }
        argv.reserve(params.size());
        for (auto &i : params)
            argv.push_back(const_cast<char *>(i.c_str()));
        interpreter_dsp_factory* factory = (interpreter_dsp_factory*)createInterpreterDSPFactoryFromFile(filename, argv.size(), argv.data(), error_msg);
        if (!factory) {
            std::cerr << "Cannot create factory : " << error_msg;
            return NULL;
        }
        return factory;
    }, nb::arg("filename"), "Create a Faust DSP factory from a DSP source code as a file.", nb::rv_policy::reference);

Does this require a custom caster as per the documentation or is there a simpler solution?

[wjakob]

What does 'this doesn't work' mean? Does it not compile? Does it raise an exception? Are you including the STL string type caster header?

[shakfu]

Apologies. It doesn't compile and I have already included #include <nanobind/stl/string.h> as a header.

Here is the error:

$HOME/nanofaust/nanofaust.cpp:64:34: error: no member named 'cast' in 'nanobind::handle'
            params.push_back(arg.cast<std::string>()); // <- this doesn't
                             ~~~ ^
$HOME/nanofaust/nanofaust.cpp:64:50: error: expected '(' for function-style cast or type construction
            params.push_back(arg.cast<std::string>()); // <- this doesn't
                                      ~~~~~~~~~~~^
$HOME/nanofaust/nanofaust.cpp:64:52: error: expected expression
            params.push_back(arg.cast<std::string>()); // <- this doesn't
                                                   ^
In file included from $HOME/nanofaust/nanofaust.cpp:1:
In file included from /opt/homebrew/lib/python3.11/site-packages/nanobind/include/nanobind/nanobind.h:51:
/opt/homebrew/lib/python3.11/site-packages/nanobind/include/nanobind/nb_func.h:67:5: error: static assertion failed due to requirement 'nargs_provided == 0 || nargs_provided + is_method_det == nargs || is_getter_det': The number of nb::arg annotations must match the argument count!
    static_assert(nargs_provided == 0 || nargs_provided + is_method_det == nargs || is_getter_det,
    ^             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/opt/homebrew/lib/python3.11/site-packages/nanobind/include/nanobind/nb_func.h:207:13: note: in instantiation of function template specialization 'nanobind::detail::func_create<false, true, (lambda at $HOME/nanofaust/nanofaust.cpp:58:55), interpreter_dsp_factory *, const std::string &, nanobind::args &, 0UL, 1UL, nanobind::scope, nanobind::name, nanobind::arg, char[61], nanobind::rv_policy>' requested here
    detail::func_create<false, true>(
            ^
/opt/homebrew/lib/python3.11/site-packages/nanobind/include/nanobind/nb_func.h:256:5: note: in instantiation of function template specialization 'nanobind::cpp_function_def<(lambda at $HOME/nanofaust/nanofaust.cpp:58:55), nanobind::scope, nanobind::name, nanobind::arg, char[61], nanobind::rv_policy, 0>' requested here
    cpp_function_def((detail::forward_t<Func>) f, scope(*this),
    ^
$HOME/nanofaust/nanofaust.cpp:58:7: note: in instantiation of function template specialization 'nanobind::module_::def<(lambda at $HOME/nanofaust/nanofaust.cpp:58:55), nanobind::arg, char[61], nanobind::rv_policy>' requested here
    m.def("create_interpreter_dsp_factory_from_file", [](const std::string& filename, nb::args& args) -> interpreter_dsp_factory* {
      ^
/opt/homebrew/lib/python3.11/site-packages/nanobind/include/nanobind/nb_func.h:67:82: note: expression evaluates to 'false || false'
    static_assert(nargs_provided == 0 || nargs_provided + is_method_det == nargs || is_getter_det,
                  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~
4 errors generated.
make[3]: *** [projects/nanofaust/CMakeFiles/nanofaust.dir/nanofaust.cpp.o] Error 1
make[2]: *** [projects/nanofaust/CMakeFiles/nanofaust.dir/all] Error 2

[wjakob]

Try nb::cast<std::string>(..)

[shakfu]

Thanks, I tried params.push_back(nb::cast<std::string>(arg)); but still got one error:

In file included from $HOME/nanofaust/nanofaust.cpp:1:
In file included from /opt/homebrew/lib/python3.11/site-packages/nanobind/include/nanobind/nanobind.h:51:
/opt/homebrew/lib/python3.11/site-packages/nanobind/include/nanobind/nb_func.h:67:5: error: static assertion failed due to requirement 'nargs_provided == 0 || nargs_provided + is_method_det == nargs || is_getter_det': The number of nb::arg annotations must match the argument count!
    static_assert(nargs_provided == 0 || nargs_provided + is_method_det == nargs || is_getter_det,
    ^             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/opt/homebrew/lib/python3.11/site-packages/nanobind/include/nanobind/nb_func.h:207:13: note: in instantiation of function template specialization 'nanobind::detail::func_create<false, true, (lambda at $HOME/nanofaust/nanofaust.cpp:58:55), interpreter_dsp_factory *, const std::string &, nanobind::args &, 0UL, 1UL, nanobind::scope, nanobind::name, nanobind::arg, char[61], nanobind::rv_policy>' requested here
    detail::func_create<false, true>(
            ^
/opt/homebrew/lib/python3.11/site-packages/nanobind/include/nanobind/nb_func.h:256:5: note: in instantiation of function template specialization 'nanobind::cpp_function_def<(lambda at $HOME/nanofaust/nanofaust.cpp:58:55), nanobind::scope, nanobind::name, nanobind::arg, char[61], nanobind::rv_policy, 0>' requested here
    cpp_function_def((detail::forward_t<Func>) f, scope(*this),
    ^
$HOME/nanofaust/nanofaust.cpp:58:7: note: in instantiation of function template specialization 'nanobind::module_::def<(lambda at $HOME/nanofaust/nanofaust.cpp:58:55), nanobind::arg, char[61], nanobind::rv_policy>' requested here
    m.def("create_interpreter_dsp_factory_from_file", [](const std::string& filename, nb::args& args) -> interpreter_dsp_factory* {
      ^
/opt/homebrew/lib/python3.11/site-packages/nanobind/include/nanobind/nb_func.h:67:82: note: expression evaluates to 'false || false'
    static_assert(nargs_provided == 0 || nargs_provided + is_method_det == nargs || is_getter_det,
                  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~
1 error generated.

[shakfu]

I think the remaining error above in unrelated to the solution. I'm closing this with thanks for your help!

[shakfu]

For of all, thanks for the help! I found something interesting here which may be a bug:

After your advised change, the following compiles without issues:

    m.def("create_interpreter_dsp_factory_from_file", [](const std::string& filename, nb::args& args) -> interpreter_dsp_factory* {
        std::vector<std::string> params;
        std::string error_msg;
        std::vector<const char *> argv;
        for (const auto &arg : args) {
            params.push_back(nb::cast<std::string>(arg));
        }
        argv.reserve(params.size());
        for (auto &i : params)
            argv.push_back(const_cast<char *>(i.c_str()));
        interpreter_dsp_factory* factory = (interpreter_dsp_factory*)createInterpreterDSPFactoryFromFile(filename, argv.size(), argv.data(), error_msg);
        if (!factory) {
            std::cerr << "Cannot create factory : " << error_msg;
            return NULL;
        }
        return factory;
    }, "Create a Faust DSP factory from a DSP source code as a file.", nb::rv_policy::reference);

However, if I add the argument annotation nb::arg("filename"), I get the "static assertion" error produced above. So this version doesn't compile:

    m.def("create_interpreter_dsp_factory_from_file", [](const std::string& filename, nb::args& args) -> interpreter_dsp_factory* {
        std::vector<std::string> params;
        std::string error_msg;
        std::vector<const char *> argv;
        for (const auto &arg : args) {
            params.push_back(nb::cast<std::string>(arg));
        }
        argv.reserve(params.size());
        for (auto &i : params)
            argv.push_back(const_cast<char *>(i.c_str()));
        interpreter_dsp_factory* factory = (interpreter_dsp_factory*)createInterpreterDSPFactoryFromFile(filename, argv.size(), argv.data(), error_msg);
        if (!factory) {
            std::cerr << "Cannot create factory : " << error_msg;
            return NULL;
        }
        return factory;
    }, nb::arg("filename"), "Create a Faust DSP factory from a DSP source code as a file.", nb::rv_policy::reference);

[shakfu]

Should I close this discussion and file a bug report? (if it is, indeed, a bug)