Can't bind function with double pointer parameter

[dangirsh]

I can successfully compile and run the example here for wrapping the function int add(int i, int j). If I change the type of j to char **, I get this compiler error:

./include/pybind11/cast.h: In instantiation of ‘ReturnValue pybind11::detail::type_caster<std::tuple<_Elements ...> >::call(Func&&, pybind11::detail::index_sequence<Index ...>) [with ReturnValue = int; Func = int (*&)(int, char**); long unsigned int ...Index = {0ul, 1ul}; Tuple = {int, char**}]’:
./include/pybind11/cast.h:694:20:   required from ‘typename std::enable_if<(! std::is_void<_Res>::value), ReturnValue>::type pybind11::detail::type_caster<std::tuple<_Elements ...> >::call(Func&&) [with ReturnValue = int; Func = int (*&)(int, char**); Tuple = {int, char**}; typename std::enable_if<(! std::is_void<_Res>::value), ReturnValue>::type = int]’
./include/pybind11/pybind11.h:133:63:   required from ‘pybind11::cpp_function::initialize(Func&&, Return (*)(Args ...), const Extra& ...) [with Func = int (*&)(int, char**); Return = int; Args = {int, char**}; Extra = {pybind11::name, pybind11::sibling, pybind11::scope, char [34], pybind11::arg, pybind11::arg}]::__lambda7’
./include/pybind11/pybind11.h:117:22:   required from ‘struct pybind11::cpp_function::initialize(Func&&, Return (*)(Args ...), const Extra& ...) [with Func = int (*&)(int, char**); Return = int; Args = {int, char**}; Extra = {pybind11::name, pybind11::sibling, pybind11::scope, char [34], pybind11::arg, pybind11::arg}]::__lambda7’
./include/pybind11/pybind11.h:117:19:   required from ‘void pybind11::cpp_function::initialize(Func&&, Return (*)(Args ...), const Extra& ...) [with Func = int (*&)(int, char**); Return = int; Args = {int, char**}; Extra = {pybind11::name, pybind11::sibling, pybind11::scope, char [34], pybind11::arg, pybind11::arg}]’
./include/pybind11/pybind11.h:46:9:   required from ‘pybind11::cpp_function::cpp_function(Return (*)(Args ...), const Extra& ...) [with Return = int; Args = {int, char**}; Extra = {pybind11::name, pybind11::sibling, pybind11::scope, char [34], pybind11::arg, pybind11::arg}]’
./include/pybind11/pybind11.h:545:80:   required from ‘pybind11::module& pybind11::module::def(const char*, Func&&, const Extra& ...) [with Func = int (*)(int, char**); Extra = {char [34], pybind11::arg, pybind11::arg}]’
example.cpp:13:87:   required from here
./include/pybind11/cast.h:711:85: error: cannot convert ‘char*’ to ‘char**’ in argument passing
             .operator typename make_caster<Tuple>::template cast_op_type<Tuple>()...);
                                                                                     ^
./include/pybind11/cast.h: In member function ‘ReturnValue pybind11::detail::type_caster<std::tuple<_Elements ...> >::call(Func&&, pybind11::detail::index_sequence<Index ...>) [with ReturnValue = int; Func = int (*&)(int, char**); long unsigned int ...Index = {0ul, 1ul}; Tuple = {int, char**}]’:
./include/pybind11/cast.h:712:5: warning: control reaches end of non-void function [-Wreturn-type]
     }
./include/pybind11/cast.h: In instantiation of ‘ReturnValue pybind11::detail::type_caster<std::tuple<_Elements ...> >::call(Func&&, pybind11::detail::index_sequence<Index ...>) [with ReturnValue = int; Func = int (*&)(int, char**); long unsigned int ...Index = {0ul, 1ul}; Tuple = {int, char**}]’:
./include/pybind11/cast.h:694:20:   required from ‘typename std::enable_if<(! std::is_void<_Res>::value), ReturnValue>::type pybind11::detail::type_caster<std::tuple<_Elements ...> >::call(Func&&) [with ReturnValue = int; Func = int (*&)(int, char**); Tuple = {int, char**}; typename std::enable_if<(! std::is_void<_Res>::value), ReturnValue>::type = int]’
./include/pybind11/pybind11.h:133:63:   required from ‘pybind11::cpp_function::initialize(Func&&, Return (*)(Args ...), const Extra& ...) [with Func = int (*&)(int, char**); Return = int; Args = {int, char**}; Extra = {pybind11::name, pybind11::sibling, pybind11::scope, char [34], pybind11::arg, pybind11::arg}]::__lambda7’
./include/pybind11/pybind11.h:117:22:   required from ‘struct pybind11::cpp_function::initialize(Func&&, Return (*)(Args ...), const Extra& ...) [with Func = int (*&)(int, char**); Return = int; Args = {int, char**}; Extra = {pybind11::name, pybind11::sibling, pybind11::scope, char [34], pybind11::arg, pybind11::arg}]::__lambda7’
./include/pybind11/pybind11.h:117:19:   required from ‘void pybind11::cpp_function::initialize(Func&&, Return (*)(Args ...), const Extra& ...) [with Func = int (*&)(int, char**); Return = int; Args = {int, char**}; Extra = {pybind11::name, pybind11::sibling, pybind11::scope, char [34], pybind11::arg, pybind11::arg}]’
./include/pybind11/pybind11.h:46:9:   required from ‘pybind11::cpp_function::cpp_function(Return (*)(Args ...), const Extra& ...) [with Return = int; Args = {int, char**}; Extra = {pybind11::name, pybind11::sibling, pybind11::scope, char [34], pybind11::arg, pybind11::arg}]’
./include/pybind11/pybind11.h:545:80:   required from ‘pybind11::module& pybind11::module::def(const char*, Func&&, const Extra& ...) [with Func = int (*)(int, char**); Extra = {char [34], pybind11::arg, pybind11::arg}]’
example.cpp:13:87:   required from here
./include/pybind11/cast.h:711:85: error: cannot convert ‘char*’ to ‘char**’ in argument passing
             .operator typename make_caster<Tuple>::template cast_op_type<Tuple>()...);
                                                                                     ^
./include/pybind11/cast.h: In member function ‘ReturnValue pybind11::detail::type_caster<std::tuple<_Elements ...> >::call(Func&&, pybind11::detail::index_sequence<Index ...>) [with ReturnValue = int; Func = int (*&)(int, char**); long unsigned int ...Index = {0ul, 1ul}; Tuple = {int, char**}]’:
./include/pybind11/cast.h:712:5: warning: control reaches end of non-void function [-Wreturn-type]
     }

Are double pointers supported? If not, do you have any workarounds for calling the main function of C++ programs (which must have signature int main(int argc, char **argv))?

[wjakob]

Double pointer arguments are not supported by pybind11.

[wjakob]

(You could try to bind a lambda function which converts from a std::vectorstd::string or so)

[dangirsh]

Thanks for the fast reply! It might be worth adding a list of known shortcomings to the main Readme to save everyone time. I'll try you suggestion now :)

[Pulsar18]

@dangirsh: This is issue is already quite old, but I have the same problem and would like to ask whether you found a solution and if so how that looked like?

[dangirsh]

Sorry @Pulsar18, too long ago to recall (and I no longer have access to the relevant code).

[levinandrew]

@Pulsar18 I am having the same issue now, were you able to find a satisfactory solution?

[Foadsf]

I had a similar issue where I had to have int argc and char** argv defined inside a function (they were required by other non-open-source functions). So I ended up changing the function from

int run(int argc, char** argv){
// some stuff
}

to

int run() {

    int argc = 1;
    char* argv[] = {"NameOfTheExecutableModule"};

// same stuff
}

more info here.