带有类成员的pybind11缓冲协议

Question

I'm trying to use pybind11 to bind a struct that looks like this

struct myStruct {
 int na;
 int nb;
 double* a;
 double* b;
}

I'm not sure the right way to go about it. The examples in the pybind11 documentation show how to attach buffer protocol semantics to an object, but not to a class member.

I don't have the luxury of changing the interface of myStruct to contain std::vector s either which would allow me to use the usual .def_readwrite() .

I've tried doing something like this

py::class_<myStruct>(m, "myStruct")
    .def_property("a",
        [](myStruct &s) {return py::array<double>({s.na}, {sizeof(double), s.a};)},
        [](myStruct &s, py::array_t<double> val) {std::copy((double*) val.request().ptr, (double*) val.request().ptr + s.na, s.a);)}
    )

Which compiles, but in python I don't see changes persist in the underlying data

print(my_struct.a[0]) # prints 0.0
my_struct.a[0] = 123.0
print(my_struct.a[0]) # still prints 0.0

Answer 1

Hey most likely not the most elegant answer, but maybe it gives you a starting point and temporary solution. I think what you need to do is use shared pointers.

Under https://github.com/pybind/pybind11/issues/1150 someone asked something similar but I was not able to adapt it to your example and only got the same result to yours with no changes to the data.

What worked for me in your specific example was using the shared_ptr and defining setter and getter functions for the pointers with a simple def_property for the pybin11 class.

class class_DATA{
    public: 
        int na;
        std::shared_ptr<double> a;  

        void set_a(double a){*class_DATA::a = a; };
        double get_a(void){return *class_DATA::a; };
};

PYBIND11_MODULE(TEST,m){
  m.doc() = "pybind11 example plugin";
 //the costum class
 py::class_<class_DATA>(m, "class_DATA", py::dynamic_attr())
  .def(py::init<>())    //needed to define constructor
  .def_readwrite("na", &class_DATA::na)
  .def_property("a", &class_DATA::get_a, &class_DATA::set_a, py::return_value_policy::copy);
}