Interpreting static_cast "static_cast<void (Pet::*)(int)>" syntax?

Question

I'm trying to understand the static cast as used in the Pybind11 docs here. Specifically, they use the syntax

static_cast<void (Pet::*)(int)>(&Pet::set)

Since I haven't seen this syntax before I'm strugling to interpret and apply to my own code so I was hoping somebody could explain what's going on here. Thanks

Edit - some context

I am creating Pybind11 bindings to an overloaded method that has two signatures, which only differ by const qualification. The class I am binding is a template so I am using this strategy to create the bindings

    template<class T>
    class Matrix {
    public:

        ...

        /**
         * get the row names
         */
        std::vector<std::string> &getRowNames() {
            return rowNames;
        }

        /**
         * get the row names (mutable)
         */
        const std::vector<std::string> &getRowNames() {
            return rowNames;
        }

    ...

My version of the helper function described in that post is this:

template<typename T>
void declare_matrix(py::module &m, const std::string &typestr) {
    using Class = ls::Matrix<T>;
    const std::string &pyclass_name = typestr;
    py::class_<Class>(m, pyclass_name.c_str(), py::buffer_protocol(), py::dynamic_attr())
            .def(py::init<unsigned int, unsigned int>())
            .def("getRowNames", static_cast<const std::vector<std::string>(ls::Matrix<T>::*)()>(&ls::Matrix<T>::getRowNames))

but the getRowNames line produces the following error:

Address of overloaded function 'getRowNames' cannot be static_cast to type 'const std::vector<std::string> (ls::Matrix<complex<double>>::*)()'

For anybody else reading this, the cast I was able to figure out thanks to the answer is:

static_cast< std::vector<std::string>& (ls::Matrix<T>::*)()>(&Class::getRowNames)

Answer 1

The meaning of:

static_cast<void (Pet::*)(int)>(&Pet::set)

• static_cast<T_1>(T_2) means we're casting type 2 to type 1.
• T_1:
  • (Pet::*) is a pointer to a class member of Pet (see https://stackoverflow.com/a/9939367/14344821 for a further discussion)
  • void (Pet::*)(int) is a pointer to a member function that takes an int parameter, returning a void
• T_2
  • &Pet::set is the memory location of Pet::set

So basically, we're stating explicitly that we are setting the integer value.

Now we can bind the set functions to python (allowing us to set both age and name):

   .def("set", static_cast<void (Pet::*)(int)>(&Pet::set), "Set the pet's age")
   .def("set", static_cast<void (Pet::*)(const std::string &)>(&Pet::set), "Set the pet's name");