Add support for describe getters && setters

[denzor200]

I suggest adding support for getters and setters to the library. I see this new functionality like this:

#include <boost/describe.hpp>
#include <iostream>

struct employee
{
    void                  set_name    (std::string name)         ;
    void                  set_age     (int age)                  ;
    const std::string&    get_name    ()                   const ;
    int                   get_age     ()                   const ;
};

BOOST_DESCRIBE_STRUCT(
    employee, (), (
        ("name", get_name, set_name),
        ("age", get_age,  set_age)))

int main()
{
    using boost::describe::describe_members;
    using boost::describe::mod_any_access;

    employee emp;
    emp.set_name("John");
    emp.set_age(20);

    boost::mp11::mp_for_each<describe_members<decltype(emp), mod_any_access>>([&](auto D){
        auto getter = D.getter; // D also have "setter" field, but a "pointer" field is unavailable
        std::cout << D.name << " = " << (emp.*getter)() << std::endl;
    }); 

    // Will print:
    //    name = John
    //    age = 20

    return 0;
}

[denzor200]

I found a better way to do this:

struct employee
{
    void                  set_name    (std::string name)         ;
    void                  set_age     (int age)                  ;
    const std::string&    get_name    ()                   const ;
    int                   get_age     ()                   const ;
    struct employee_data& data        ()                         ;
};

BOOST_DESCRIBE_STRUCT(employee, (), (
    (set_name, (setter, "name")),
    (set_age,  (setter, "age")),
    (get_name, (getter, "name")),
    (get_age,  (getter, "age")),
    (data),    (accessor, "data")
))

int main()
{
    using boost::describe::describe_members;
    using boost::describe::mod_getters;
    using boost::describe::mod_accessors;
    using boost::describe::is_setter;
    using boost::describe::get_encapsulated_field_name;

    employee emp;
    emp.set_name("John");
    emp.set_age(20);
    emp.data() = employee_data();

    boost::mp11::mp_for_each<describe_members<decltype(emp), mod_getters | mod_accessors>>([&](auto D){
        const auto pointer = D.pointer;
        std::cout << get_encapsulated_field_name(D) << " = " << (emp.*pointer)() << std::endl;
    }); 

    // Will print:
    //    name = John
    //    age = 20
    //    data = employee_data

    return 0;
}

Also, its compatible with https://github.com/boostorg/describe/issues/13. We can start by making https://github.com/boostorg/describe/issues/13 and then separately declare the setter, getter and accessor as service attributes.

[klemens-morgenstern]

How's that different from a function on the language level?

[denzor200]

How's that different from a function on the language level?

I'm not getting your question. Please reword or submit a little better example.

[klemens-morgenstern]

Getters and setter don't exist in C++, this isn't C# or python. You're trying to describe functions, which you already can do with boost.describe.

[denzor200]

Getters and setter don't exist in C++, this isn't C# or python. You're trying to describe functions, which you already can do with boost.describe.

I'm trying to suggest alternative for BOOST_HANA_ADAPT_ADT. I haven't thought about C# or Python getters and setters.

It's useful for https://github.com/boostorg/describe/pull/32