What is the equivalent of BOOST_FUSION_ADAPT_ASSOC in Boost.Hana?

Question

In the good old days we used to adapt a struct into a Boost.Fusion container or an associative container with

#include <boost/fusion/adapted/struct/adapt_struct.hpp>
#include <boost/fusion/adapted/struct/adapt_assoc_struct.hpp>

struct Person{
    std::string name;
    int age;
};

BOOST_FUSION_ADAPT_STRUCT(
    Person,
    (std::string, name)
    (int, age)
);

struct tag_name;
struct tag_age;

BOOST_FUSION_ADAPT_ASSOC_STRUCT(
    Person,
    (std::string, name, tag_name)
    (int, age, tag_age));

int main(){}

Now with Boost.Hana we can also adapt

#include <boost/hana/adapt_struct.hpp>
BOOST_HANA_ADAPT_STRUCT(Person, 
    name,
    age
);

The question: Is there a sort of BOOST_HANA_ADAPT_ASSOC_STRUCT (equivalent to BOOST_FUSION_ADAPT_ASSOC_STRUCT) in Boost.Hana? Or is this done differently now?

Bonus question: Is there a BOOST_HANA_ADAPT_TPL?

Jason Rice · Accepted Answer · 2018-01-23T18:36:56.093

According to the documentation of Boost.Fusion the BOOST_FUSION_ADAPT_ASSOC_STRUCT macro generates boiler plate for modeling Random Access Sequence and Associative Sequence.

With Boost.Hana these additional macros are not needed as you get additional functionality for free without any boiler plate.

For "Associative Sequence", Boost.Hana has the Searchable concept that provides the at_key function.

While Boost.Hana does not have iterators for "Random Access Sequence", it does have the functions at and at_c in Boost.Hana's Iterable concept. Unfortunately Boost.Hana's Struct does not implement these although I believe it could. Perhaps that could be added.

As for BOOST_HANA_ADAPT_TPL, Boost.Hana has no macro for facilitating template structs, but users can still implement hana::accessors to make a Struct without any macro.

You can search boost::hana::Structs members by their name with a compile-time string:

#define BOOST_HANA_CONFIG_ENABLE_STRING_UDL
#include <boost/hana.hpp>
#include <string>

namespace hana = boost::hana;
using namespace hana::literals;

struct Person {
  std::string name;
  int age;
};

BOOST_HANA_ADAPT_STRUCT(
  Person,
  name,
  age
);

int main()
{
  Person person{"Joe", 42};

  // hana::Searchable search by key
  hana::at_key(person, "name"_s);
  hana::at_key(person, "age"_s);
}

What is the equivalent of BOOST_FUSION_ADAPT_ASSOC in Boost.Hana?

1 Answers1