Re: [Boost-users] Get fields from struct

Subject: Re: [Boost-users] Get fields from struct
From: Vicente Botet (vicente.botet_at_[hidden])
Date: 2012-01-13 10:44:14

Matthijs Möhlmann wrote
> 
> Hello all,
> 
> I'm currently busy to get things like this working:
> 
> struct person {
>   std::string name;
>   int age;
> };
> 
> int main(int argc, char **argv) {
> 
> 
>   person p;
>   p.name = "Matthijs"
>   p.age = 1234;
> 
>   std::string key = "name";
> 
>   // Now try to fetch the value based on the key string.
>   std::cout << p.{key} << std::endl; // does not compile
> 
>   return 0;
> }
> 
> Of course above doesn't work for obvious reasons.
> 
> 	std::cout << "Name: " << boost::fusion::at_key<keys::name>(x) <<
> std::endl;
> 	std::cout << "Age:  " << boost::fusion::at_key<keys::age>(x) <<
> std::endl;
> 
> Now my question, is there maybe another way to achieve this?
> 

I guess that what you want is a some kind of run-time reflection. 

int main(int argc, char **argv) {
  person p;
  p.name = "Matthijs"
  p.age = 1234;
  std::string key = "name";
  std::cout << field_at_key(p, key) << std::endl; 
  return 0;
}

The problem is that we don't know which type person_field_at_key should
return. We could type erase the result type and get it lazily when the value
is required.

The following maps to a field reference
 template <typename T>
 field_ref<T> field_at_key(T const& p, std::string const& key) 
 {
   return field_ref<T>(p,key);
 }

We could overload an apply function specific for field_ref<person>

template <typename Visitor>
void apply(Visitor& v, field_ref<person> const&p)
{
   if (key=="name") v(p.get_person().name);
   else if (key=="age")  v(p.get_person().age);
}

The preceding code could be generated by a macro, as it is done STRUCT_ADAPT

We could the define a OSTREAM visitor as follows

  template <typename OSTREAM>
  struct ostream_visitor {
    OSTREAM& os_;
    otream_visitor(OSTREAM& os) : _os(os)
    template <typename T>
    operator(T const& v) { os_ << v; }
  };

  template <typename OSTREAM, typename T>
  OSTREAM& operator<<(OSTREAM& os, field_ref<T> const& p) 
  {
    ostream_visitor v(std::cout);
    apply(v,p);
    return *this;
  }

There are surely some boost related reflection libraries that I don't know
that it could be worth looking for as e.g. Boost.Mirror.

I don't know if you are looking for this behavior.

HTH,
Vicente

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