Q:

How to get value from reference in c++

I have a func which returns a const& to an Object. Let say,
class Y {
    private:
        Id id;
    public:
        Id const& GetId() const;
}  

Now this Y Object is part of another class, let say class X, which needs to be serialized by boost.
class X {
    template <class Archive>
    void serialize(Archive& ar, uint32 version)
     {
         ar & y->GetId();
     }
     Y y;
}

However, boost complains about this and gives error:
error C2027: use of undefined type 'boost::STATIC_ASSERTION_FAILURE<x>'

C:\Users\Y.h(580) : see reference to function template instantiation 'Archive &boost::archive::detail::interface_iarchive<Archive>::operator &<const Id>(T &)' being compiled
            with
            [

How can I serialize the Id?

A:

 ar & y->GetId(); 

should be
  ar & y.GetId();

A:

The purpose of boost serialization is to use references to serialize to an Archive.
Problem is it receives the values as references and not CONST references. 
Id const& GetId() const; // Returns a const reference

Try using the following
Id & GetId(); 

And then one of the following
ar & y.GetId();
ar << y.GetId();

If you don't like the breaking of encapsulation then either use protected, friends, or copy it to another value and pass the value as reference
Id copy = y.GetId();
ar & copy;

