如何改善实例化此对象的方式？

Question

Consider this "design" (implementation details left out):

struct service {};

struct sub {
  sub(service&) {}
  service& s_;
};

struct sub_a: sub {
  sub_a(service&) {}
};

struct sub_b: sub {
  sub_b(service&) {}
};

struct top {
  service svc_;
  sub_ptr sub_;
};

Basically, my top object holds sub_ptr , but the instance held by this pointer requires access to svc_ in order to be instantiated, which is also in top . However, I want a user of top to be able to choose the type of sub (here sub_a or sub_b ), without giving access to svc_ .

My solution so far is to have my constructor of top like this:

template <typename SubType>
top(std::unique_ptr<SubType>): sub_(new SubType(svc_)) {}

Here is the usage:

auto t = top(std::unique_ptr<sub_b>());

But I don't feel like this solution is elegant. I could write functions (eg. make_top_sub_b() ) to hide the details, but I was wondering how I could avoid giving a dummy pointer to my constructor? Making top a templated class is not an option.

Any idea how I could improve this? Thanks,

Answer 1

The Liskov substitution principle would hear suggest that users of sub_ptr would not need to be aware of which implementation it held.

As sub_ptr is wrapped within top , you can say that top is a user.

The construction of the sub needs to be done via its factory and the service it references must have a lifetime that is longer than that of the class that holds the reference.

In your case the lifetime of the service object appears to be in top which means the sub must have a shorter lifetime.

You could have a sub_factory class that is abstract and creates one from a service.

eg

struct sub_factory
{
   virtual sub* create( service_ & ) const = 0;
};


struct sub_a_factory : sub_factory 
{ 
       // detail, creates sub_a
};

struct sub_b_factory : sub_factory
{
     // detail, creates sub_b
};

top::top( sub_factory const & fact ) :
      sub_( fact.create( svc_ ) )
{
}

sub_factory_a afact;
sub_Factory_b bfact;
top ta(  afact );
top tb( bfact );

Note that you can use the initialiser list here as svc_ appears above sub_ in your class so gets initialised first.