Best pattern for functions that apply to a preexisting class hierarchy

Question

I have an existing class hierarchy, that I can not modify

ie a phylogenetic tree:

base
├── animal
│   ├── invertebrate
│   └── vertebrate
│       ├── amphibian
│       ├── bird
│       ├── fish
│       ├── mammal
│       └── reptile
└── plant

I need to build a library of functions for objects in that hierarchy, ie print functions:

print_mammal_A()
print_mammal_B()
print_fish_A()
print_vertebrate_A()
print_vertebrate_B()
print_animal_A()

• These functions will be developed as they're needed.

• There may be more than one function for each class.

The obvious solution is to make a class hierarchy of wrappers that maps the target hierarchy. Each wrapper implementing it's own function, ie:

class base_wrapper(){
  base * base_ptr;
}

class animal_wrapper : base_wrapper{
  void print_animal_A();
}

class vertebrate_wrapper : animal_wrapper {
  void print_vertebrate_A();
  void print_vertebrate_B();
}

I'd like to know of design patterns that allow either of the following items:

• Removing wrapper inheritance (so the library developer doesn't need to know of the target class bases)

• Automatically wrap with the most specialised wrapper for the target class (so the library user doesn't need to know of the target class bases)

I'm interested in either, C# or C++ solutions. Not sure if there are patterns that can be implemented easily in one and not the other.

Answer 1

I am assuming you cannot modify the library it self so you cannot add methods to each class?

If I am understanding this correctly I think you could use extension methods. In C# it would be something like this.

public static class FishExtension
{
    public static void Print(this Fish fish)
    {
     // Now you can work with the fish object.
    }
}   

The user of fish will then be able to do:

Fish fish = new Fish();
fish.Print();

This way there is no additional hierarchy (no wrappers) The extension class in never used directly, the user only has to import its namespace.

See the Extension Methods (C# Programming Guide) https://msdn.microsoft.com/en-us//library/bb383977.aspx