class Object with Dependency Injection

[Salih Dincer via Digitalmars-d-learn]

Hi, below is an example of DI-dependency injection with 3 
versions nested in the code. If you remove the leading // 
characters, you will not get the "no property `deliver` for 
`service` of type `object.Object`" error.  Because version-2I 
with interface wants its methods to depend on Object..

```d
//abstract class /* toggle-code
interface //* ^---version 2A */
ITransport
{
  string deliver();
}
class Ship : ITransport
{
  override string deliver()
  {
    return "Ship Deliver";
  }
}

class Truck : ITransport
{
  override string deliver()
  {
    return "Truck Deliver";
  }
}

abstract class Logistics
{
  ITransport createTransport();

  auto operations()
  {
    return createTransport.deliver();
  }
}

class RoadLogistics : Logistics
{
  override ITransport createTransport()
  {
    return new Truck();
  }
}

class SeaLogistics : Logistics
{
  override ITransport createTransport()
  {
    return new Ship();
  }
}

import std.stdio;
void main()
{
  // DI version 1:
  auto sl = new SeaLogistics;
  auto rl = new RoadLogistics;

  auto logistics = [ sl, rl ];
  foreach(deliver; logistics)
  {
    auto str = deliver.operations();
    str.length.writeln(": ", str);
  }

  import std.range : repeat;
  "÷ ".repeat(9).writefln!"%-(%s%)";

  // A->I version 2:
  auto truck = new Truck;
  auto ship = new Ship;

  auto services = [ truck, ship ];
  foreach(service; services)
  {
    auto str = service.deliver();
    str.length.writeln(": ", str);
  }
}
```
Maybe using an abstract class instead of interface or not using 
auto will solve the problem, but I can't accept the situation!  I 
wonder what makes the interface special?

SDB@79