CODEWITHSUNDEEP
c++dependency-injection
Mathew Joy
Mathew Joy

Reputation: 121

C++ How to do dependency injection on a derived class?

I have a base class B derived from an abstract class A. The idea of having abstract class A is to facilitate Dependency Injection and Mocking while unit testing. So A have no implementation. Almost like C# interface. All works fine.

Now I have a derived class D that is inherited from B. The problem now is D does not have an abstract class to facilitate DI or Mocking. Here is a little code that explains the problem:

class A // Abstract
{
public:
  virtual String Identify() = 0;
};

class B : public A
{
public:
  String Identify() {return "B"; }
};

class D : public B
{
public:
  String D_Work() {return "D_Work"; }
};

So to use B, I do A *b = new B(); which is fine. The using class does not know the type B. new() is done by a factory or the object is just passed in.

But to use D, I either need the actual type (which I am trying to get away) or use type A and cast to call methods like A *d = new D(); ((D*)d)->D_Work()) (using c-style for simplicity) which again will require the user to know the type. Any thoughts on the design?

Upvotes: 1

Views: 883

Answers (2)

Gem Taylor
Gem Taylor

Reputation: 5613

The other approach is that you have multiple interfaces I, J ,K, and pure methods in A like GetI() that return pointers to that interface.

Then All your implementation classes implement A, plus any other interfaces plus any of the GetX() methods.

Your interface is A,I,J,K, and that is all that is exposed. That is all that needs to be mocked.

Ultimately, the continuous updating of A to add new classes gets tiresome, so you invest in a different mechanism for getting the interfaces, probably based on a token declared in the interface class, and a template method in A that resolves the token and return type by calling an implementation method that does the same. Or you just give up and let the caller dynamic_cast to the interfaces.

Sometime soon you end up with COM, or something very like it.

class A;
{
  virtual String Identify() = 0;
  virtual I* GetI() { return nullptr;}
  virtual J* GetJ() { return nullptr;}
  virtual K* GetK() { return nullptr;}
};
class I
{
   virtual void Work() =0;
};
class J
{
   virtual void MoreWork() =0;
};

class B: A, I
{
  void Work() {}
  I* GetI() { return this;}
};
class C: A, J
{
  void MoreWork() {}
  I* GetJ() { return this;}
};

Upvotes: 0

scohe001
scohe001

Reputation: 15446

You can add another abstract class in between B and D:

class B : public A {
public:
  string Identify() {return "B"; }
};

class C : public B {
public:
    virtual string D_Work() = 0;    
};

class D : public C {
public:
  string D_Work() {return "D_Work"; }
};

See it work here: ideone

It looks like I may have misunderstood what you want. It seems you want to be able to continue using only an A pointer. In that case, I'd add a virtual D_Work() function to A that throws:

class A {
public:
  virtual string Identify() = 0;
  virtual string D_Work() { throw; } // If it's not defined, throw
};

See it work here: ideone

Upvotes: 1

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