C++ Solving Diamond Inheritance Without Virtual Inheritance

Question

I have the following diamond class structure that does not compile:

class Base{
  int a;
public:
  virtual void doSomething();
};

class NotMineToTouch : public Base {};

class MyParentClass : public Base {};

class EvilDiamond : public NotMineToTouch, public MyParentClass {};

// I need these methods (that I cannot necessarily edit) to work for an EvilDiamond
void processBase (Base* b) { b->doSomething; /*...*/} // Cannot edit

void processParent (MyParentClass* p) { p->doSomething; /*...*/} // Can edit

void processNotMine (NotMineToTouch* n) { n->doSomething; /*...*/} // Cannot edit

I know the normal solution is to inherit virtually from Base; however, I am not allowed to change NotMineToTouch (or Base). Is there another solution? I am allowed to change MyParentClass and EvilDiamond at my pleasure; however, EvilDiamond must inherit from MyParentClass and NotMineToTouch, and MyParentClass must inherit from Base and may not inherit from EvilDiamond.

I only get a warning not an error. Can you give us the actual compiler error you are getting or post code that reproduces the problem. BTW it isn't a diamond if you don't use virtual inheritance. Virtual inheritance is what merges the two bases into one creating the diamond shape if you draw it in a diagram. — Eelke
– Eelke, Commented Jul 6, 2017 at 6:13
We call the virtual method getA() in other places on Bases and MyParentClass (in code I am not always allowed to change). It may not be diamond inheritance, but it is certainly the diamond problem (see the description on the tag). Compiler error when I call getA() on an EvilDiamond: error: request for member 'getA' is ambiguous note: candidates are: virtual int Base::getA() note: virtual int Base::getA() — Hirsch Alter
– Hirsch Alter, Commented Jul 6, 2017 at 6:28
Can you not cast to the right type when you pass the object? — Passer By
– Passer By, Commented Jul 6, 2017 at 7:38
Passer By - I cannot cast an object to something it is not (hence why I need to multiple inherit). I see no reason to cast - if my object subclasses a base class, I can pass it where a pointer of that class is called for. This is not an overload issue. — Hirsch Alter
– Hirsch Alter, Commented Jul 6, 2017 at 8:12
The following code would give that error void processEvilDiamond (EvilDiamond* n) { n->doSomething(); } the solution is to specify which one you want: void processEvilDiamond (EvilDiamond* n) { n->MyParentClass::doSomething(); } — Eelke
– Eelke, Commented Jul 6, 2017 at 12:39

score 4 · Accepted Answer · 2017-07-06 06:51:48Z

4

I challenge the following assertion:

EvilDiamond must inherit from MyParentClass and NotMineToTouch

You can probably do something along these lines (depending on your architecture):

class EvilDiamond;

class NotMineToTouchImpl : public NotMineToTouch {
  EvilDiamond* tgt_;
public:
  NotMineToTouchImpl(EvilDiamond* tgt) : tgt_(tgt) {}

  ... implement NotMineToTouch here, using tgt_ where you would have used this
};

class MyParentClassImpl : public MyParentClass {
  EvilDiamond* tgt_;
public:
  MyParentClassImpl(EvilDiamond* tgt) : tgt_(tgt) {}

  ... implement Base here, using tgt_ where you would have used this
};

class EvilDiamond {
  friend class NotMineToTouchImpl;
  friend class MyParentClassImpl;

  // Creating permanent instances of the API classes 
  // may or may not be appropriate in your case.
  NotMineToTouchImpl nmti_;
  MyParentClassImpl pci_;
public:
  EvilDiamond () : nmti_(this), pci_(this) {}

  NotMineToTouchImpl* getAsNotMineToTOuch() {return &nmti_;}
  MyParentClassImpl * getAsParentClass() {return &pci_;}
};

edited Jul 6, 2017 at 6:51

answered Jul 6, 2017 at 6:13

user4442671

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1 Comment

Hirsch Alter Over a year ago

Edited the original post. I have other methods that accept a Base where I need to pass an EvilDiamond.

Jarod42 · Accepted Answer · 2017-07-06 11:38:03Z

2

You don't have diamond as you don't use virtual inheritance.
you have some "Y" inheritance currently (EvilDiamond has 2 Base).

Without changing your classes, you may add overloads to instruct compiler what to do:

void processBase (EvilDiamond* evil) {
    processBase(static_cast<NotMineToTouch*>(evil)); // Use NotMineToTouch::Base
    processBase(static_cast<MyParentClass*>(evil));  // Use MyParentClass::Base
}

answered Jul 6, 2017 at 11:38

Jarod42

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