Multiple inheritance and singleton design pattern

Question

I have the following class hierarchy set up and would like the print() function of the non-singleton base object OtherBase to be called which in turn calls the printSymbol() from one of the child classes, in this case SingletonChild. I understand this is a complicated and somewhat unnecessary looking hierarchy and way of doing things but this is an assignment and I am required to do it in this way.

An example of my problem is as follows:

#include <iostream>
using namespace std;

class Object
{
    virtual void print() = 0;
};

class SingletonBase : public Object
{
private:
    static SingletonBase* theOnlyTrueInstance;
protected:
    SingletonBase()
    {
        if(!theOnlyTrueInstance)
            theOnlyTrueInstance = this;
    }
    virtual ~SingletonBase(){}
public:
    static SingletonBase* instance()
    {
        if (!theOnlyTrueInstance) initInstance();
        return theOnlyTrueInstance;
    }
    void print()
    { cout<<"Singleton"<<endl; }
    static void initInstance()
    { new SingletonBase; }
};

SingletonBase* SingletonBase::theOnlyTrueInstance = 0;

class OtherBase : public Object
{
public:
    virtual string printSymbol() = 0;
    void print()
    { cout<<printSymbol(); }
};

class SingletonChild : public SingletonBase , public OtherBase
{
public:
    string printSymbol()
    {
        return "Test";
    }
    static void initInstance()
    { new SingletonChild; }
};

int main() {
    SingletonChild::initInstance();
    OtherBase* test = (OtherBase*) SingletonChild::instance();
    test->print();
    return 0;
}

How can I get the instance test to call the print function of the one base class OtherBase and not the Singleton base class SingletonBase?

I have tried test->OtherBase::print(), but this did not work.

You just want your test object to call the OtherBase's print method instead of SingletonBase's? — Ahmad Khan
– Ahmad Khan, Commented Oct 3, 2016 at 13:49

Vaughn Cato · Accepted Answer · 2016-10-03 14:13:04Z

3

@MuhammadAhmad's answer is basically right. I would like to add that the main issue here is that a C-style cast is allowing you to do something that you really don't want to do. Because you can't statically cast a SingletonBase to an OtherBase, the C-style cast is doing a reinterpret_cast instead, and calling print() on the resulting pointer is undefined behavior. If you had used a static_cast, you would have gotten an error:

OtherBase* test = static_cast<OtherBase*>(SingletonChild::instance());

error: invalid static_cast from type ‘SingletonBase*’ to type ‘OtherBase*’

This may have led you to realize you needed to do things a bit differently. For example, you can use a dynamic_cast to cast sideways like this.

edited Oct 3, 2016 at 14:13

answered Oct 3, 2016 at 14:07

Vaughn Cato

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Comments

Ahmad Khan · Accepted Answer · 2016-10-03 13:59:57Z

2

SingletonChild is inheriting it's instance method from SingletonBase, which is returning a pointer to SingletonBase.
So calling SingletonChild::instance(); will get you a SingletonBase*, which you can't simply cast to OtherBase*

Try casting it to SingletonChild* first, then to OtherBase*:

OtherBase* test = (OtherBase*)((SingletonChild*)SingletonChild::instance());

And then call the print method simply like this: test->print();

See the code on ideone.

EDIT:

You can also achieve this like this:

SingletonChild* test = (SingletonChild*)SingletonChild::instance();
test->OtherBase::print();

See this method in action too.

edited Oct 3, 2016 at 13:59

answered Oct 3, 2016 at 13:54

Ahmad Khan

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Comments

alessandrolenzi · Accepted Answer · 2016-10-03 14:19:13Z

What you are trying to do is casting an object of type SingletonBase* to type OtherBase*, which is not possible because SingletonBase does not derive from OtherBase. If you had used dynamic_cast rather than old, deprecated C-style cast, you would have recognized this situation rather immediately.

To solve the issue, you need to modify the code as follows:

class Object
{
    virtual void print() = 0;
};

class SingletonBase : public Object
{
private:
    static Object* theOnlyTrueInstance;
protected:
    SingletonBase()
    {
        if(!theOnlyTrueInstance)
            theOnlyTrueInstance = this;
    }
    virtual ~SingletonBase(){}
public:
    static Object* instance()
    {
        if (!theOnlyTrueInstance) initInstance();
        return theOnlyTrueInstance;
    }
    void print()
    { cout<<"Singleton"<<endl; }
    static void initInstance()
    { new SingletonBase; }
};

Object* SingletonBase::theOnlyTrueInstance = 0;

class OtherBase : public Object
{
public:
    virtual string printSymbol() = 0;
    void print()
    { cout<<printSymbol(); }
};

class SingletonChild : public SingletonBase , public OtherBase
{
public:
    string printSymbol()
    {
        return "Test";
    }
    static void initInstance()
    { new SingletonChild; }
};

int main() {
    SingletonChild::initInstance();
    OtherBase* test = dynamic_cast<OtherBase*>(SingletonChild::instance());
    test->print();
    return 0;
}

You should avoid C-style casts, as you could end-up manipulating object like something they are not.

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