How to achieve dynamic polymorphism (run-time call dispatch) on unrelated types?

Question

GOAL:

I would like to achieve type-safe dynamic polymorphism (i.e. run-time dispatch of a function call) on unrelated types

Answer 1

I once solved this by simulating .NET delegates:

template
class Delegate
{
    //static_assert(false, "T must be a function type");
};

template
class Delegate
{
private:
    class HelperBase
    {
    public:
        HelperBase()
        {
        }

        virtual ~HelperBase()
        {
        }

        virtual ReturnType operator()() const = 0;
        virtual bool operator==(const HelperBase& hb) const = 0;
        virtual HelperBase* Clone() const = 0;
    };

    template
    class Helper : public HelperBase
    {
    private:
        Class* m_pObject;
        ReturnType(Class::*m_pMethod)();

    public:
        Helper(Class* pObject, ReturnType(Class::*pMethod)()) : m_pObject(pObject), m_pMethod(pMethod)
        {
        }

        virtual ~Helper()
        {
        }

        virtual ReturnType operator()() const
        {
            return (m_pObject->*m_pMethod)();
        }

        virtual bool operator==(const HelperBase& hb) const
        {
            const Helper& h = static_cast(hb);
            return m_pObject == h.m_pObject && m_pMethod == h.m_pMethod;
        }

        virtual HelperBase* Clone() const
        {
            return new Helper(*this);
        }
    };

    HelperBase* m_pHelperBase;

public:
    template
    Delegate(Class* pObject, ReturnType(Class::*pMethod)())
    {
        m_pHelperBase = new Helper(pObject, pMethod);
    }

    Delegate(const Delegate& d)
    {
        m_pHelperBase = d.m_pHelperBase->Clone();
    }

    Delegate(Delegate&& d)
    {
        m_pHelperBase = d.m_pHelperBase;
        d.m_pHelperBase = nullptr;
    }

    ~Delegate()
    {
        delete m_pHelperBase;
    }

    Delegate& operator=(const Delegate& d)
    {
        if (this != &d)
        {
            delete m_pHelperBase;
            m_pHelperBase = d.m_pHelperBase->Clone();
        }

        return *this;
    }

    Delegate& operator=(Delegate&& d)
    {
        if (this != &d)
        {
            delete m_pHelperBase;
            m_pHelperBase = d.m_pHelperBase;
            d.m_pHelperBase = nullptr;
        }

        return *this;
    }

    ReturnType operator()() const
    {
        (*m_pHelperBase)();
    }

    bool operator==(const Delegate& d) const
    {
        return *m_pHelperBase == *d.m_pHelperBase;
    }

    bool operator!=(const Delegate& d) const
    {
        return !(*this == d);
    }
};

You can use it much like .NET delegates:

class A
{
public:
    void M() { ... }
};

class B
{
public:
    void M() { ... }
};

A a;
B b;

Delegate d = Delegate(&a, &A::M);
d(); // calls A::M

d = Delegate(&b, &B::M);
d(); // calls B::M

This works with methods that have no arguments. If you can use C++11, you can modify it to use variadic templates to handle any number of parameters. Without C++11, you need to add more Delegate specializations to handle specific numbers of parameters:

template
class Delegate
{
    ...
};

template
class Delegate
{
    ...
};

With this Delegate class you can also emulate .NET events, which are based on delegates.