Using parameter packs to automate dynamic_cast checks

Question

I'm trying to implement a communication system inside of a GUI. I would like to avoid the visitor pattern for maintainability reasons. Likewise making a dynamic_cast if else statement is not maintainable. The closest I've come is implementing multiple dispatch with tables from Scott Meyers' More Effective C++.

So far I have:

SubmitCommand(BaseCommand* pCommand)
{
    m_Distpatcher->Dispatch<DerivedCommand1>(pCommand);
    m_Distpatcher->Dispatch<DerivedCommand2>(pCommand);
    m_Distpatcher->Dispatch<DerivedCommand3>(pCommand);
}

Where I would like to be is:

SubmitCommand(BaseCommand* pCommand)
{
    m_Distpatcher->Dispatch<DerivedCommand1,
                            DerivedCommand2,
                            DerivedCommand3>(pCommand);
}

Where dispatch is a automated way of checking dynamic_cast results for incoming commands.

    template<typename K>
    void Dispatch(ICommand* pCommand)
    {
        auto pConcreteCommand = dynamic_cast<K*>(pCommand);
        if (pConcreteCommand)
        {
            //call Recieve on the interface of the owner class
            m_pInstance->Recieve(pConcreteCommand);
        }
    }

In this case the specific module would be checked at compile time to make sure it has a function for each argument in the template. Is code block 2 possible?

Could you share the definition of `Dispatch`? – cigien May 20 '20 at 12:48 — cigien, May 20 '20 at 12:48
Also, what is `DerivedCommand1`? – Aykhan Hagverdili May 20 '20 at 12:49 — Aykhan Hagverdili, May 20 '20 at 12:49

score 0 · Accepted Answer · answered May 20 '20 at 12:59

0

You might do something like:

template <typename ... Ts>
void Distpatcher::Dispatch(BaseCommand* pCommand)
{
    (DispatchUnique<Ts>(pCommand), ...); // C++17
}

so

m_Distpatcher->Dispatch<DerivedCommand1,
                        DerivedCommand2,
                        DerivedCommand3>(pCommand);

would be equivalent to

m_Distpatcher->DispatchUnique<DerivedCommand1>(pCommand);
m_Distpatcher->DispatchUnique<DerivedCommand2>(pCommand);
m_Distpatcher->DispatchUnique<DerivedCommand3>(pCommand);

answered May 20 '20 at 12:59

Jarod42

203,559
14
181
302

Worked perfectly. What is the operation associated with the outside round bracket operation in calling the instances of UniqueDispatch? Thanks! Accepted Answer! – dHubley May 20 '20 at 13:08
folding expression with comma operator. – Jarod42 May 20 '20 at 13:10

Using parameter packs to automate dynamic_cast checks

1 Answers1