template multiple variadic inheritance with variadic argument types

问题

I need to inherit multiple times the following class, taking variadic arguments as template parameters.

template <class SignalDispatcherClass, class ... ArgTypes>
class ISignalMap
{
//private
public:

    void RegisterSlot(SignalAddress pSignalFunc, ISlotInvoker<ArgTypes...>* pSlotInvoker)
    {
        //implementation
    }
};

So far I can expand a parameter pack and get multiple class specializations, but with functions taking only one argument.

template <class SignalDispatcherClass, class ... ArgTypes>
class ISignalStorage : public ISignalMap<SignalDispatcherClass, ArgTypes>...
{
};

///////
ISignalStorage<SignalA, int, double, bool> iss; 

For now this allows me to register slot functions with a single argument (int, double or bool - accordingly). What I need is something that would look like:

ISignalStorage<SignalA, <int, double, bool>, <int, int>, <const char*>> iss;

So far I've been looking into other questions and one appears to be somewhat close to the topic, though I failed to implement or understand it. Wish there were a simplier way (Variadic variadic template templates)

added: code example

struct IDummySlot
{
    void FuncDbl(double)
    {}
    void FuncInt(int)
    {}
    void FuncIntDbl(int, double)
    {}
};

template <class ... Args>
struct ISlotInvoker
{};

template <class SignalDispatcherClass, class ... ArgTypes>
class ISignalMap
{
public:

    void RegisterSlot(void(IDummySlot::*pSignalFunc)(ArgTypes...), ISlotInvoker<ArgTypes...>* pSlotInvoker)
    {
        return;
    }
};

template <class SignalDispatcherClass, class ... ArgTypes>
class ISignalStorage : public ISignalMap<SignalDispatcherClass, ArgTypes>...
{
};


int main()
{
    ISignalStorage<IDummySlot, int, double> iss;

    ISlotInvoker<int> slot_int;
    ISlotInvoker<double> slot_double;
    ISlotInvoker<int, double> slot_intDouble;

    //iss.RegisterSlot(&IDummySlot::FuncInt, &slot_int); //ambigous
    /*Appears to be that I didn't test it, I just saw that inheritance worked as I expected, but didn't try to invoke*/

    return 0;
}

回答1:

The fundamental problem here is that there is no syntax for "squeezing" multiple variadic parameter packs out of a single parameter pack.

The usual approach in these kinds of situations is to use a std::tuple to wrap each individual parameter pack, and make a parameter pack out of those tuples:

    ISignalStorage<foo, std::tuple<int, double>, std::tuple<double, int>> a;

Then, it becomes a simple matter of unwrapping each parameter pack from the std::tuple using a specialization:

#include <tuple>


template <class SignalDispatcherClass, class ... ArgTypes>
class ISignalMap
{
};

// Take a class, and a tuple. Give me an ISignalMap for the class, and
// what's in the tuple.

template<typename cl, typename tuple_t> struct tuple_expansion;

template<typename cl, typename ...tuple_types>
struct tuple_expansion<cl, std::tuple<tuple_types...>> {

    typedef ISignalMap<cl, tuple_types...> type;
};

// Syntactic sugar.    
template<typename cl, typename tuple_t>
using tuple_expansion_t=typename tuple_expansion<cl, tuple_t>::type;

// And a variadic parameter pack of tuples...

template <class SignalDispatcherClass, class ... ArgTypes>
class ISignalStorage : public tuple_expansion_t<SignalDispatcherClass,
                        ArgTypes>...
{
};

class foo;

void bar()
{

    // Note the syntax: pass each "inner" parameter pack wrapped into a
    // tuple.
    ISignalStorage<foo, std::tuple<int, double>, std::tuple<double, int>> a;

    ISignalMap<foo, int, double> &b=a;
    ISignalMap<foo, double, int> &c=a;
}

回答2:

For now this allows me to register slot functions with a single argument (int, double or bool - accordingly). What I need is something that would look like:

ISignalStorage<SignalA, <int, double, bool>, <int, int>, <const char*>> iss;

As better explained by Sam Varshavchik, this type of needs is usually managed wrapping packs of types as template arguments for another class. Usually is used std::tuple, that offer some handy tools to manage pack of types, but you can also define a trivial template class/struct as follows (TW for "type wrapper")

template <typename...>
struct TW
 { };

I propose a slightly different solution that is wrapper neutral (so you can use with classic std::tuple, with a custom TW or also mixing they)

template <typename, typename>
struct ISignalStorageHelper;

template <typename T, template <typename...> class C, typename ... Ts>
struct ISignalStorageHelper<T, C<Ts...>> : public ISignalMap<T, Ts...>
 { };

template <typename SignalDispatcherClass, typename ... Tuples>
class ISignalStorage
   : public ISignalStorageHelper<SignalDispatcherClass, Tuples>...
 { };

This way we can avoid the using to extract the type inside the intermediate struct but ISignalStorage inherit also from ISignalStorageHelper structs (I hope isn't a problem).

You can define ISignalStorage objects using tuples

ISignalStorage<foo, std::tuple<int, double>, std::tuple<double, int>> a;

custom pack wrapper as TW

ISignalStorage<foo, TW<int, double>, TW<double, int>> a;

or also mixing

ISignalStorage<foo, std::tuple<int, double>, TW<double, int>> a;

来源：https://stackoverflow.com/questions/53434669/template-multiple-variadic-inheritance-with-variadic-argument-types