Applying CTAD with multiple template parameter packs

Question

Following some of the solutions provided here How can I have multiple parameter packs in a variadic template?, I'm hoping to apply multiple parameter packs to a classes and use CTAD to make the class more usable.

Here is what I came up with (on Coliru), but this gives:

error: class template argument deduction failed

Here is the code I tried:

// A template to hold a parameter pack
template < typename... >
struct Typelist {};

// Declaration of a template
template< typename TypeListOne 
        , typename TypeListTwo
        > 
struct Foo;     

// A template to hold a parameter pack
template <typename... Args1, typename... Args2>
struct Foo< Typelist < Args1... >
                 , Typelist < Args2... >
                 >{
    template <typename... Args>
    struct Bar1{
        Bar1(Args... myArgs) {
            _t = std::make_tuple(myArgs...);
        }
        std::tuple<Args...> _t;
    };

    template <typename... Args>
    struct Bar2{
        Bar2(Args... myArgs) {
            _t = std::make_tuple(myArgs...);
        }
        std::tuple<Args...> _t;
    };

    Bar1<Args1...> _b1;
    Bar2<Args2...> _b2;

    Foo(Bar1<Args1...>& b1, Bar2<Args2...>& b2) {
        _b1 = b1;
        _b2 = b2;
    }
};

int main()
{

    Foo{Foo::Bar1(1, 2.0, 3), Foo::Bar2(100, 23.4, 45)};
    return 0;
}

IMHO there's no way for the compiler, even with deduction guide, to understand `Foo::Bar1(1, 2.0, 3)`. — YSC, Mar 18 '20 at 09:19

score 6 · Accepted Answer · answered Mar 18 '20 at 09:32

First, CTAD is not done when using the scope resolution operator, so Foo:: can never be used. You must specify the template arguments for this Foo explicitly.

I suggest you simply move Bar1 and Bar2 outside the Foo partial specialization into the namespace scope and use

Foo{Bar1(1, 2.0, 3), Bar2(100, 23.4, 45)};

instead.

Then you will get an error that deduction for Foo fails. This is because only constructors in the primary template are considered for implicit deduction guides. But the constructor that you want to use is in the partial specialization, not the primary template.

So you will need to add an appropriate deduction guide yourself, e.g.:

template <typename... Args1, typename... Args2>
Foo(Bar1<Args1...>, Bar2<Args2...>) -> Foo<Typelist<Args1...>, Typelist<Args2...>>;

Then you will get an error that no constructor is viable. This is because you are taking Bar1<Args1...>& b1 and Bar2<Args2...>& b2 as non-const lvalue references, but you provide prvalues for them. Non-const lvalue references cannot bind to rvalues and so you get an error. Either take the parameters by-value or by const lvalue reference.

Finally you will get an error that _b1 and _b2 do not have default constructors, which is true. They are needed because you are default-initializing _b1 and _b2 in your constructor. You only later assign them values.

So either add default constructors to Bar1 and Bar2 or better use initialization instead of assignment:

Foo(const Bar1<Args1...>& b1, const Bar2<Args2...>& b2) : _b1(b1), _b2(b2) { }

After all these steps the code should compile. I am not sure what exactly your goal is, so not entirely sure whether this will do what you want.

Thanks here is a working implementation incorporating your answer: http://coliru.stacked-crooked.com/a/25f02a58966e185b — tangy, Mar 18 '20 at 10:32

Applying CTAD with multiple template parameter packs

1 Answers1