Expand parameter pack into tuple with tuple_cat

Question

Godbolt link: https://godbolt.org/z/18nseEn4G

I have a std::map of various types of vectors (cast to void*) and a T& get<T> method that gives me a reference to an element in one of the vectors in the map.

class Container {
public:
    Container() {
        auto v1 = new std::vector<int>({1, 2, 3, 4, 5});
        auto v2 = new std::vector<char>({'a','b','c','d','e'});
        auto v3 = new std::vector<double>({1.12, 2.34, 3.134, 4.51, 5.101});

        items.insert({
            std::type_index(typeid(std::vector<int>)),
            reinterpret_cast<void*>(v1)
        });
        items.insert({
            std::type_index(typeid(std::vector<char>)),
            reinterpret_cast<void*>(v2)
        });
        items.insert({
            std::type_index(typeid(std::vector<double>)),
            reinterpret_cast<void*>(v3)
        });
    }

    template<typename T>
    T& get(int index) {
        auto idx = std::type_index(typeid(std::vector<T>));
        auto ptr = items.at(idx);
        auto vec = reinterpret_cast<std::vector<T>*>(ptr);
        return (*vec)[index];
    }

private:
    std::map<std::type_index, void*> items {};
};

I want to be able to use structured binding to get back references to 3 elements all at the same index but in difference vectors, but I'm not sure how to create a tuple with multiple calls to the T& get<T> method. Something like this;

auto [a, b, c] = myContainer.get_all<int, char, double>(1); // get a reference to an int, a char, and a double from myContainer at index 1.

I'm currently trying to make use of repeated calls to T& get<T> for each parameter in a parameter pack, but I can't figure out the correct syntax.

template<typename... Ts>
auto get_all(int index) {
    return std::tuple_cat<Ts...>(
        std::make_tuple<Ts>(get<Ts>(index)...)
    );

How could I make this work? Here is a link to my current attempt: https://godbolt.org/z/18nseEn4G

Alternatively, is there a "better way" to achieve this?

Using `void*` and `typeid` are usually bad signs. Especially that there is no RAII. This could potentially be implemented in a much safer way using type erasure. — janekb04, Apr 08 '21 at 16:12
Sure, I could use std::any over void* but I’m just doing a quick and dirty example on godbolt to use as a base for a more concrete implementation later down the road. Same for the reason I’m not deleting any of these allocations or chucking them in smart pointers, I just wanted a quick and dirty baseline. — Acorn, Apr 08 '21 at 17:02

janekb04 · Accepted Answer · 2021-04-08T18:31:46.617

I would suggest using type erasure. Here is an example:

#include <vector>
#include <typeindex>
#include <memory>
#include <any>
#include <unordered_map>
#include <iostream>
#include <experimental/propagate_const>

// If no library implementation is availble, one may be copied from libstdc++
template<class T>
using propagate_const = std::experimental::propagate_const<T>;

class Container
{
public:
    Container() {
        std::unique_ptr<Eraser> v1{ static_cast<Eraser*>(new ErasedVector<int>(1, 2, 3, 4, 5)) };
        std::unique_ptr<Eraser> v2{ static_cast<Eraser*>(new ErasedVector<char>('a','b','c','d','e')) };
        std::unique_ptr<Eraser> v3{ static_cast<Eraser*>(new ErasedVector<double>(1.12, 2.34, 3.134, 4.51, 5.101)) };

        items[std::type_index(typeid(int))] = std::move(v1);
        items[std::type_index(typeid(char))] = std::move(v2);
        items[std::type_index(typeid(double))] = std::move(v3);
    }

    template<typename... Ts>
    std::tuple<Ts&...> get(size_t index)
    {
        return {
            std::any_cast<std::reference_wrapper<Ts>>((*items.find(std::type_index{typeid(Ts)})->second)[index]).get()...
        };
    }
    template<typename... Ts, typename = std::enable_if_t<(std::is_const_v<Ts> && ...)>>
    std::tuple<Ts&...> get(size_t index) const
    {
        return {
            std::any_cast<std::reference_wrapper<Ts>>((*items.find(std::type_index{typeid(Ts)})->second)[index]).get()...
        };
    }
private:
    class Eraser
    {
    public:
        virtual std::any operator[](size_t index) = 0;
        virtual std::any operator[](size_t index) const = 0;
        virtual ~Eraser() = default;
    };
    template <typename T>
    class ErasedVector : public Eraser
    {
    public:
        template <typename... Args>
        ErasedVector(Args&&... args) :
            data{ std::forward<Args>(args)... }
        {
        }

        virtual std::any operator[](size_t index) override final
        {
            return std::reference_wrapper{ data[index] };
        };
        virtual std::any operator[](size_t index) const override final
        {
            return std::reference_wrapper{ data[index] };
        }
    private:
        std::vector<T> data;
    };

    std::unordered_map<std::type_index, propagate_const<std::unique_ptr<Eraser>>> items;
};

It works properly on this example:

int main()
{
    Container co;
    auto [i0_0, c0_0, d0_0] = co.get<int, char, double>(0);
    std::cout << i0_0 << ' ' << c0_0 << ' ' << d0_0 << '\n';
    i0_0 = 3; // is a reference
    d0_0 = 42; // is a reference
    auto [i0_1, d0_1] = static_cast<const Container&>(co).get<const int, const double>(0); // works on const Container
    std::cout << i0_1 << ' ' << d0_1; // original values modified
    // i0_1 = 0xDEADBEEF; can be const too
}

And outputs:

1 a 1.12
3 42

Demo

score 1 · Answer 2 · answered Apr 08 '21 at 16:09

1

Simply:

template<typename... Ts>
auto get_all(int index) {
    return std::tuple<Ts&...>(get<Ts>(index)...);
}

Demo

answered Apr 08 '21 at 16:09

Jarod42

203,559
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score 1 · Answer 3 · answered Apr 08 '21 at 16:09

1

You can use std::tie to take all of the "returns" from get<Ts>(index) can pack them into a tuple of references. That would look like

template<typename... Ts>
auto get_all(int index) {
    return std::tie(get<Ts>(index)...);
}

answered Apr 08 '21 at 16:09

NathanOliver

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Expand parameter pack into tuple with tuple_cat

3 Answers3