Vector of queues of different types

Steve Brown 0
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I have a type, let's call it T. It can be of different types, and I do not know them in advance: int, double, bitmap pointer... Now, I have a class, let's call it C. It should maintain a vector of queues of T. Each queue has its unique type of elements, but it is possible that C has a vector (or whatever other way of maintaining multiple queues) with few queues of different types:

queue, queue, ...

So my question is: is there a way to pass variable number types to a class? Example: I need to get <int, double, double, int> and in a constructor, to create corresponding queues, plus I will need a function to get/put data to a particular queue (say, queue No 3 in a list).

I know about variadic templates, but can not figure how to use it: it looks like I have to do it recursively, and I am not that advanced...

I can do it using "any", but then get/put will have "any" as well, and I do not know how to do a type check, so the user doesn't send a double instead of string, or something like that.

Any ideas? Thanks.

templates c++
2 Answers

It's actually not that complicated. You don't even need recursion. Just std::tuple

#include <tuple>
#include <queue>

template <class... Args>
struct C
{
    std::tuple<std::queue<Args>...> queues_;

    template <std::size_t I>
    auto& queue_at()
    {
        return std::get<I>(queues_);
    }
};

auto test()
{
    C<int, double, bool> c;

    std::queue<int>& q1 = c.queue_at<0>();
    std::queue<double>& q2 = c.queue_at<1>();
}
bolov0

You can use variadic template with recursive inheritance:

#include <queue>
#include <string>

template <typename First, typename... Rest>
struct Base : Base<First>,  Base<Rest...> {
};

template <typename First>
struct Base<First> {
    std::queue<First> q;
};


int main (){
    auto f = Base<int, std::string>{};

    static_cast<Base<int>>(f).q.emplace(32);
    static_cast<Base<std::string>>(f).q.emplace("sdf");
   
    // not allowed
    // static_cast<Base<float>>(f).q.emplace(1231.2);
}
slsy0
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