Create C-style array form parameter pack

Question

How can I create an array from the parameter pack?

    template<typename T, typename... Tpack>
    void covert(Tpack ...pack){
        T *arr = new T[???]; //TODO: how to get Tpack size?
        // TODO: how to fill array?
    }

How to fill the array with what? You made an array of `T`, but `pack` could have various different types. — interjay, Jun 10 '21 at 13:36
Is `Tpack` supposed to be a pack of types or should it really be `T... Tpack` in the template parameter list? — G.M., Jun 10 '21 at 13:37
This sounds like an XY problem: what are you actually trying to achieve? Maybe there's a better way to solve it... — JHBonarius, Jun 10 '21 at 13:38
You an use `sizeof...(pack)` to get the number of elements in the pack, but I would forget about the array entirely and just use `std::vector arr{pack...};`. — François Andrieux, Jun 10 '21 at 13:40
thanks for the comments, I'm trying to figure out the parameter pack. This is a simple practice example. — sergej.p, Jun 10 '21 at 13:45

score 2 · Accepted Answer · answered Jun 10 '21 at 13:39

2

You might do:

template<typename T, typename... Tpack>
void covert(Tpack ...pack){
    T *arr = new T[sizeof...(Tpack)]{pack...};
    // ...
    delete[] arr;
}

Demo

but your function is strange as-is. std::tuple might be more appropriate, or change input parameter to std::initializer_list<T> or std::array<T, N>.

answered Jun 10 '21 at 13:39

Jarod42

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Mr. quick answer man.... it's not even clear what the OP wants to achieve... I mean, why not just do something like `int t[] = { 1, 2, 3, 4, 5 };` – JHBonarius Jun 10 '21 at 13:43
I want to call the function like this: convert (1, 2, 3, 4, 5) – sergej.p Jun 10 '21 at 13:50
1

@JHBonarius: We ask for MCVE, and OP provides it and question is answerable. That reduction might make the example with poor meaning, I agree. And so now we can speculate about what OP really wants. – Jarod42 Jun 10 '21 at 13:51

Create C-style array form parameter pack

1 Answers1