C++17 can variadic template class define new type and variadic data member?

Question

#include <iostream>

template <typename T0, typename T1>
struct Clazz {
    void doSomething(const std::shared_ptr<T0>& p0,
            const std::shared_ptr<T1>& p1) {}
    T0 t0;
    T1 t1;
};
template <typename T0, typename T1, typename T2>
struct Clazz3 {
    void doSomething(const std::shared_ptr<T0>& p0,
            const std::shared_ptr<T1>& p1,
            const std::shared_ptr<T1>& p2) {}
    T0 t0;
    T1 t1;
    T2 t1;
};

template <typename... Ts>
struct ClazzN {
    // Q1: one part: define types in doSomething
    // Q2: two part: define data members
    // using param_type = typename (std::shared_ptr<Ts>, ...) // fold expression? can it works on type part, or only value?

    // Ts... ts; // ??? pack it by tuple ???
};

int main() {
    Clazz<int, double> c2;
    Clazz3<int, double, char> c3;

    return 0;
}

I have a class ClazN, and I:

want a variadic template form, instead of limiting the class template type size.
want define different data members as the template type correspodingly.
the T1, T2 ... Tn have no relationship.

How to write ClazzN? It's too stupid to write a lot repeated codes.

How could I do this? Need your help, thanks a lot!!!

"pack it by tuple ???" why not? Did you try to have a `std::tuple` member? — 463035818_is_not_an_ai, Jan 17 '22 at 08:52
It's all right for me, only if it works. You can ommit my comments. I comment because I don not know how to do it. — xishvai, Jan 17 '22 at 09:05
"it works" is not a useful description for an objective. Have you tried it? Does it fulfils your expectations? — YSC, Jan 17 '22 at 09:07
when you have code that has an error, then you better show that code and the error message. I posted an answer, but I don't know if this really adresses the issue in your acutal code — 463035818_is_not_an_ai, Jan 17 '22 at 09:10

463035818_is_not_an_ai · Answer 1 · 2022-01-17T09:19:20.887

1

You can use std::tuple, also for the param_type. In case you want a tuple of shared pointers, that is:

#include <tuple>
#include <iostream>
#include <memory>

template <typename... Ts>
struct Clazz {
    using param_type = typename std::tuple<std::shared_ptr<Ts>...>; 
    std::tuple<Ts...> ts;
};

int main() {
    Clazz<int, double> c0;
}

edited Jan 17 '22 at 09:19

answered Jan 17 '22 at 09:10

463035818_is_not_an_ai

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Great! But how can I use the paramers seperatedly in function `doSomething` as you shows: void doSomething(param_type params) { // use every params at any will } – xishvai Jan 17 '22 at 09:20
@xishvai whats the problem with `void doSomething(param_type params)` ? – 463035818_is_not_an_ai Jan 17 '22 at 09:26
It compiles ok(includes the doSomething part). But I don not know how to get each parameter's type and value? How can i make it? – xishvai Jan 17 '22 at 09:29
@xishvai if you want to know how to deal with `std::tuple` you can open another question – 463035818_is_not_an_ai Jan 17 '22 at 09:30
I think your answer solved my problem. Thanks a lot !!! ^_^ – xishvai Jan 17 '22 at 11:08

score 0 · Answer 2 · answered Jan 17 '22 at 09:30

You could use std::tuple_element to access the list of types:

#include <cstdint>
#include <tuple>
#include <string>

template <typename... Ts>
struct Clazz {
    template<std::size_t I>
    using T = std::tuple_element_t<I, std::tuple<Ts...>>;
};


using Test = Clazz<int, int&, double, std::string>;
static_assert(std::is_same_v<Test::T<0>, int>, "");
static_assert(std::is_same_v<Test::T<1>, int&>, "");
static_assert(std::is_same_v<Test::T<2>, double>, "");
static_assert(std::is_same_v<Test::T<3>, std::string>, "");

https://gcc.godbolt.org/z/dvadxrbEM

C++17 can variadic template class define new type and variadic data member?

2 Answers2