CODEWITHSUNDEEP
c++templatesc++11
m.s.
m.s.

Reputation: 16334

convert variadic iterator arguments to tuple of value types

How can I convert a template parameter pack of IteratorTypes to a tuple of the respective value_types? I tried the following, but it failed with

error: wrong number of template arguments (2, should be 1)
using ValueTypes = typename std::iterator_traits<IteratorTypes...>::value_type;
                                                                              ^

#include <type_traits>
#include <vector>
#include <tuple>

template <typename ... IteratorTypes>
void foo(IteratorTypes&&...its)
{
    using ValueTypes = typename std::iterator_traits<IteratorTypes...>::value_type;
    static_assert(std::is_same<std::tuple<ValueTypes>,std::tuple<int,float>>::value, "types do no match");
}

int main() {
    std::vector<int> int_vec(1);
    std::vector<float> float_vec(1);

    foo(int_vec.begin(), float_vec.begin());
    return 0;
}

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Upvotes: 1

Views: 192

Answers (2)

T.C.
T.C.

Reputation: 137345

The ... goes immediately after the pattern you want to expand.

using ValueTypes = std::tuple<typename std::iterator_traits<IteratorTypes>::value_type...>;
static_assert(std::is_same<ValueTypes, std::tuple<int,float>>::value, "types do no match");

Also, you should probably std::decay IteratorTypes before passing it to iterator_traits, or take them by value rather than by forwarding reference.

Upvotes: 4

Richard Hodges
Richard Hodges

Reputation: 69892

...or to make it easier to read:

#include <type_traits>
#include <vector>
#include <tuple>
#include <utility>

template <typename ... IteratorTypes>
void foo(IteratorTypes&&...its)
{
    using value_type_tuple = std::tuple<typename IteratorTypes::value_type...>;
    using desired_tuple = std::tuple<int, float>;
    static_assert(std::is_same<value_type_tuple , desired_tuple>::value, "types do no match");
}

int main() {
    std::vector<int> int_vec(1);
    std::vector<float> float_vec(1);

    foo(int_vec.begin(), float_vec.begin());
    return 0;
}

Upvotes: 0

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