Concatenating a sequence of std::arrays

You may do the following:

template <typename F, typename T, typename T2>
auto func(F f, T&& t, T2&& t2)
{
    return f(std::forward<T>(t), std::forward<T2>(t2));
}

template <typename F, typename T, typename T2, typename ... Ts>
auto func(F f, T&& t, T2&& t2, Ts&&...args)
{
    return func(f, f(std::forward<T>(t), std::forward<T2>(t2)), std::forward<Ts>(args)...);
}

With usage

struct concatenater
{
    template<typename T, std::size_t N, std::size_t M>
    auto operator()(const std::array<T, N>& ar1, const std::array<T, M>& ar2) const
    {
        std::array<T, N+M> result;
        std::copy (ar1.cbegin(), ar1.cend(), result.begin());
        std::copy (ar2.cbegin(), ar2.cend(), result.begin() + N);
        return result;
    }
};

and

auto result = func(concatenater{}, ar1, ar2, ar3, ar4);

C++14 Demo
C++11 Demo

Given a sequence of std::array<T, length1>, std::array<T, length2>, ..., std::array<T, lengthK>, how can I write a function which concatenates the sequence into an std::array<T, sum(lengths)>?

Here's a C++17 solution. It can very probably be shortened and improved, working on it.

template <std::size_t Last = 0, typename TF, typename TArray, typename... TRest>
constexpr auto with_acc_sizes(TF&& f, const TArray& array, const TRest&... rest)
{
    f(array, std::integral_constant<std::size_t, Last>{});

    if constexpr(sizeof...(TRest) != 0)
    {
        with_acc_sizes<Last + std::tuple_size_v<TArray>>(f, rest...); 
    }
}

template<typename T, std::size_t... Sizes>
constexpr auto concat(const std::array<T, Sizes>&... arrays)
{
    std::array<T, (Sizes + ...)> result{};

    with_acc_sizes([&](const auto& arr, auto offset)
    {
        std::copy(arr.begin(), arr.end(), result.begin() + offset);
    }, arrays...);

    return result;
}

Usage:

std::array<int, 3> ar1 = {1, 2, 3};
std::array<int, 2> ar2 = {4, 5};
std::array<int, 3> ar3 = {6, 7, 8};
auto result = concat(ar1, ar2, ar3);

live wandbox example

Works with both g++7 and clang++5.

Here is a simple C++17 solution via fold expressions:

#include <array>
#include <algorithm>

template <typename Type, std::size_t... sizes>
auto concatenate(const std::array<Type, sizes>&... arrays)
{
    std::array<Type, (sizes + ...)> result;
    std::size_t index{};

    ((std::copy_n(arrays.begin(), sizes, result.begin() + index), index += sizes), ...);

    return result;
}

Example of using:

const std::array<int, 3> array1 = {{1, 2, 3}};
const std::array<int, 2> array2 = {{4, 5}};
const std::array<int, 4> array3 = {{6, 7, 8, 9}};

const auto result = concatenate(array1, array2, array3);

Live demo