Zip Class in C++ (Internal Object Lifetime)

Question

I have the following Zip class in C++, which works the same as Python's zip.

When I run the below code, I get this output:

1 | 11
2 | 22
3 | 33

1 | 11 | 0             <--- problematic
2 | 22 | 6.91092e-317  <--- problematic 
3 | 33 | 9

In the second case, where I additionally create a temporary std::vector<int>{7, 8, 9}, it appears that the lifetime of this vector ends as soon as the Zip constructor is exited. Which is why we see the wrong values 0 and 6.91092e-317.

I wonder why that is. Because internally Zip stores its elements as const Ts&. So a const reference. Should that not extend the lifetime of the temporary vector until the parent Zip object is destructed (= after the for loop)? Is there any way to fix this?

---

Sample code, https://godbolt.org/z/4n6j83fn5:

#include <tuple>
#include <vector>
#include <iostream>

template<class... Ts>
class Zip
{
public:
    explicit Zip(const Ts&... objs)
        : m_data(objs...) { }

    struct ZipIterator
    {
    public:
        explicit ZipIterator(const std::tuple<const Ts&...>& data, std::size_t idx)
            : m_data(data), m_idx(idx) { }

        ZipIterator& operator++()
        {
            ++m_idx;
            return *this;
        }

        bool operator!=(const ZipIterator& rhs) const
        {
            return m_idx != rhs.m_idx;
        }

        auto operator*() const
        {
            return std::apply([this](auto const&... obj) { return std::forward_as_tuple(obj.at(m_idx)...); }, m_data);
        }

    private:
        const std::tuple<const Ts&...>& m_data;
        std::size_t m_idx;
    };

    ZipIterator begin() const
    {
        return ZipIterator(m_data, 0);
    }

    ZipIterator end() const
    {
        return ZipIterator(m_data, std::get<0>(m_data).size());
    }

private:
    std::tuple<const Ts&...> m_data;
};


int main()
{
    const std::vector<double> vec1{1,  2,  3};
    const std::vector<double> vec2{11, 22, 33};

    for (const auto& [v1, v2] : Zip(vec1, vec2))
    {
        std::cout << v1 << " | " << v2 << std::endl;
    }
    std::cout << std::endl;

    for (const auto& [v1, v2, v3] : Zip(vec1, vec2, std::vector<double>{7, 8, 9}))
    {
        std::cout << v1 << " | " << v2 << " | " << v3 << std::endl;
    }

    return 0;
}

Shrum

Using class members that are references, or contain references, frequently ends in tears. – Domett

Answer 1

@BoP answers your question perfectly regarding the "Why". This answer provides a possible fix to your issue.

The fix involves a MaybeOwning class, which stores by reference if it is constructed from a non-temporary and stores by value if it is constructed from a temporary. To get this to work, you need to wrap the constructor in a factory function zip that properly forwards the correct types. That's why the constructor of Zip in this implementation is private.

#include <tuple>
#include <vector>
#include <iostream>

template <typename E>
struct MaybeOwning
{};

template <typename E>
struct MaybeOwning<E&&>
{
    MaybeOwning(E&& e) : _val(e) {}
    E& get() { return _val; };
    E const& get() const { return _val; };
    E _val;
};

template <typename E>
struct MaybeOwning<E&>
{
    MaybeOwning(E& e) : _val(e) {}
    E& get() { return _val; };
    E const& get() const { return _val; };
    E& _val;
};


template<class... Ts>
class Zip
{
    template <typename... Args>
    friend auto zip(Args&&...);

private:
    explicit Zip(Ts&&... objs)
        : m_data(MaybeOwning<Ts>(std::forward<Ts>(objs))...) { }

public:
    struct ZipIterator
    {
    public:
        explicit ZipIterator(const std::tuple<MaybeOwning<Ts>...>& data, std::size_t idx)
            : m_data(data), m_idx(idx) { }

        ZipIterator& operator++()
        {
            ++m_idx;
            return *this;
        }

        bool operator!=(const ZipIterator& rhs) const
        {
            return m_idx != rhs.m_idx;
        }

        auto operator*() const
        {
            return std::apply([this](auto const&... obj) { return std::forward_as_tuple(obj.get().at(m_idx)...); }, m_data);
        }

    private:
        const std::tuple<MaybeOwning<Ts>...>& m_data;
        std::size_t m_idx;
    };

    ZipIterator begin() const
    {
        return ZipIterator(m_data, 0);
    }

    ZipIterator end() const
    {
        return ZipIterator(m_data, std::get<0>(m_data).get().size());
    }

private:
    std::tuple<MaybeOwning<Ts>...> m_data;
};

template <typename... Ts>
auto zip(Ts&&... ts){
    return Zip<Ts&&...>(std::forward<Ts>(ts)...);
}


int main()
{
    const std::vector<double> vec1{1,  2,  3};
    const std::vector<double> vec2{11, 22, 33};

    for (const auto& [v1, v2] : zip(vec1, vec2))
    {
        std::cout << v1 << " | " << v2 << std::endl;
    }
    std::cout << std::endl;

    for (const auto& [v1, v2, v3] : zip(vec1, vec2, std::vector<double>{7, 8, 9}))
    {
        std::cout << v1 << " | " << v2 << " | " << v3 << std::endl;
    }

    return 0;
}

1 | 11
2 | 22
3 | 33

1 | 11 | 7
2 | 22 | 8
3 | 33 | 9

https://godbolt.org/z/Tsa8W31v7

Grace

@Grace I did indeed ask how to resolve this issue. So thank you for detailed answer. – Shrum

@Shrum oh you are right! Let me just reword the beginning of my anser than. Happy I could help – Grace

In MaybeOwning(E&& e), would it be better to use _val(std::move(e)) instead of just _val(e), or are there any bad implications? – Shrum

To my understanding std::move is equivalent to a static_cast to an rvalue reference type. So in this scenario it just wouldn't do anything. – Grace

Answer 2

explicit Zip(const Ts&... objs)
    : m_data(objs...) { }

The lifetime extension works for a direct binding only, the temporary is bound to the objs parameter.

The lifetime extension is not transferred to the m_data member.

Tyrrell

Answer 3

Here's a supplement based on @joergbrech's answer. Use class template argument deduction instead of the wrapper function.

template<class... Ts>
class Zip
{
public:
    template <typename... T>
    explicit Zip(T&&... objs)
        : m_data(MaybeOwning<T&&>(std::forward<T>(objs))...) {}
};

template <typename... Ts> Zip(Ts&&...) -> Zip<Ts&&...>;

Demo

Efficiency

Nice! Really good use for deduction guides – Grace