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;
}
@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