error use of deleted function when trying to pass rvalue to a tuple

Question

Original context: I am trying to pass a tuple of (object, expected_value_of_some_property) to a test function

I created a simple class to reproduce the error I am facing:

template <typename T> class Vector { private: size_t m_size; std::unique_ptr<std::vector<int>> m_vector; public: Vector(); Vector(const Vector<int>&); ~Vector() = default; void push_back(T); T get(int) const; size_t size() const; }; template <typename T> Vector<T>::Vector() :m_size(0), m_vector(new std::vector<T>) {} template <typename T> Vector<T>::Vector(const Vector<int>& v) :m_size(v.size()), m_vector(new std::vector<T>) { for (size_t i = 0; i < v.size(); i++) { this->push_back(v.get(i)); } } template <typename T> void Vector<T>::push_back(T value) { m_vector->push_back(value); m_size ++; } template <typename T> T Vector<T>::get(int index) const { return m_vector->operator[](index); } template <typename T> size_t Vector<T>::size() const { return m_size; }

The error is triggered when trying to produce a tuple of Vector object and an int somewhere in a test code:

int main(int argc, char const *argv[]) { std::tuple<Vector<int>, int> vector_test; vector_test = std::make_tuple(Vector<int>{}, 0); int size = std::get<0>(vector_test); Vector<int> vector = std::get<1>(vector_test); // .. test code return 0; }

Output:

error: use of deleted function ‘std::tuple<Vector<int>, int>& std::tuple<Vector<int>, int>::operator=(const std::tuple<Vector<int>, int>&)’ 20 | vector_test = std::make_tuple(Vector<int>{}, 0); | ^ ^

What am I doing wrong here?

not the cause of the error, but it looks wrong that only Vector<int> can be copied to all other Vector<T>s, for example you have no copy constructor for Vector<double> but a conversion from Vector<int> to Vector<double>. I suppose you want Vector(const Vector&); rather than Vector(const Vector<int>&); — 463035818_is_not_an_ai
– 463035818_is_not_an_ai, Commented Nov 11, 2022 at 13:12
I haven't paid attention to this. I created this class only to reproduce the error I am getting. — Zaki
– Zaki, Commented Nov 11, 2022 at 13:27

Cory Kramer · Accepted Answer · 2022-11-11 13:08:47Z

Since your Vector class has a unique_ptr member, that means your class itself has the copy-assignment implicitly deleted. Therefore this assignment will fail

std::tuple<Vector<int>, int> vector_test; vector_test = std::make_tuple(Vector<int>{}, 0); // <--- this

Instead you can just directly initialize in one step

std::tuple<Vector<int>, int> vector_test = std::make_tuple(Vector<int>{}, 0);

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