Can't initialize std::vector

Question

Why is the code below require ProcessTask to have copy constructor?

#include <utility>
#include <vector>

template<typename T>
struct ProcessTask
{
    using Handle = int*;

    ProcessTask() : m_h(nullptr) {}

    ProcessTask(Handle handle) noexcept : m_h(handle) {}

    ProcessTask(ProcessTask&& other) noexcept : m_h(std::exchange(other.m_h, nullptr)) {}

    ProcessTask& operator=(ProcessTask&& other) noexcept
    {
        free();

        m_h = std::exchange(other.m_h, nullptr);

        return *this;
    }

    ~ProcessTask()
    {
        free();
    }

    void free()
    {
        delete m_h;
    }

    Handle m_h;
};

ProcessTask<int> loadData()
{
    return {};
}

int main()
{
    using Task = ProcessTask<int>;

    std::vector<Task> order_tasks
    {
        loadData(),
        loadData()
    };

    return 0;
}

What is the difference between the initializer list and

order_tasks.push_back(loadData());

that does not require copy constructor?

EDIT1

The same effect with std::unique_ptr:

std::vector<std::unique_ptr<int>>
{
    std::make_unique<int>(5)
};

You are asking about the code that creates a vector from the tasks in an initializer list, yes? — Drew Dormann, Aug 11 '22 at 20:37
`std::initializer_list` makes a copy of elements passed to it. https://en.cppreference.com/w/cpp/utility/initializer_list — Yksisarvinen, Aug 11 '22 at 20:38
The tasks are _copied_ from the initializer list you created into the vector you created. That requires a copy constructor. — Drew Dormann, Aug 11 '22 at 20:38
When you have an initializer list like `{ foo, bar, baz }` the compiler create a `std::initializer_list` from that and the storage it wraps for those initializers is a temporary `const T[N]` array. Since that array elements are const they cannot be moved from, as moving is a modifying operation. — NathanOliver, Aug 11 '22 at 20:41
If you are asking why the objects are not _moved_ but instead _copied_, the duplicate should answer your question. If that's not you question, you may need to [edit] your question to add some clarity about what you are asking. — Drew Dormann, Aug 11 '22 at 20:41

Can't initialize std::vector

0 Answers0