std::vector of structs which require a constructor

Question

The problem only appears when adding the ~MyClass() function. I think the error is undefined behaviour or has something to do with the const MyClass& maybe this kind reference doesn't work with emplace_back. Comments explain the rest:

#include <vector>
#include <iostream>
#include <memory>

struct MyClass
{
    const int x;

    MyClass(int _x) : x(_x)
    {
    }
};

struct MyClass2
{
    std::unique_ptr<MyClass> my;

    MyClass2(const MyClass& my2)
    {
        my.reset(new MyClass(my2.x));
    }
    
    // when adding this function
    // error: static assertion failed: result type must be constructible from input type
    ~MyClass2()
    {
        printf("deleting\n");
    }
};

int main()
{
    MyClass main(12);

    std::vector<MyClass2> ts;

    for (int i = 0; i < 10; ++i)
    {
        ts.emplace_back(main);
    }

    printf("%i\n", ts[0].my->x);

    return 0;
}

Haven't tested it but surely it should be `printf("%i\n", *(ts[0].x));` You need to dereference the smart pointer to get the integer. — john, May 18 '23 at 13:55
`*x.get() = y;` can be replaced with `*x = y;`, no need for `get()`. — john, May 18 '23 at 13:58
There's a close button above. And when it's closed there's a delete button. — john, May 18 '23 at 14:00
OK, I guess that's because you are the author. Press it if you want the post deleted. — john, May 18 '23 at 14:05
@deanqx Only higher reputation users see the "close" button. But you can use "delete". It will make the question invisible to most users. You can later "undelete" it if you e.g. have edited it. — user17732522, May 18 '23 at 14:06
Adding a destructor removes the implicit move constructor (and assignment operator) that you need for the `std::unique_ptr` member. You can add them back in as `MyClass2(MyClass2&&) = default; MyClass2& operator=(MyClass2&&) = default;` — Caleth, May 18 '23 at 15:31

score 0 · Answer 1 · answered May 18 '23 at 13:57

0

You need to dereference the std::unique_ptr in order to access the data.

int main()
{
    std::vector<MyClass> ts;

    for (int i = 0; i < 10; ++i)
    {
        ts.emplace_back(123);
    }

    // error: use of deleted function 'std::unique_ptr'
    printf("%i\n", *ts[0].x);

    return 0;
}

answered May 18 '23 at 13:57

RandomBits

4,194
1
17
30

that was a typo from copying, fixed that now – deanqx May 18 '23 at 14:31

score -1 · Answer 2 · answered May 18 '23 at 15:24

-1

Calling the emplace_back function attempts to create a new MyClass2 object and copy that object into the vector. However, unique_ptr objects cannot be copied. Therefore, you need to write a copy constructor for MyClass2 like this

MyClass2(const MyClass2& my2)
{
    my.reset();
}

answered May 18 '23 at 15:24

Barış İşbiliroğlu

119
3

emplace moves, not copies – Caleth May 18 '23 at 15:31
If this is true it could be fixed with a raw pointer but that would lead to bad code readability – deanqx May 18 '23 at 15:31
Actually emplace functions not move. They are creates new object by using perfect forwarding. https://en.cppreference.com/w/cpp/container/vector/emplace – Barış İşbiliroğlu May 18 '23 at 18:33
I think shared_ptr is better option for this situation. Because shared_ptr can be copied. – Barış İşbiliroğlu May 18 '23 at 18:39

std::vector of structs which require a constructor

2 Answers2