error use of deleted function - when pushing back a class with a vector<unique_ptr<base>> member

Blitz00 (2)
Disclaimer; I realize some users are against "using namespace std;" and I don't use it in my actual code, I just thought it was more clear for the example.

Most of the code is pretty straight forward. I have a vector<Base> that can hold a list of pointers to derived classes to avoid slicing. One of those derived classes actually holds a vector<Base> itself. The problem comes when I try to push this class (called "DerivedArray" in the code below) into another vector<Base>.

if I comment out the "derived_items" member code I can push_back without problems.

http://coliru.stacked-crooked.com/a/3e14fb946d912ba2

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#include <stdio.h>
#include <vector>
#include <iostream>
#include <memory>

using namespace std;

class Base {};

class Derived : public Base {};

class DerivedArray : public Base {
    public:
    DerivedArray(vector<unique_ptr<Base>>& vector) {
        derived_items = move(vector);
    };
    
    private:
    vector<unique_ptr<Base>> derived_items;
};


int main()
{   
    Derived d1;
    vector<unique_ptr<Base>> items;
    items.push_back(make_unique<Derived>(d1));
    
    DerivedArray da{items}; 
    
    // re-use items, which is now empty after the elements have been moved in DerivedArray's constructor
    items.push_back(make_unique<DerivedArray>(da)); // error

    return 0;
}


Why does this not work?

I have a hunch that there is somewhere a copy going on inside "make_unique()" that violates the uniqueness of the "unique_ptr"'s stored "derived_items", but I can't reason why because the function name suggests I get a unique reference to my DerivedArray "da".

Note that I'm not very proficient in plowing through the big list of errors of the template functions.

Thanks in advance.
dutch (2548)
This works:

 
 
    items.push_back(make_unique<DerivedArray>(move(da)));

Last edited on
Blitz00 (2)
Could you please elaborate why this works/is needed?

I'm missing the logic as to why I'd need to move() the DerivedArray object. Thanks
dutch (2548)
Could you please elaborate why this works/is needed?

make_unique creates a copy of da, which contains unique_ptr's.
You could try this instead:

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    // DerivedArray da{items};
    items.push_back(make_unique<DerivedArray>(items)); // construct in place

TheToaster (628)
Unique_ptrs do not have a copy constructor. They cannot be copied, only moved. Think about it, if a unique_ptr could be copied (which means a single resource can have more than one pointer pointing to it), that wouldn't make the unique_ptr very "unique", would it?
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