I can't push_back a unique_ptr after passed it into a function

Question

I'm working on my little project about pointers, I ran into problem when I was trying to push a unique_ptr to a derived class, to a vector of unique_ptr to base class. It keeps saying that I'm "attempting to reference to a deleted function". I tried to use push_back with and without std::move but nothing worked...

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

using namespace std;

class Base {};
class Derived : public Base {};

void add(vector<unique_ptr<Base>> list, const unique_ptr<Base>& ptr);

int main()
{
    vector<unique_ptr<Base>> list;
    unique_ptr<Base> derived(new Derived);
    add(list, derived);
}

void add(vector<unique_ptr<Base>> list, const unique_ptr<Base>& ptr)
{
    list.push_back(move(ptr));
}

`add(vector> list` is a copy, I guess you don't intend it. – 273K May 03 '20 at 03:35 — 273K, May 03 '20 at 03:35
`list.push_back(move(ptr))` const can't be moved. – 273K May 03 '20 at 03:35 — 273K, May 03 '20 at 03:35

bolov · Answer 1 · 2020-05-03T03:43:45.957

There are quite a few issues with your code:

please don't use using namespace std. See Why is "using namespace std;" considered bad practice?

you copy the vector. Because it contains non-copyable items, the vector itself is non-copyable:

void add(vector<unique_ptr<Base>> list, const unique_ptr<Base>& ptr);
//                              ^

you can't move from const objects:

void add(vector<unique_ptr<Base>> list, const unique_ptr<Base>& ptr);
//                                      ^~~~~                 ~

use std::make_unique. See Differences between std::make_unique and std::unique_ptr with new
unique_ptr cannot be copied so you need to move it:
```
add(list, derived);
```

Here is the code corrected:

void add(std::vector<std::unique_ptr<Base>>& list, std::unique_ptr<Base> ptr)
{
    list.push_back(std::move(ptr));
}
int main()
{
    std::vector<std::unique_ptr<Base>> list;
    std::unique_ptr<Base> derived = std::make_unique<Derived>();

    add(list, std::move(derived));
}

// or

int main()
{
    std::vector<std::unique_ptr<Base>> list;

    add(list, std::make_unique<Derived>());
}

@DucNguyen you can pass by ref, just not by constant ref, nor lvalue ref. You can pass by rvalue ref (a.k.a. `&&`). The reason is because moving from an object modifies that object (to reflect the new state - moved from) so you cannot move from const. — bolov, May 03 '20 at 03:58

I can't push_back a unique_ptr after passed it into a function

1 Answers1