push_back(move(struct.unique_ptr)) causes compile errors

Question

I have a function which is like

for (auto&& i : v) {
    ...
    this->fifoQueue.push_back(move(i.p));
    ...
}

where fifoQueue is of type vector<unique_ptr<T> >, v is of type vector<struct s>, and struct s is

struct s {
    ...
    std::unique_ptr<T> p;
    ...
};

The statement this->fifoQueue.push_back(move(i.p)); has caused a compiler error "... error: declared here unique_ptr(const unique_ptr&) = delete; ...". What's wrong with this statement?

I have another working statement basically the same as it, except that statement is like this->fifoQueue.push_back(move(p)). So i.p is the evil? Or auto&&?

Update:

As Petr inferred, v is indeed a const reference, which caused the error.

Are you sure that `v` in your first code is not `const`? And that there is no other place from where `i.p`can become `const`? — Petr, Jul 10 '15 at 12:10
Just curious, what about `this->fifoQueue.push_back(move(i).p)`? — fredoverflow, Jul 10 '15 at 12:11
Holy... Yeah, `v` is `const`. My bad. Have been staying up and now a little dizzy. Thanks. — vincentvangaogh, Jul 10 '15 at 12:14
[This](http://coliru.stacked-crooked.com/a/6780311ab8ed83f6) doesn't give any errors. — NathanOliver, Jul 10 '15 at 12:17

score 3 · Answer 1 · edited May 23 '17 at 12:14

(Expanding my comment)

Your error message, unique_ptr(const unique_ptr&), seems to indicate that i.p becomes const, thus push_back has to make its copy (or in fact even std::move has to make its copy, see this answer). However, unique_ptr can not be constructed as a copy of const unique_ptr&, because this is absolutely senseless given the nature of unique_ptr. So you should look where does it become const from.

I thought the most probable source of this constness is from v, because you have not shown us that declaration.

score 0 · Answer 2 · edited May 23 '17 at 11:51

As said above, seems like compiler wants to make a copy of your struct. Because of this unique_ptr inside it cannot.

When you think about it, gues what would mean auto here? It would be rather yourStruct & than yourStruct, because you can actually modify the struct inside this vector. And bacause of reference collapsing you get yourStruct & from auto &&.

All in all, instead of thinking how to avoid this situation with push_back simply use emplace_back which enforces construction inside vector.

push_back(move(struct.unique_ptr)) causes compile errors

2 Answers2