How to declare a vector of unique_ptr's as class data member?

Question

I'd like to have a vector of unique_ptr's as a member of a class I'm making.

class Foo {
    [...]

private:
    vector<unique_ptr<Bar>> barList;
}

But then I start getting cryptic error messages from the VS2010 compiler:

error C2248: 'std::unique_ptr<_Ty>::operator =' : cannot access private member declared in class 'std::unique_ptr<_Ty>'

Along with a handful of error lines below that which dive into Microsoft's implementation of std::_Copy_impl<>...

I changed the member declaration to

vector<unique_ptr<Bar>>* barList;

And it compiles. But I can't help but wonder why I can't do it the way I originally wanted? For grins, I tried this and it works fine:

vector<Bar> barList;

But now I lose the convenience of unique_ptr. I want my cake and I want to eat it too!

Answer 1

Often a std::move(iUniquePtr) is missing somewhere (e. g. when using push_back).

Answer 2

unique_ptr doesn't have copy semantics, so you can't use any methods that would copy the contained object. You can do this with rvalue references by using std::move in the place(s) it's trying to make a copy. Without seeing your code I can't say where that would be.

If it compiles in the second form either you didn't exercise the same code or there's a compiler bug. Both should fail the same way.

Your third example, storing by value is the simplest way unless your objects are large and expensive to store/copy around by value.