Using assignment operator with vector of unique_ptr

Question

If I have a std::vector of std::unique_ptr and resize it, and wanted to add element by index, what is the best way to add them using operator=?

std::vector<std::unique_ptr<item>> _v;
_v.resize(100);
// is it safe to use the assignment operator? 
_v[20] = new item;

most tutorial talk about using ways to use unique_ptr and ways to avoid just making sure there is no drawback with the = operator. — devcodexyz, Oct 20 '17 at 18:52
Be careful with preceding underscores. They are often reserved for use by the library implementation. Handy reading: [What are the rules about using an underscore in a C++ identifier?](https://stackoverflow.com/questions/228783/what-are-the-rules-about-using-an-underscore-in-a-c-identifier) — user4581301, Oct 20 '17 at 18:58
@MoradMohammad As mentioned in the answers, there is a drawback for using the assignment operator in a way that you show in your example: [it doesn't compile](https://ideone.com/7OXVw7). — Algirdas Preidžius, Oct 20 '17 at 19:01

score 1 · Answer 1 · edited Oct 20 '17 at 18:52

1

std::unique_ptr does not have an assignment operator that takes a raw pointer.

But it does have an assignment operator that moves from another std::unique_ptr, which you can create using std::make_unique():

_v[20] = std::make_unique<item>();

edited Oct 20 '17 at 18:52

Remy Lebeau

answered Oct 20 '17 at 18:48

Chris Drew

2

Note that `std::make_unique()` was added in C++14. For C++11, you can use `_v[20] = std::unique_ptr(new item);` instead. – Remy Lebeau Oct 20 '17 at 18:51

score 1 · Accepted Answer · answered Oct 20 '17 at 18:50

You could use std::make_unique if you are using C++14, like that

_v[20] = std::make_unique<item>(/* Args */);

Otherwise if you are under C++14, you can make your own implementation of std::make_unique, or use the constructor of std::unique_ptr

_v[20] = std::unique_ptr<item>(new item(/* Args */));

2 Answers2