Are there any advantages of using std::unique_ptr<T>& instead of std::unique_ptr<T>?

Question

Are there any advantages of using std::unique_ptr<T>& instead of std::unique_ptr<T>? For example, in function arguments?

Answer 1

There are a few different situations

1. You want to transfer ownership to the function
   • Use std::unique_ptr<T>
2. You want to allow the function to modify the pointer
   • Use std::unique_ptr<T> &
3. You want to allow the function to modify the pointee
   • Use T &, and dereference at the call site
   • If the pointer may be null, instead use T * and call unique_ptr::get at the call site
4. You want to allow the function to observe the pointee
   • Use const T &, and dereference at the call site
   • If the pointer may be null, instead use const T * and call unique_ptr::get at the call site
5. You want to allow the function to have a copy of the pointee
   • Use T, and dereference at the call site
6. You have a ranged-for loop (or <algorithm> call) over a collection of unique_ptr<T>, and want to observe the values, and don't have ranges::indirect (or similar)
   • Use auto &, and know that it is inferred as std::unique_ptr<T> & (or const std::unique_ptr<T> & if the collection is const), so you have to dereference in the loop body