58

What are recommended uses of a std::unique_ptr as to specifically where, when, and how is it is best used?

I discovered:

About unique_ptr performances

I already know:

  1. std::unique_ptr was developed in C++11 as a replacement for std::auto_ptr
  2. That a std::unique_ptr has no reference counting and "owns" object it points to
  3. There is no copy/assign with a std::unique_ptr
  4. When I need a unique pointer, std::unique_ptr is the go to structure

What I would like to know:

  1. Is using a std::unique_ptr ever preferable (other than uniqueness) to something
    else? What do I gain in this situation?
  2. If so, under what circumstances and when?
  3. Given the need for move semantics, would this make a std::unique_ptr less favorable
    overall?
  4. If a std::shared_ptr would suffice for dynamic memory management in nearly every situation, why does having at my disposal a std::unique_ptr matter (again, other
    than uniqueness)?
Improve this question
4
  • 2
    since the type is called unique_ptr, I find it strange you're searching for other use than being unique... Commented Mar 18, 2013 at 12:34
  • 2
    And Which kind of pointer do I use when? Commented Mar 18, 2013 at 12:41
  • 1
    @Mushy: The chief question you need to answer is that of ownership. Nothing else matters in the choice of a smart pointer, and that is answered very well in the questions I linked to. The question you linked to had my downvote (from earlier) because it shows nothing definitive at all and asks the wrong question about the "data". Commented Mar 18, 2013 at 12:49
  • stackoverflow.com/questions/8706192/… is probably closest to my question in overall meaning and intent. I did not see stackoverflow.com/questions/8706192/… when researching or posting my question from the closest recommendations offered. Commented Mar 18, 2013 at 12:53

1 Answer 1

Reset to default
29

In theory, you should use unique_ptr for all pointers unless you know you want to share it, in which case you should use shared_ptr. The reason is that unique_ptr has less overhead since it doesn't count references.

However, a unique_ptr is movable but not copyable, so using one as a member variable can require you to write more code (eg a move constructor), passing one by value means you'll need to use std::move and so on. As a result some people use shared_ptr out of laziness, because it's just easier, and the perf difference may not be significant for their app.

Finally a raw pointer is fine for observation - pointer uses that can never affect lifetime. Careful choice of the right pointer type can give those who read your code a good understanding of what you are doing. For more, see Herb Sutter's essay, Elements of C++ Style, specifically the "no delete" section.

Improve this answer
Sign up to request clarification or add additional context in comments.

4 Comments

=default and implicit move members can easily take care of the extra code.
If you didn't need an explicit move constructor before adding a unique_ptr, you don't need an explicit move constructor afterwards. The only situation in which you'd have to write more code is when you'd have done something that disables the implicit move constructor.
@DeadMG I'm still waiting for =default in my compiler, but yes
@KateGregory Use another compiler ;-)

Start asking to get answers

Find the answer to your question by asking.

Ask question

Explore related questions

See similar questions with these tags.