Smart Pointers: Closing Words

Shared Pointers: When To Use Which?

Now when to use which pointer?

⟶ no definitive answer, but …

Answer 1: performance, and designwise correctness

• Always use std::unique_ptr<> ⟶ clearly defined ownership

• Pass object around as pointer (uptr->get())

• Use std::shared_ptr<> only if we have real shared ownership

Answer 2: programming efficiency

• Don’t waste a thought on ownership, simply write it

• Always use std::shared_ptr<>

Resource Usage

How do std::shared_ptr and std::unique_ptr compare?

• std::unique_ptr

  • Small - size of a pointer

  • Operations compile away entirely

  • No excuse not to use it

  • Have to think more though

• std::shared_ptr

  • Size of two pointers

  • Copying manipulates the resource count. Expensive: atomic instructions - memory barriers

  • Copying manipulates non-adjacent memory locations

  • Usage is very easy (no std::move and such)

  Attention

  • Cyclic references possible!

  • No garbage collection as in Java

  • ⟶ Leak!!

  See below …