Smart Pointers

The Covariant Return Type of a member function allows an overriding member function to return a narrower type. This is particularly useful when you implement the Prototype Pattern.

In my last post, I introduced latches in C++20. A latch enables its threads to wait until a counter becomes zero. Additionally to a latch, its big sibling barrier can be used more than once. Today, I write about barriers and present atomic smart pointers.

Passing smart pointers is a critical topic that is seldom addressed. This ends with the C++ core guidelines because they have six rules for passing std::shared_ptr and std::unique_ptr.

There were a lot of C++ experts who said that smart pointers were the essential feature of C++11. Today, I will write about smart pointers in C++.

C++20 will have atomic smart pointers. To be exact, we will get a std::atomic_shared_ptr and a std::atomic_weak_ptr. But why? std::shared_ptr and std::weak_ptr are already thread-safe. Sort of. Let me dive into the details.

std::unique_ptr models the concept of exclusive ownership, std::shared_ptr the concept of shared ownership. If I stick to this picture then std::weak_ptr models the concept of temporary ownership because it borrows the resource from a std::shared_ptr. There is one dominant reason for having a std::weak_ptr in C++: breaking of cyclic references of std::shared_ptr's.

After I draw the big picture of a std::shared_ptr in the last post, I want to present two special aspects of this smart pointer in this post. First, I show with std::shared_from_this how to create a std::shared_ptr from an object; second, I'm interested in the question to the answer: Should a function take a std::shared_ptr by copy or by reference? The numbers are quite interesting.

std::shared_ptr's share the resource. The shared reference counter counts the number of owners. Copying a std::shared_ptr increases the reference count by one. Destroying a std::shared_ptr decreases the reference count by one. If the reference count becomes zero, the resource will automatically be released. 

According to the RAII idiom, a std::unique_ptr manages automatically and exclusively the lifetime of its resource. std::unique_ptr should be your first choice because it does its work without memory or performance overhead.

C++11 offers four different smart pointers. I will have a closer look in this post regarding memory and performance overhead on two of them. My first candidate, std::unique_ptr takes care of the lifetime of one resource exclusively; std::shared_ptr shares the ownership of a resource with another std::shared_ptr. I will state the result of my tests before I show you the raw numbers: There are only a few reasons in modern C++ justifying the memory management with new and delete.