C++ Smart Pointer

Overview

• defined in the std namespace in the <memory> header file.
  They are crucial to the RAII or Resource Acquisition Is Initialization programming idiom.

unique_ptr

A unique_ptr has exclusive ownership of the object it points to.

unique_ptr<T> name(new T(args));
 
name.reset(); // Free the memory before we exit function block.

shared_ptr

Use reference counter, used when you want to assign one raw pointer to multiple owners. The raw pointer is not deleted until all shared_ptr owners have gone out of scope or have otherwise given up ownership.

It use extra size: 2 pointers, one for object and one for the shared control block that contains the reference count.

• std::shared_ptr - cppreference.com

weak_ptr

provides access to an object that is owned by one or more shared_ptr instances, but does not participate in reference counting.

Use when you want to observe an object, but do not require it to remain alive. Required in some cases to break circular references between shared_ptr instances.

Comparison

• Difference Between unique_ptr and shared_ptr
  We can only have 1 unique_ptr but we can have multiple shared_ptr. However, shared_ptr may introduce some small overhead.