Move Semantics and Smart Pointers recap

These advanced features enable efficient resource management and are essential for modern C++ programming. Let's review the key concepts.

Smart Pointers

A smart pointer class is a composition class designed to manage dynamically allocated memory and ensure that memory gets deleted when the smart pointer object goes out of scope.

Copy Semantics

Copy semantics allow our classes to be copied. This is done primarily via the copy constructor and copy assignment operator.

Move Semantics

Move semantics mean a class will transfer ownership of the object rather than making a copy. This is done primarily via the move constructor and move assignment operator.

std::auto_ptr

std::auto_ptr is deprecated and should be avoided.

R-value References

An r-value reference is a reference designed to be initialized with an r-value. An r-value reference is created using a double ampersand. It's fine to write functions that take r-value reference parameters, but you should almost never return an r-value reference.

L-values vs R-values in Construction and Assignment

If we construct an object or do an assignment where the argument is an l-value, the only thing we can reasonably do is copy the l-value. We can't assume it's safe to alter the l-value, because it may be used again later in the program. If we have expression "a = b", we wouldn't reasonably expect b to change in any way.

However, if we construct an object or do an assignment where the argument is an r-value, then we know that r-value is just a temporary object of some kind. Instead of copying it (which can be expensive), we can simply transfer its resources (which is cheap) to the object we're constructing or assigning. This is safe to do because the temporary will be destroyed at the end of the expression anyway, so we know it will never be used again!

Disabling Copy Semantics

You can use the delete keyword to disable copy semantics for classes you create by deleting the copy constructor and copy assignment operator.

std::move

std::move allows you to treat an l-value as r-value. This is useful when we want to invoke move semantics instead of copy semantics on an l-value.

std::unique_ptr

std::unique_ptr is the smart pointer class that you should probably be using. It manages a single non-shareable resource. std::make_unique() (in C++14) should be preferred to create new std::unique_ptr. std::unique_ptr disables copy semantics.

std::shared_ptr

std::shared_ptr is the smart pointer class used when multiple objects need to access the same resource. The resource won't be destroyed until the last std::shared_ptr managing it is destroyed. std::make_shared() should be preferred to create new std::shared_ptr. With std::shared_ptr, copy semantics should be used to create additional std::shared_ptr pointing to the same object.

std::weak_ptr

std::weak_ptr is the smart pointer class used when you need one or more objects with the ability to view and access a resource managed by a std::shared_ptr, but unlike std::shared_ptr, std::weak_ptr isn't considered when determining whether the resource should be destroyed.

Key Terminology

  • Smart pointer: Class managing dynamically allocated memory automatically
  • Copy semantics: Rules for copying objects (copy constructor, copy assignment)
  • Move semantics: Rules for transferring ownership (move constructor, move assignment)
  • l-value: Expression with persistent identity
  • r-value: Temporary expression without persistent identity
  • r-value reference: Reference designed to bind to r-values (&&)
  • std::move: Casts l-value to r-value to enable move semantics
  • Move constructor: Constructor that transfers resources from another object
  • Move assignment operator: Assignment operator that transfers resources
  • std::auto_ptr: Deprecated smart pointer (do not use)
  • std::unique_ptr: Smart pointer for exclusive ownership
  • std::make_unique: Preferred way to create std::unique_ptr
  • std::shared_ptr: Smart pointer for shared ownership
  • std::make_shared: Preferred way to create std::shared_ptr
  • std::weak_ptr: Non-owning smart pointer for observing shared resources
  • Reference counting: Tracking how many shared_ptrs manage a resource

Looking Forward

Move semantics and smart pointers represent a major shift from traditional C++ memory management. By understanding when objects can safely have their resources transferred versus copied, you can write more efficient code while maintaining safety. Smart pointers eliminate entire categories of bugs related to manual memory management. These concepts are fundamental to modern C++ and will serve you throughout your programming career.