Chapter 4. Smart Pointers Flashcards
Item 18: Use std::unique_ptr for exclusive-ownership resource management.
std::unique_ptr is a small, fast, move-only smart pointer for managing resources with exclusive-ownership semantics.
By default, resource destruction takes place via delete, but custom deleters can be specified. Stateful deleters and function pointers as deleters increase the size of std::unique_ptr objects.
Converting a std::unique_ptr to a std::shared_ptr is easy.
Item 19: Use std::shared_ptr for shared-ownership resource management.
std::shared_ptrs offer convenience approaching that of garbage collection for the shared lifetime management of arbitrary resources.
Compared to std::unique_ptr, std::shared_ptr objects are typically twice as big, incur overhead for control blocks, and require atomic reference count manipulations.
Default resource destruction is via delete, but custom deleters are supported. The type of the deleter has no effect on the type of the std::shared_ptr. Avoid creating std::shared_ptrs from variables of raw pointer type.
Item 20: Use std::weak_ptr for std::shared_ptr-like pointers that can dangle.
Use std::weak_ptr for std::shared_ptr-like pointers that can dangle.
Potential use cases for std::weak_ptr include caching, observer lists, and the prevention of std::shared_ptr cycles.
Item 21: Prefer std::make_unique and std::make_shared to direct use of new.
Compared to direct use of new, make functions eliminate source code duplication, improve exception safety, and, for std::make_shared and std::allocate_shared, generate code that’s smaller and faster.
Situations where use of make functions is inappropriate include the need to specify custom deleters and a desire to pass braced initializers.
For std::shared_ptrs, additional situations where make functions may be ill-advised include (1) classes with custom memory management and (2) systems with memory concerns, very large objects, and std::weak_ptrs that outlive the corresponding std::shared_ptrs.
Item 22: When using the Pimpl Idiom, define special member functions in the implementation file.
The Pimpl Idiom decreases build times by reducing compilation dependencies between class clients and class implementations.
For std::unique_ptr pImpl pointers, declare special member functions in the class header, but implement them in the implementation file. Do this even if the default function implementations are acceptable.
The above advice applies to std::unique_ptr, but not to std::shared_ptr.
