deleters

Deleters are callables responsible for releasing a resource when it is no longer needed. In programming languages that use automatic resource management, a deleter defines how the resource is destroyed or reclaimed and may be tied to an owning handle or smart pointer.

In C++, the term is most commonly used with smart pointers such as unique_ptr and shared_ptr. The deleter determines how the pointee is destroyed when the smart pointer releases ownership. The default deleter for unique_ptr is to call the delete operator on the stored pointer; a version for arrays uses delete[]. Both unique_ptr and shared_ptr can be configured with a custom deleter, which can be a function pointer, a function object (functor), or a lambda. A deleter may carry state, such as a memory pool pointer or a resource manager, which is captured in the functor and kept with the owning pointer. Custom deleters enable safe management of resources beyond plain memory, such as file handles, sockets, or opaque handles returned by libraries.

Best practices for deleters include ensuring they do not throw exceptions and that they correctly match the resource type they manage. Deleters should be lightweight and, when possible, idempotent to handle multiple releases gracefully. They may also perform resource reclamation from specialized allocators or pools, or release resources in a particular order required by the application.

Beyond C++, the general concept of a deleter appears in various resource-management patterns where a user-supplied cleanup function is invoked when ownership ends. In that sense, a deleter is a modular, customizable policy for resource destruction that helps implement RAII and robust resource lifetimes.