selectoncontainercopyconstruction

Select_on_container_copy_construction is a hook in the C++ standard library allocator interface that determines which allocator a container should use when it is copy-constructed. It affects how memory management and resource ownership are transferred or reset during container duplication. The mechanism is accessed through the allocator interface, and most containers consult it via allocator_traits to obtain the allocator for the new, copied container.

In practice, when a container such as a vector or map is copy-constructed, the library asks for an allocator to use for the new instance. If the allocator type provides a dedicated select_on_container_copy_construction function, that allocator may be used to create a new allocator instance for the copied container. If the allocator does not provide such a hook, the standard library supplies a default behavior, typically using a copy of the source allocator.

The primary purpose of the hook is to support stateful allocators that manage resources beyond raw memory, such as memory pools, statistics, or external handles. By customizing the select_on_container_copy_construction method, an allocator can decide whether the copied container shares the same resource, starts with a fresh resource, or uses a partially reinitialized state. This gives library writers and user-defined allocators control over resource ownership without changing container copy semantics.

Support for the hook is part of allocator_traits, which provides a uniform way for containers to query the appropriate allocator for a copied container. While not every allocator needs to implement the hook, those that do can influence container copy behavior in a predictable, centralized way.