A container hierarchy is a tree of containers for the purposes of sharing the registrations of dependency injections. Service types registered to a parent container can be resolved in its child containers too. Use init(parent: Container) to instantiate a child container while specifying its parent container:
let parentContainer = Container()
parentContainer.register(Animal.self) { _ in Cat() }
let childContainer = Container(parent: parentContainer)
let cat = childContainer.resolve(Animal.self)
print(cat != nil) // prints "true"In contrast, service types registered to a child container are not resolved in its parent container:
let parentContainer = Container()
let childContainer = Container(parent: parentContainer)
childContainer.register(Animal.self) { _ in Cat() }
let cat = parentContainer.resolve(Animal.self)
print(cat == nil) // prints "true"