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AnyObject is a built-in protocol in Swift to which all concrete class types implicitly conform. It represents an instance of any class type and serves as a universal reference type, distinguishing reference semantics from value semantics within Swift’s type system.

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Type System Integration

At the compiler level, AnyObject acts as a type-erased wrapper specifically for reference types. While the Any type can represent any instance (including structs, enums, and closures via existential containers), AnyObject is strictly limited to instances allocated on the heap and managed by Automatic Reference Counting (ARC). In the Swift standard library, it is defined conceptually as an empty protocol: 
public protocol AnyObject {}
Only concrete class types implicitly conform to this protocol. Protocols can inherit from AnyObject to restrict their conformance to classes, but protocols themselves do not conform to it. Furthermore, class-only existentials (including the AnyObject existential itself) do not conform to AnyObject because, in Swift’s type system, existentials cannot conform to protocols. Attempting to pass an AnyObject existential to a generic function constrained to <T: AnyObject> will result in a compiler error (Protocol type 'AnyObject' cannot conform to 'AnyObject').

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Protocol Bounding and Memory Management

A primary mechanical function of AnyObject is to constrain protocols to reference types. By inheriting from AnyObject, a protocol guarantees to the compiler that only classes can conform to it. 
protocol ReferenceBoundProtocol: AnyObject {
    func execute()
}
This constraint is mandatory for ARC memory management modifiers. Because value types (structs and enums) are copied on assignment and do not have reference counts, Swift requires the AnyObject bound to safely apply weak or unowned modifiers to protocol-typed variables. 
class Node {
    // 'weak' is only valid because ReferenceBoundProtocol inherits from AnyObject.
    // Without the AnyObject bound, the compiler throws an error.
    weak var delegate: ReferenceBoundProtocol?
}

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Type Casting and Dynamic Dispatch

Because AnyObject erases the concrete type of the instance, accessing standard Swift properties or methods requires dynamic type checking and downcasting using the conditional cast (as?) or forced cast (as!) operators. 
class CustomClass {
    func invokeMethod() {}
}

let opaqueReference: AnyObject = CustomClass()

// Dynamic downcasting is required to access the concrete Swift implementation
if let concreteInstance = opaqueReference as? CustomClass {
    concreteInstance.invokeMethod()
}
However, AnyObject possesses a specific language feature regarding Objective-C interoperability: the Swift compiler permits direct access to any known @objc method or property on an AnyObject instance without prior downcasting. The compiler utilizes dynamic dispatch for these calls and returns the result as an implicitly unwrapped optional. This evaluates at runtime and will trigger a crash if the underlying instance does not actually implement the invoked member. 
import Foundation

class ObjCClass: NSObject {
    @objc func performAction() -> String { 
        return "Action Executed" 
    }
}

let anyObj: AnyObject = ObjCClass()

// Direct invocation of an @objc method without downcasting.
// The compiler allows this and returns an implicitly unwrapped optional (String!).
let result = anyObj.performAction()

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Objective-C Interoperability

Historically, AnyObject served as the direct Swift equivalent to the Objective-C id pointer. Since Swift 3 (SE-0116), Objective-C id is imported as Any rather than AnyObject to better accommodate bridged value types. Despite this change, AnyObject remains integral to interoperability. When the Foundation framework is imported, Swift automatically bridges certain native value types (such as String, Array, or Dictionary) to their Objective-C class counterparts (NSString, NSArray, NSDictionary) when they are explicitly cast to AnyObject. 
import Foundation

let swiftString: String = "Bridged String"
// Implicitly bridges String (value type) to NSString (reference type)
let bridgedObject: AnyObject = swiftString as AnyObject
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