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Method overriding is an object-oriented mechanism in Swift that allows a subclass to provide a custom implementation of an instance method or type method inherited from its superclass. By explicitly marking a method with the override keyword, the compiler verifies that a matching declaration exists in the superclass chain, ensuring type safety and preventing accidental method shadowing.

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Syntax and Covariant Return Types

To override a method, prefix the func declaration with the override keyword. The method signature—including the name, parameter labels, and parameter types—must exactly match the superclass implementation. However, Swift supports covariant return types. This means an overridden method in a subclass can return a more specific type (such as a subclass) than the return type defined in the superclass method. 
class BaseResult {}
class SpecificResult: BaseResult {}

class BaseClass1 {
    func process() -> BaseResult {
        return BaseResult()
    }
}

class DerivedClass1: BaseClass1 {
    // Valid override utilizing a covariant return type
    override func process() -> SpecificResult {
        return SpecificResult()
    }
}

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Accessing the Superclass Implementation

When overriding a method, the subclass implementation completely replaces the superclass implementation for instances of that subclass. To invoke the original superclass behavior within the overridden method, use the super prefix. 
class BaseClass2 {
    func execute() {
        // Original behavior
    }
}

class DerivedClass2: BaseClass2 {
    override func execute() {
        // Invoke the superclass implementation
        super.execute() 
        
        // Additional subclass-specific behavior
    }
}

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Compiler Rules and Constraints

  • Mandatory Keyword: The override keyword is strictly enforced. Attempting to declare a method in a subclass with the identical signature of a superclass method without the override modifier results in a compile-time error.
  • Existence Verification: Applying override to a method that does not exist in the superclass (due to a typo or signature mismatch) triggers a compile-time error.
  • Access Control: The access level of an overridden method is bounded by the access level of the subclass. An overridden method cannot be less accessible than its superclass method, unless the subclass itself has a more restrictive access level. For example, an internal subclass overriding a public method defaults to internal access, and explicitly declaring the override as internal is perfectly valid. Conversely, a public subclass overriding a public method must maintain public access and cannot restrict the override to internal. Additionally, an open method can be overridden with a public method, which maintains visibility but restricts further overriding outside the defining module.
  • Throwing Methods: A subclass can override a throwing method with a non-throwing method, but it cannot override a non-throwing method with a throwing method.

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Overriding Type Methods

Swift distinguishes between two types of type-level methods: static and class. While both are called on the type itself rather than an instance, only methods marked with class participate in dynamic dispatch and can be overridden. Methods marked with static are implicitly final. 
class BaseClass3 {
    class func overridableTypeMethod() { }
    static func staticTypeMethod() { }
}

class DerivedClass3: BaseClass3 {
    override class func overridableTypeMethod() { }
    
    // ERROR: Cannot override static method
    // override static func staticTypeMethod() { } 
}

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Preventing Overrides

To explicitly prevent a method from being overridden by any future subclasses, apply the final modifier to the method declaration in the superclass. Attempting to override a final method results in a compile-time error. 
class BaseClass4 {
    final func lockedMethod() {
        // Implementation guaranteed not to change in subclasses
    }
}

class DerivedClass4: BaseClass4 {
    // ERROR: Instance method overrides a 'final' instance method
    // override func lockedMethod() { }
}
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