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Any is a built-in existential type in Swift that can represent an instance of any type whatsoever, including value types (structs and enumerations), reference types (classes), and function types (closures). It acts as a universal type-erased container, allowing variables to hold disparate underlying concrete types by masking their specific type signatures from the compiler.

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Type Erasure and Existential Containers

When a concrete type is assigned to an Any variable, Swift wraps the value in an opaque existential container. The variable’s static type (resolved at compile time) becomes Any, while its dynamic type (resolved at runtime) remains the original concrete type. Because the static type is opaque, the compiler prohibits direct access to the underlying type’s properties, methods, or protocol conformances.

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Syntax and Declaration

You declare a variable of type Any using standard type annotation. The compiler permits reassignment to entirely different types: 
var typeErasedValue: Any = 42
typeErasedValue = "A string literal"
typeErasedValue = { (x: Int) -> Int in return x * 2 }

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Downcasting

To interact with the underlying dynamic type, you must explicitly downcast the Any instance back to a concrete type. This is achieved using the conditional cast operator (as?), the forced cast operator (as!), or a switch statement utilizing type-casting patterns. 
let unknownValue: Any = 3.14159

// Conditional downcasting
if let doubleValue = unknownValue as? Double {
    print(doubleValue.rounded())
}

// Type-casting in a switch statement
switch unknownValue {
case let intValue as Int:
    print("Integer: \(intValue)")
case let stringValue as String:
    print("String: \(stringValue)")
case let doubleValue as Double:
    print("Double: \(doubleValue)")
default:
    print("Unmatched type")
}

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Optionals and Any

Because an Optional is a value type (an enumeration) in Swift, assigning an optional value to an Any variable boxes the Optional itself, rather than the underlying wrapped value. This behavior frequently triggers a specific compiler warning: Expression implicitly coerced from 'T?' to 'Any'. To explicitly acknowledge this coercion and silence the warning, you must cast the optional using as Any. 
let optionalInt: Int? = 42

// Triggers compiler warning: Expression implicitly coerced from 'Int?' to 'Any'
let implicitAny: Any = optionalInt

// Silences the warning by explicitly boxing the Optional enum
let explicitAny: Any = optionalInt as Any

// To box the unwrapped value instead, unwrap it first
if let unwrappedInt = optionalInt {
    let unwrappedAny: Any = unwrappedInt
}

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Any vs. AnyObject

It is critical to distinguish Any from its counterpart, AnyObject, at the compiler level:
  • Any: Represents an instance of any type at all (structs, enums, classes, tuples, closures).
  • AnyObject: A protocol to which all classes implicitly conform. It can only represent instances of reference types (classes).

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Memory Layout (Under the Hood)

For values assigned to Any, Swift utilizes an opaque existential container. On a 64-bit system, MemoryLayout<Any>.size is exactly 32 bytes (4 words). This container consists of:
  1. Inline Value Buffer (3 words): Used to store the actual value if it fits within 24 bytes. If the concrete value exceeds this size, Swift automatically allocates the value on the heap and stores a pointer to that heap allocation inside this buffer.
  2. Type Metadata Pointer (1 word): Points to the metadata of the concrete type. This metadata provides access to the Value Witness Table (VWT) for memory lifecycle management (allocation, copying, destruction) and allows the runtime to perform type-checking during as? or as! downcasting operations.
Note: Unlike standard protocol existentials (which require a 5th word for a Protocol Witness Table pointer), Any has no protocol requirements and therefore consists solely of the value buffer and the type metadata pointer.
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