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A lazy stored property is a property whose initial value is not calculated until the exact moment it is first accessed. It defers the evaluation of an expression or the instantiation of an object, ensuring that the memory allocation and initialization logic only occur if the property is explicitly read during the program’s lifecycle.

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Syntax and Declaration

A lazy property is declared by prepending the lazy modifier to a variable declaration. It is commonly initialized using a direct assignment, a method call, or an Immediately Invoked Function Expression (IIFE) via a closure. 
class ConfigurationManager {
    // Direct assignment
    lazy var defaultTimeout: Int = 30
    
    // Initialization via an Immediately Invoked Function Expression (closure)
    lazy var environmentSettings: [String: String] = {
        var settings = [String: String]()
        settings["theme"] = "dark"
        settings["version"] = "1.0.4"
        return settings
    }()
}

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Technical Constraints and Behavior

Mutability Requirement (var vs let) Lazy properties must always be declared as variables (var). They cannot be declared as constants (let). Swift’s initialization rules dictate that constant properties must possess a valid value before the initialization phase of an instance completes. Because a lazy property’s value is resolved post-initialization, it inherently violates the constant initialization contract. Structs and Mutating Contexts When a lazy property is defined within a value type (struct), accessing it for the first time alters the internal state of the instance. Consequently, the struct instance must be mutable (assigned to a var). Furthermore, any instance method that accesses the lazy property must be explicitly marked with the mutating keyword. 
struct DataParser {
    lazy var parserID: String = "ID-99"
    
    mutating func getID() -> String {
        // Accessing parserID mutates the struct on first read
        return parserID 
    }
}
Thread Safety Swift’s lazy modifier does not provide inherent thread synchronization. If a lazy property has not yet been initialized and is accessed by multiple threads simultaneously, there is no guarantee that the property will be initialized only once. This race condition can lead to multiple evaluations of the initialization expression, potentially causing memory leaks or inconsistent state. Implicit self Access When initializing a lazy property using a closure, the closure executes after the instance has completed its primary initialization phase. Because of this, the closure can safely reference self and access other properties or methods on the instance. Furthermore, because the closure is executed and immediately deallocated upon the property’s first access, it does not create a strong reference cycle. Explicit capture lists (e.g., [weak self]) are not required. 
class NetworkClient {
    var baseURL: String = "https://api.example.com"
    
    lazy var endpoint: String = {
        // 'self' is safely accessible here without a capture list
        return self.baseURL + "/v1/data"
    }()
}
Property Observers Lazy properties cannot be used in conjunction with property observers (willSet and didSet). The Swift compiler prohibits this combination because the initial access to a lazy property is an initialization event, not a standard mutation event.
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