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References

It is possible to create references to objects, i.e. resources or structures. A reference can be used to access fields and call functions on the referenced object.

References are copied, i.e. they are value types.

References have the type &T, where T is the type of the referenced object.

References are created using the & operator. The reference type must be explicitly provided, for example through a type annotation on a variable declaration, or a type assertion using the as operator.

let hello = "Hello"
// Create a reference to the `String` `hello`.
// Provide the reference type `&String` using a type assertion
//
let helloRef = &hello as &String
helloRef.length // is `5`
// Create another reference to the `String` `hello`.
// Provide the reference type `&String` using a type annotation instead
//
let alsoHelloRef: &String = &hello
// Invalid: Cannot create a reference without an explicit type
//
let unknownRef = &hello

The reference type must be a supertype of the referenced object's type.

// Invalid: Cannot create a reference to `hello`
// typed as `&Int`, as it has type `String`
//
let intRef = &hello as &Int

When creating a reference to an optional value, the result is an optional reference. If the referenced value is nil, the resulting reference itself will be nil. If the referenced value exists, then forcing the optional reference will yield a reference to that value:

let nilValue: String? = nil
let nilRef = &nilValue as &String? // r has type &String?
let n = nilRef! // error, forced nil value
let strValue: String? = ""
let strRef = &strValue as &String? // r has type &String?
let n = strRef! // n has type &String

References are covariant in their base types. For example, &T is a subtype of &U, if T is a subtype of U.

// Declare a resource interface named `HasCount`,
// that has a field `count`
//
resource interface HasCount {
    count: Int
}
// Declare a resource named `Counter` that conforms to `HasCount`
//
resource Counter: HasCount {
    access(all) var count: Int
    access(all) init(count: Int) {
        self.count = count
    }
    access(all) fun increment() {
        self.count = self.count + 1
    }
}
// Create a new instance of the resource type `Counter`
// and create a reference to it, typed as `&Counter`,
// so the reference allows access to all fields and functions
// of the counter
//
let counter <- create Counter(count: 42)
let counterRef: &Counter = &counter as &Counter
counterRef.count  // is `42`
counterRef.increment()
counterRef.count  // is `43`

References can be freely upcasted and downcasted, and are covariant in their referenced type. So, for example, for some struct S, &S is a subtype of &AnyStruct, but not of &Int.

// Create an reference to the counter,
// typed with the intersection type `&{HasCount}`,
// i.e. some resource that conforms to the `HasCount` interface
//
let countRef = &counter as &{HasCount}
countRef.count  // is `43`
// Invalid: The function `increment` is not available
// for the type `&{HasCount}`
//
countRef.increment()
// We can conditionally downcast `countRef` to a `Counter` if it has
// that type at runtime.
//
let counterRef2: &Counter = countRef as? &Counter
counterRef2.increment()

References are ephemeral, i.e. they cannot be stored. Instead, consider storing a capability and borrowing it when needed.

Authorized References

By default, references are unauthorized. However, they may also be authorized to a set of entitlements

Authorized references have the auth modifier, along with the set of entitlements to which they are authorized. The full syntax is: auth(E, F, G) &T for a reference authorized to E, F and G, or auth(E | F | G) &T for a refernece authorized to E, F, or G. Authorized references are subtypes of unauthorized references.

Entitlements can only be given to references when they are created, and references to a value can only be created by the owner of the value. When creating a reference, that reference can be given any set of entitlements the value owner wishes to add.

Possessing an entitlement allows a reference to have access to functions and fields on its referenced type that require that entitlement. E.g, if we extended the HasCount interface with a function:

entitlement Reset
resource interface HasCount {
    count: Int
    access(Reset) fun resetCount()
}

Then an unauthorized reference of type &{HasCount} would be unable to call resetCount. However, we can create a reference that can, like so:

let authCountRef: auth(Reset) &{HasCount} = &counter
// Valid, because `authCountRef` is authorized to `Reset`
authCountRef.resetCount()

It is important to note that while references are covariant (and downcastable) with respect to their reference type, the authorization portion of the reference can never be downcast. In fact, the only way to "add" entitlements to a reference is to do so at the time of its creation, like in the example above. A reference will never have any more entitlements than the set with which it was created, and the set of entitlements on a reference at runtime will always match the set expressed in its static type. One implication of this is that upcasting an authorized reference actually changes its runtime type:

let authCountRef: auth(Reset) &{HasCount} = &counter
let unauthCountRef = authCountRef as &{HasCount}
let authCountRef2 = unauthCountRef as? auth(Reset) &{HasCount}
// Invalid: `authCountRef2` is `nil`, as the upcast of `authCountRef` cleared the
// `Reset` entitlement from the reference, meaning that it cannot be regained on downcasting.
authCountRef2.resetCount()

The benefit of this is that there is never any "surprising" behavior with regards to entitlements, every reference value is transparent about what it is capable of at runtime.

While entitlement sets on references cannot be downcast, they can be upcast, or used in places expecting supertypes, and have special subtyping rules based on whether they are | or ,-separated sets.

In general, an entitlement set {Us} is a subtype of an entitlement set {Vs} when {Us} has more entitlements in it than {Vs}, and when both are ,-separated (as they will be in most cases), this is the rule exactly: {Us} is a subset of {Vs} when it is a superset of {Vs}.

Conversely, if both are |-separated, the rule is reversed: {Us} is a subset of {Vs} when it is a subset of {Vs}. It may be helpful to think of this as saying that {Us} is more specific than {Vs} in this case; {Vs} expresses a set of entitlements that the reference might possess, while {Us} is expressing a more specific set of potential entitlements.

Lastly, if {Us} is ,-separated while {Vs} is |-separated, then {Us} is a subset of {Vs} when any of the Us also appears in {Vs}. To see why, consider again that {Vs} expresses a set of entitlements that the reference might possess, and as long as at least one of these entitlements is in {Us} (which is a set of entitlements that we know the reference has), then the description provided by {Vs} is correct.

As an example to illustrate these rules:

let eRef: auth(E) &T = ...
let efRef: auth(E, F) &T = ...
let eOrFRef: auth(E | F) &T = ...
// Invalid, `eRef` only has `E` but `F` is required
eRef as auth(F) &T
// Invalid, `eRef` only has `E` but both `E` and `F` are required
eRef as auth(E, F) &T
// Valid, `eRef` definitely has `E` and either `E` or `F` is sufficient
eRef as auth(E | F) &T
// Valid, `efRef` both `E` and `F` but only `F` is required
efRef as auth(F) &T
// Valid, `efRef` both `E` and `F`, and either is sufficient
efRef as auth(E | F) &T
// Invalid, `eOrFRef` has one of `E` or `F` but we need to definitely have `F`
eOrFRef as auth(F) &T
// Invalid, `eOrFRef` has one of `E` or `F` but we need both
eOrFRef as auth(E, F) &T

References and Entitlement Mappings

In most situations, an entitlement mapping is valid in the auth portion of a reference type. However, in certain specific circumstances in the definition of a field or function on a composite type, an entitlement mapping may be used in an auth modifier.

When a field is defined with an entitlement mapping:

entitlement mapping M {
    // omitted
}
resource interface I {
    access(M) let foo: auth(M) &T
}

Here, the M in auth(M) &T indicates that the entitlements that the reference produced by an iRef.foo access will have are determined by the entitlements to I that iRef has, for some iRef value that is a reference to {I}. Conceptually, it creates a correspondence between the "output" reference's type and the "input" access modifier.

When an accessor function is defined with an entitlement mapping:

entitlement mapping M {
    // omitted
}
resource I {
    access(self) let myField: T
    access(M) fun getMyField(): auth(M) &T {
        return &self.myField as auth(M) &T
    }
}

The M in the auth(M) &T of the function's return type annotation indicates the same thing as in the field case. However, in this example M is also used in a reference type within the body of the function. Inside the body of function with entitlement-mapped access, the name of the entitlement mapping may be used as a stand-in for the output entitlements of the map.

Field and Index Access

References to container types (structs/resources, dictionaries and arrays) can be used to access (read/write) fields or elements of the container.

When a field/index is read through a reference, it will return:

A reference, if the field / value at index is also container-typed.
Or, the concrete value, if the value is a primitive type.

For example, consider the below Collection resource which has two fields: one (id) is String-typed, and the other (ownedNFTs) is dictionary-typed.

resource Collection {
    // Primitive-typed field
    access(all) var id: String
    // Dictionary typed field
    access(all) var ownedNFTs: @{UInt64: NFT}
}

Thus,

var collectionRef: &Collection = ...
// `collectionRef.ownedNFTs` would return a reference of type `&{UInt64: NFT}`.
var ownedNFTsRef: &{UInt64: NFT} = collectionRef.ownedNFTs
// Whereas, `collectionRef.id` would return the value, since it is a primitive type.
var id: String = collectionRef.id

Similarly, accessing an element of an array/dictionary will return a reference.

// Index-access to an array reference would return a reference to the element.
var resourceArrayRef: &[AnyResource] = ...
var elementRef: &AnyResource = collectionArrayRef[2]
// Whereas, if the array is of a primitive type, it will return the concrete value.
var intArrayRef: &[Int] = ...
var element: Int = intArrayRef[2]

// Index-access to a dictionary reference would return a reference to the value.
var resourceDictionaryRef: &{String: AnyResource} = ...
var valueRef: &AnyResource? = resourceDictionaryRef["two"]
// Whereas, if the dictionary values are of a primitive type, it will return the concrete value.
var intDictionaryRef: &{String: Int} = ...
var value: Int? = intDictionaryRef["two"]

It is also important to note that, in the above examples, the returned references have no entitlements. i.e: they are non-auth references.

To get entitled references for struct/resource fields, they must be defined with entitlement mappings. However, accessing a value at an index/key of an array/dictionary reference would always return a non-auth reference.

Index Assignment

Assigning to an index of an array or a dictionary reference is an entitled-operation. In other words, the assignment operator for arrays/dictionaries would also have the Mutate and Insert built-in entitlements.

Think of assignment as a built-in function with Mutate or (Insert, Remove) entitlements. e.g:

access(Mutate | (Insert, Remove)) set(keyOrIndex, value) { ... }

Note that the syntax for having nested entitlements in access modifiers like (Mutate | (Insert, Remove)) is not currently supported, but this is for illustration purpose only.

Thus,

var arrayRef = &array as &[String]
arrayRef[2] = "John"               // Static Error: updating via a read-only reference
var mutableArrayRef = &array as auth(Mutate) &[String]
mutableArrayRef[2] = "John"        // OK
var insertableArrayRef = &array as auth(Insert) &[String]
insertableArrayRef[2] = "John"     // Static Error: doesn't have the required entitlement
var removableArrayRef = &array as auth(Remove) &[String]
removableArrayRef[2] = "John"     // Static Error: doesn't have the required entitlement
var insertableAndRemovableArrayRef = &array as auth(Insert, Remove) &[String]
insertableAndRemovableArrayRef[2] = "John"        // OK

Reference Validity

Ephemeral references stay valid throughout the course of the program. However, references to resources can become invalid during the execution of a program, if the referenced resource is moved or destroyed after taking the reference.

let r <-create R()
// Take a reference to resource.
let ref = &r as &R
// Then transfer the resource into an account.
// This will invalidate all the references taken to the resource `r`.
account.save(<-r, to: /storage/r)
// Static error, since the referenced resource has been moved.
ref.id = 2

A reference is invalidated upon the first transfer of the underlying resource, regardless of the origin and the destination.

let ref = &r as &R
// Moving a resource to a different variable invalidates all references to it.
let r2 <- r
// Static error, since the referenced resource has been moved.
ref.id = 2

info

Invalidations of storage references are not statically caught, but only at run-time.

Authorized References​

References and Entitlement Mappings​

Field and Index Access​

Index Assignment​

Reference Validity​

Authorized References

References and Entitlement Mappings

Field and Index Access

Index Assignment

Reference Validity