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Access control

Access control allows making certain parts of the program accessible/visible and making other parts inaccessible/invisible.

In Flow and Cadence, there are two types of access control:

Access control on objects in account storage using capability security.

Within Flow, a caller is not able to access an object unless it owns the object or has a specific reference to that object. This means that nothing is truly public by default. Other accounts can not read or write the objects in an account unless the owner of the account has granted them access by providing references to the objects.
Access control within contracts and objects using access(all) and access keywords.

For the explanations of the following keywords, we assume that the defining type is either a contract, where capability security doesn't apply, or that the caller would have valid access to the object governed by capability security.

The high-level reference-based security (point 1 above) will be covered in a later section.

Top-level declarations (variables, constants, functions, structures, resources, interfaces) and fields (in structures, and resources) are always only able to be written to and mutated (modified, such as by indexed assignment or methods like append) in the scope where it is defined (self).

There are four levels of access control defined in the code that specify where a declaration can be accessed or called.

Public or access(all) means the declaration is accessible/visible in all scopes.

This includes the current scope, inner scopes, and the outer scopes.

For example, a public field in a type can be accessed using the access syntax on an instance of the type in an outer scope. This does not allow the declaration to be publicly writable though.

An element is made publicly accessible / by any code by using the access(all) or access(all) keywords.
access(account) means the declaration is only accessible/visible in the scope of the entire account where it is defined. This means that other contracts in the account are able to access it,

An element is made accessible by code in the same account (e.g. other contracts) by using the access(account) keyword.
access(contract) means the declaration is only accessible/visible in the scope of the contract that defined it. This means that other types and functions that are defined in the same contract can access it, but not other contracts in the same account.

An element is made accessible by code in the same contract by using the access(contract) keyword.
Private or access(self) means the declaration is only accessible/visible in the current and inner scopes.

For example, an access(self) field can only be accessed by functions of the type is part of, not by code in an outer scope.

An element is made accessible by code in the same containing type by using the access(self) keyword.

Access level must be specified for each declaration

To summarize the behavior for variable declarations, constant declarations, and fields:

Declaration kind	Access modifier	Read scope	Write scope	Mutate scope
`let`	`access(self)`	Current and inner	None	Current and inner
`let`	`access(contract)`	Current, inner, and containing contract	None	Current and inner
`let`	`access(account)`	Current, inner, and other contracts in same account	None	Current and inner
`let`	`access(all)`	All	None	Current and inner
`var`	`access(self)`	Current and inner	Current and inner	Current and inner
`var`	`access(contract)`	Current, inner, and containing contract	Current and inner	Current and inner
`var`	`access(account)`	Current, inner, and other contracts in same account	Current and inner	Current and inner
`var`	`access(all)`	All	Current and inner	Current and inner

To summarize the behavior for functions:

Access modifier	Access scope
`access(self)`	Current and inner
`access(contract)`	Current, inner, and containing contract
`access(account)`	Current, inner, and other contracts in same account
``access(all)`	All

Declarations of structures, resources, events, and contracts can only be public. However, even though the declarations/types are publicly visible, resources can only be created from inside the contract they are declared in.

// Declare a private constant, inaccessible/invisible in outer scope.
//
access(self) let a = 1
// Declare a public constant, accessible/visible in all scopes.
//
access(all) let b = 2

// Declare a public struct, accessible/visible in all scopes.
//
access(all) struct SomeStruct {
    // Declare a private constant field which is only readable
    // in the current and inner scopes.
    //
    access(self) let a: Int
    // Declare a public constant field which is readable in all scopes.
    //
    access(all) let b: Int
    // Declare a private variable field which is only readable
    // and writable in the current and inner scopes.
    //
    access(self) var c: Int
    // Declare a public variable field which is not settable,
    // so it is only writable in the current and inner scopes,
    // and readable in all scopes.
    //
    access(all) var d: Int
    // Arrays and dictionaries declared without (set) cannot be
    // mutated in external scopes
    access(all) let arr: [Int]
    // The initializer is omitted for brevity.
    // Declare a private function which is only callable
    // in the current and inner scopes.
    //
    access(self) fun privateTest() {
        // ...
    }
    // Declare a public function which is callable in all scopes.
    //
    access(all) fun publicTest() {
        // ...
    }
    // The initializer is omitted for brevity.
}
let some = SomeStruct()
// Invalid: cannot read private constant field in outer scope.
//
some.a
// Invalid: cannot set private constant field in outer scope.
//
some.a = 1
// Valid: can read public constant field in outer scope.
//
some.b
// Invalid: cannot set public constant field in outer scope.
//
some.b = 2
// Invalid: cannot read private variable field in outer scope.
//
some.c
// Invalid: cannot set private variable field in outer scope.
//
some.c = 3
// Valid: can read public variable field in outer scope.
//
some.d
// Invalid: cannot set public variable field in outer scope.
//
some.d = 4
// Invalid: cannot mutate a public field in outer scope.
//
some.f.append(0)
// Invalid: cannot mutate a public field in outer scope.
//
some.f[3] = 1
// Valid: can call non-mutating methods on a public field in outer scope
some.f.contains(0)