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
pub
andaccess
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 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
pub
oraccess(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
The (set)
suffix can be used to make variables also publicly writable.
To summarize the behavior for variable declarations, constant declarations, and fields:
Declaration kind
Access modifier
Read scope
Write scope
let
priv
/ access(self)
Current and inner
None
let
access(contract)
Current, inner, and containing contract
None
let
access(account)
Current, inner, and other contracts in same account
None
let
pub
,access(all)
All
None
var
access(self)
Current and inner
Current and inner
var
access(contract)
Current, inner, and containing contract
Current and inner
var
access(account)
Current, inner, and other contracts in same account
Current and inner
var
pub
/ access(all)
All
Current and inner
var
pub(set)
All
All
To summarize the for functions:
Access modifier
Access scope
priv
/ access(self)
Current and inner
access(contract)
Current, inner, and containing contract
access(account)
Current, inner, and other contracts in same account
pub
/ 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.
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