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Scope and introduction
This document defines the policy for applying access control modifiers and
related naming conventions for the Swift standard library and overlays.
In this document, "stdlib" refers to the core standard library and
overlays for system frameworks written in Swift.
Swift has three levels of access control --- private, internal
and public. As currently implemented, access control is only
concerned with API-level issues, not ABI. The stdlib does not have a stable ABI,
and is compiled in "non-resilient" mode with inlining into user code; thus, all
stdlib symbols are considered ABI and stdlib clients should be recompiled after
*any* change to the stdlib.
User-visible APIs should be marked public.
Unfortunately, the compiler has bugs and limitations that the stdlib
must work around by defining additional public symbols not intended
for direct consumption by users. For example:
.. parsed-literal::
// Workaround.
public protocol **_Collection** { ... }
// Symbol intended for use outside stdlib.
public protocol Collection : **_Collection** { ... }
These symbols are hidden using the `leading underscore rule`_.
Because Swift does not yet support a notion of SPI, any implementation
details that are shared across the stdlib's various sub-modules must
also be public. These names, too, use the `leading underscore rule`_.
To document the reason for marking symbols public, we use comments:
* symbols used in tests::
public // @testable
func _foo() { ... }
* symbols that are SPIs for the module X::
public // SPI(X)
func _foo() { ... }
In Swift, `internal` is an implied default everywhere--except within
`public` extensions and protocols. Therefore, `internal` should be used
explicitly everywhere in the stdlib to avoid confusion.
.. Note:: No declaration should omit an access
To create a "single point of truth" about whether a name is intended
for user consumption, the following names should all use the `leading
underscore rule`_:
* module-scope `private` and `internal` symbols::
var _internalStdlibConstant: Int { ... }
* `private` and `internal` symbols nested within `public` types::
public struct Dictionary {
var _representation: _DictionaryRepresentation
The `private` modifier cannot be used in the stdlib at least until
rdar://17631278 is fixed.
Leading Underscore Rule
Variables, functions and typealiases should have names that start with an
var _value: Int
func _bridgeSomethingToAnything(_ something: AnyObject) -> AnyObject
typealias _InternalTypealias = HeapBuffer<Int, Int>
To apply the rule to an initializer, one of its label arguments *or*
internal parameter names must start with an underscore::
public struct Foo {
init(_count: Int) {}
init(_ _otherInitializer: Int) {}
.. Note:: the identifier that consists of a single underscore ``_`` is not
considered to be a name that starts with an underscore. For example, this
initializer is public::
public struct Foo {
init(_ count: Int) {}
The compiler and IDE tools may use the leading underscore rule,
combined with additional heuristics, to hide stdlib symbols that users
don't need to see.
Users are prohibited to use leading underscores symbols in their own source
code, even if these symbols are visible through compiler diagnostics
or IDE tools.