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Title: Private symbol visibility
DIP: 22
Version: 3
Status: Draft
Created: 2013-01-28
Last Modified: 2013-12-20
Authors: Михаил Страшун (m.strashun gmail.com) (Dicebot)
Martin Nowak
Links: Access specifiers and visibility : data gathered before creating proposal


This proposal attempts to solve one of important issues with current protection attribute design: senseless name clashes between private and public symbols. So change of private related name resolution rules is proposed.


private is an encapsulation tool. If it is not intended to be used by "outsiders", it should not interfere with them at all. It creates no new limitations and reduces amount of code breakage by changes in other modules.


  • Private restricts the visibility of a symbol.
 A private symbol will not interact with other modules.
 In case look-up for a symbol fails the compiler might suggest private symbols similar to how spell checking works.
  • The least protected symbol determines the visibility for an overload set.
 After overload resolution an access check will be performed. Thereby overload resolution is independent of look-up origin.
  • Meta programming tools like __traits and .tupleof can access private symbols.
 This is necessary for some generic code, e.g. serialization.
  • All changes apply for modules as well as for classes.
 Protection has module granularity so looking up private
 members of a base class from a different module follows
 the same rules as accessing other private symbols from
 a different module.
 Additionally protected allows access from derived classes
 but not from other modules.
  • Alias protection overrides the protection of the aliased symbol.
 A public alias to a private symbol makes the symbol
 accessibly through the alias. The alias itself needs to be
 in the same module, so this doesn't impair protection control.

other protection attribute changes

  • public stays the same
  • package matches private changes from the point of view of other packages
  • extern stays the same
  • protected matches private changes, descendants still treat protected symbols as public ones.

Possible code breakage and solutions

No previously valid code will become illegal in normal use cases, as this proposal is more permissive than current behavior. As __traits and .tupleof will still work for private as before, any library that relies on them should not break.

Walters concerns

original comment

1. what access means at module scope

"Does this symbol is ignored when doing symbol name look-up?". All protection attributes boil down to simple answer (Yes/No) depending on symbol origins and place look-up is made from. In example:

Symbol origin:               module a;
Look-up origin:              not module a;
Symbol protection attribute: private
Answer:                      No

2. at class scope

D minimal encapsulation unit is a module. Private class members are, technically, private module members and thus have the same behavior. Same for package and public. Protected is only special case that takes additional parameter into consideration.

3. at template mixin scope

No changes here. For templates look-up origin is definition module. For mixin templates - instantiation module. Other than that, usual rules apply.

4. backwards compatibility

See "Possible code breakage and solutions"

5. overloading at each scope level and the interactions with access

See "Description".

6. I'd also throw in getting rid of the "protected" access attribute completely, as I've seen debate over that being a useless idea

I have found no harm in keeping it. This will break code for sure and is irrelevant to this DIP topic.

7. there's also some debate about what "package" should mean

This is also irrelevant to this DIP. While there may be debates on meaning of package concept, meaning of package protection attribute is solid: encapsulation within set of modules belonging to same package, whatever they are.


This document has been placed in the Public Domain.