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Title: Private symbol visibility
DIP: 22
Version: 1
Status: Draft
Created: 2013-01-28
Last Modified: 2013-01-28
Author: Михаил Страшун (m.strashun gmail.com) (Dicebot)
Links: Access specifiers and visibility : data gathered before creating proposal


There are two important issues with current protection attribute design:

  • Senseless name clashes between private and public symbols
  • No way to specify internal linkage storage class

This DIP addresses both of them with two decoupled proposals:

  • Change of private related name resolution rules
  • Definition of static storage class for module-scope symbols.


private is private

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

static is internal

If private is a source code level encapsulation tool then static is object file / binary level encapsulation tool. It allows even more struct separation of implementation-specific code and cleans up object files from unneeded symbols, providing more freedom for compiler optimizations.


private-related changes

  • All changes are same for both classes and modules: it breaks no code but leaves consistent approach.
  • Compiler errors upon access to private symbol are changed from "%s is not accessible" to "undefined identifier %s"
  • Overload resolution takes place after protection attribute questions are settled. private symbols do not take part in overload resolution.
  • As privates are already non-virtual, override resolution does not need to change in this regard.
  • All __traits still show private symbols. Those are advanced user tools and having this may be essential to library code.

new meaning for static

Currently static storage class is no-op for global symbols. It is a big luck as we can use it in the way similar to plain C:

  • All global static symbols are implicitly private
  • It is forbidden to leak static symbols outside of the module, i.e.
static struct Internal { }
void func1(Internal) { } // compile error
private void func2(Internal) { } // compile error, private still leaks its symbols to object file
void func3() { Internal t; } // fine, does not leak to func3 signature
alias Internal External; // compile error
static alias Internal InternalPrim; // fine
  • As can have just been seen, nested symbols can also be marked as static in case there were already static entities: enums, structs, classes, aliases. Variables and functions already have static defined for other usaged
  • No static symbols can be found in resulting object file. They are not shown in traits like __traits(allMembers). From the point of view of other modules they simply do not exist and may be all inlined or optimized away or whatever.
  • Only module-level symbols can be marked with global static, for example, nested struct types can't. It is minor limitation that greatly simplifies interaction with other language features like alias this.

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, descendant still treat protected symbols as public ones. It is a rare guest in idiomatic D code but does no harm and may be useful for transition from other languages.

other name resolution changes

  • UFCS functions should also take priority over private class functions when names conflict
  • alias this protection attribute overrides protection attribute of aliased symbol ( not transitive )

Possible code breakage and solutions

  • global static had no meaning before and should not have been used like that. It may happen that is used by accident or in generic code gen. To be absolutely sure this brings no surprises I suggest to create a dmd option that will find and print all usages of static in global context. It should appear in a release _before_ this DIP release and kept for one release after, then removed.
  • private simply replaces one error with another and allows new legal code. Code breakage very unlikely.
  • as __traits 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 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.