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This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 119a. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2025-12-20

2588. friend declarations and module linkage

[Accepted as a DR at the June, 2024 meeting.]

Consider:

  export module Foo;  class X {    friend void f(X); // #1 linkage?  };

Subclause 11.8.4 [class.friend] paragraph 4 gives #1 external linkage:

A function first declared in a friend declaration has the linkage ofthe namespace of which it is a member (6.7 [basic.link]).

(There is no similar provision for friend classes first declared ina class.)

However, 6.7 [basic.link] bullet 4.8 gives it module linkage:

... otherwise, if the declaration of the name is attached to a namedmodule (10.1 [module.unit]) and is not exported(10.2 [module.interface]), the name has module linkage;

Subclause 10.2 [module.interface] paragraph 2 does not apply:

A declaration isexported if it is declared withinanexport-declaration and inhabits a namespace scope or it is

• anamespace-definition that contains an exporteddeclaration, or
• a declaration within a header unit(10.3 [module.import]) that introduces at least one name.

Also consider this related example:

  export module Foo;  export class Y;  // maybe many lines later, or even a different partition of Foo  class Y {    friend void f(Y); // #2 linkage?  };

• Should the friend's linkage be affected by the linkage of thebefriending class? In this example, #2 would therefore have externallinkage, asY is exported.
• Or should the friend's linkage be affected by the presence orabsence ofexport on the class definition itself? In thisexample, #2 would thus have module linkage.
• Or should the friend's linkage be determined ignoring anyenclosingexport and ignoring whether the enclosing class isexported, per 11.8.4 [class.friend] paragraph 4 (alone)?
• Or should the friend's linkage be as-if the declaration inhabitedits nearest enclosing namespace scope, withoutthefriend?

Seeissue 2607 for a similar questionabout enumerators.

Additional note (May, 2022):

Forwarded to EWG withpaper issue 1253,by decision of the CWG chair.

EWG telecon 2022-06-09

Consensus: "A friend's linkage should be affected by thepresence/absence of export on the containing class definition itself,but ONLY if the friend is a definition", pending confirmation byelectronic polling.

Proposed resolution (June, 2022) (approved by CWG 2023-01-27) [SUPERSEDED]:

1. Change in 6.7 [basic.link] paragraph 4 as follows:

   ... The name of an entity that belongs to a namespace scope that has notbeen given internal linkage above and that is the name of

   • a variable; or
   • a function; or
   • a named class (11.1 [class.pre]), or an unnamed classdefined in a typedef declaration in which the class has the typedefname for linkage purposes (9.2.4 [dcl.typedef]); or
   • a named enumeration (9.8.1 [dcl.enum]), or an unnamedenumeration defined in a typedef declaration in which the enumerationhas the typedef name for linkage purposes(9.2.4 [dcl.typedef]); or
   • an unnamedenumeration that has an enumerator as a name for linkage purposes(9.8.1 [dcl.enum]); or
   • a template

   has its linkagedetermined as follows:

   • if the entity is a function or function template firstdeclared in a friend declaration and that declaration is a definition,the name has the same linkage, if any, as the name of the enclosingclass (11.8.4 [class.friend]);
   • otherwise, if the entity is a function or function templatedeclared in a friend declaration and a corresponding non-frienddeclaration is reachable, the name has the linkage determined fromthat prior declaration,
   • otherwise, if the enclosing namespace has internallinkage, the name has internal linkage;
   • otherwise, if thedeclaration of the name is attached to a named module(10.1 [module.unit]) and is not exported(10.2 [module.interface]), the name has module linkage;
   • otherwise, the name has external linkage.

2. Remove 11.8.4 [class.friend] paragraph 4:

   A function first declared in a friend declaration has thelinkage of the namespace of which it is a member(6.7 [basic.link]). Otherwise, the function retains itsprevious linkage (9.2.2 [dcl.stc]).

EWG electronic poll 2022-06

Consensus for "A friend's linkage should be affected by thepresence/absence of export on the containing class definition itself,but ONLY if the friend is a definition (option #2, modified by Jason'ssuggestion). This resolves CWG2588."Seevote.

Proposed resolution (approved by CWG 2024-03-20):

1. Change in 6.7 [basic.link] paragraph 4 as follows:

   ... The name of an entity that belongs to a namespace scope that has notbeen given internal linkage above and that is the name of

   • a variable; or
   • a function; or
   • a named class (11.1 [class.pre]), or an unnamed classdefined in a typedef declaration in which the class has the typedefname for linkage purposes (9.2.4 [dcl.typedef]); or
   • a named enumeration (9.8.1 [dcl.enum]), or an unnamedenumeration defined in a typedef declaration in which the enumerationhas the typedef name for linkage purposes(9.2.4 [dcl.typedef]); or
   • an unnamedenumeration that has an enumerator as a name for linkage purposes(9.8.1 [dcl.enum]); or
   • a template

   has its linkagedetermined as follows:

   • if the entity is a function or function template firstdeclared in a friend declaration and that declaration is a definitionand the enclosing class is defined within anexport-declaration,the name has the same linkage, if any, as the name of the enclosingclass (11.8.4 [class.friend]);
   • otherwise, if the entity is a function or function templatedeclared in a friend declaration and a corresponding non-frienddeclaration is reachable, the name has the linkage determined fromthat prior declaration,
   • otherwise, if the enclosing namespace has internallinkage, the name has internal linkage;
   • otherwise, if thedeclaration of the name is attached to a named module(10.1 [module.unit]) and is not exported(10.2 [module.interface]), the name has module linkage;
   • otherwise, the name has external linkage.

2. Remove 11.8.4 [class.friend] paragraph 4:

   A function first declared in a friend declaration has thelinkage of the namespace of which it is a member(6.7 [basic.link]). Otherwise, the function retains itsprevious linkage (9.2.2 [dcl.stc]).