10 Modules [module]

10.5 Private module fragment [module.private.frag]

private-module-fragment:
	module : private ; top-level-declaration-seq
A private-module-fragment shall appear only in a primary module interface unit ([module.unit]).
A module unit with a private-module-fragment shall be the only module unit of its module; no diagnostic is required.
Note
:
A private-module-fragment ends the portion of the module interface unit that can affect the behavior of other translation units.
A private-module-fragment allows a module to be represented as a single translation unit without making all of the contents of the module reachable to importers.
The presence of a private-module-fragment affects:
— end note
 ]
Example
:
export module A;
export inline void fn_e();      // error: exported inline function fn_­e not defined
                                // before private module fragment
inline void fn_m();             // OK, module-linkage inline function
static void fn_s();
export struct X;
export void g(X *x) {
  fn_s();                       // OK, call to static function in same translation unit
  fn_m();                       // OK, call to module-linkage inline function
}
export X *factory();            // OK

module :private;
struct X {};                    // definition not reachable from importers of A
X *factory() {
  return new X ();
}
void fn_e() {}
void fn_m() {}
void fn_s() {}
— end example
 ]