10 Modules [module]

Translation unit #1:

export module A;
export import :Foo;
export int baz();

Translation unit #2:

export module A:Foo;
import :Internals;
export int foo() { return 2 * (bar() + 1); }

Translation unit #3:

module A:Internals;
int bar();

Translation unit #4:

module A;
import :Internals;
int bar() { return baz() - 10; }
int baz() { return 30; }

Translation unit #1:

module B:Y;                     // does not implicitly import B
int y();

Translation unit #2:

export module B;
import :Y;                      // OK, does not create interface dependency cycle
int n = y();

Translation unit #3:

module B:X1;                    // does not implicitly import B
int &a = n;                     // error: n not visible here

Translation unit #4:

module B:X2;                    // does not implicitly import B
import B;
int &b = n;                     // OK

Translation unit #5:

module B;                       // implicitly imports B
int &c = n;                     // OK

Source file "a.h":

export int x;

Translation unit #1:

module;
#include "a.h"                  // error: declaration of x is not in the
                                // purview of a module interface unit
export module M;
export namespace {}             // error: does not introduce any names
export namespace {
  int a1;                       // error: export of name with internal linkage
}
namespace {
  export int a2;                // error: export of name with internal linkage
}
export static int b;            // error: b explicitly declared static
export int f();                 // OK
export namespace N { }          // OK
export using namespace N;       // error: does not declare a name

Source file "b.h":

int f();

Importable header "c.h":

int g();

Translation unit #1:

export module X;
export int h();

Translation unit #2:

module;
#include "b.h"
export module M;
import "c.h";
import X;
export using ::f, ::g, ::h;     // OK
struct S;
export using ::S;               // error: S has module linkage
namespace N {
  export int h();
  static int h(int);            // #1
}
export using N::h;              // error: #1 has internal linkage

export module M;
struct S { int n; };
typedef S S;
export typedef S S;             // OK, does not redeclare an entity
export struct S;                // error: exported declaration follows non-exported declaration

Interface unit of M:

export module M;
export struct X {
  static void f();
  struct Y { };
};

namespace {
  struct S { };
}
export void f(S);               // OK
struct T { };
export T id(T);                 // OK

export struct A;                // A exported as incomplete

export auto rootFinder(double a) {
  return [=](double x) { return (x + a/x)/2; };
}

export const int n = 5;         // OK, n has external linkage

Implementation unit of M:

module M;
struct A {
  int value;
};

Main program:

import M;
int main() {
  X::f();                       // OK, X is exported and definition of X is reachable
  X::Y y;                       // OK, X​::​Y is exported as a complete type
  auto f = rootFinder(2);       // OK
  return A{45}.value;           // error: A is incomplete
}

Translation unit #1:

module M:Part;

Translation unit #2:

export module M;
export import :Part;    // error: exported partition :Part is an implementation unit

module M;
import M;               // error: cannot import M in its own unit

Interface unit of M1:

export module M1;
import M2;

Interface unit of M2:

export module M2;
import M3;

Interface unit of M3:

export module M3;
import M1;              // error: cyclic interface dependency $M3\to M1\to M2\to M3$

const int size = 2;
int ary1[size];                 // unspecified whether size is decl-reachable from ary1
constexpr int identity(int x) { return x; }
int ary2[identity(2)];          // unspecified whether identity is decl-reachable from ary2

template<typename> struct S;
template<typename, int> struct S2;
constexpr int g(int);

template<typename T, int N>
S<S2<T, g(N)>> f();             // S, S2, g, and ​::​ are decl-reachable from f

template<int N>
void h() noexcept(g(N) == N);   // g and ​::​ are decl-reachable from h

Source file "foo.h":

namespace N {
  struct X {};
  int d();
  int e();
  inline int f(X, int = d()) { return e(); }
  int g(X);
  int h(X);
}

Module M interface:

module;
#include "foo.h"
export module M;
template<typename T> int use_f() {
  N::X x;                       // N​::​X, N, and ​::​ are decl-reachable from use_­f
  return f(x, 123);             // N​::​f is decl-reachable from use_­f,
                                // N​::​e is indirectly decl-reachable from use_­f
                                //   because it is decl-reachable from N​::​f, and
                                // N​::​d is decl-reachable from use_­f
                                //   because it is decl-reachable from N​::​f
                                //   even though it is not used in this call
}
template<typename T> int use_g() {
  N::X x;                       // N​::​X, N, and ​::​ are decl-reachable from use_­g
  return g((T(), x));           // N​::​g is not decl-reachable from use_­g
}
template<typename T> int use_h() {
  N::X x;                       // N​::​X, N, and ​::​ are decl-reachable from use_­h
  return h((T(), x));           // N​::​h is not decl-reachable from use_­h, but
                                // N​::​h is decl-reachable from use_­h<int>
}
int k = use_h<int>();
  // use_­h<int> is decl-reachable from k, so
  // N​::​h is decl-reachable from k

Module M implementation:

module M;
int a = use_f<int>();           // OK
int b = use_g<int>();           // error: no viable function for call to g;
                                // g is not decl-reachable from purview of
                                // module M's interface, so is discarded
int c = use_h<int>();           // OK

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() {}

Translation unit #1:

export module stuff;
export template<typename T, typename U> void foo(T, U u) { auto v = u; }
export template<typename T, typename U> void bar(T, U u) { auto v = *u; }

Translation unit #2:

export module M1;
import "defn.h";        // provides struct X {};
import stuff;
export template<typename T> void f(T t) {
  X x;
  foo(t, x);
}

Translation unit #3:

export module M2;
import "decl.h";        // provides struct X; (not a definition)
import stuff;
export template<typename T> void g(T t) {
  X *x;
  bar(t, x);
}

Translation unit #4:

import M1;
import M2;
void test() {
  f(0);
  g(0);
}

Translation unit #1:

export module A;
struct X {};
export using Y = X;

Translation unit #2:

module B;
import A;
Y y;                // OK, definition of X is reachable
X x;                // error: X not visible to unqualified lookup