module, import, export

The module, import, and export declarations are available in C++20 and require the compiler switch /std:c++20 or later. For more information, see Overview of modules in C++.

module

Place a module declaration at the beginning of a module implementation file to specify that the file contents belong to the named module.

module ModuleA;

export

Use an export module declaration for the module's primary interface file, which has an extension .ixx by default. If you want to use a different extension, use the /interface switch to compile it as a module interface.

export module ModuleA;

In an interface file, use the export modifier on names that are intended to be part of the public interface:

// ModuleA.ixx

export module ModuleA;

namespace ModuleA_NS
{
   export int f();
   export double d();
   double internal_f(); // not exported
}

Nonexported names aren't visible to code that imports the module:

import ModuleA;

int main() {
  ModuleA_NS::f(); // OK
  ModuleA_NS::d(); // OK
  ModuleA_NS::internal_f(); // Ill-formed: error C2065: 'internal_f': undeclared identifier
}

The export keyword may not appear in a module implementation file. When export is applied to a namespace name, all names in the namespace are exported.

import

Use an import declaration to make a module's names visible in your program. The import declaration must appear after the module declaration and after any #include directives, but before any declarations in the file.

module ModuleA;

#include "custom-lib.h"
import std;
import myModule;

// begin declarations here:
template <class T>
class Baz
{...};

Remarks

Both import and module are treated as keywords only when they appear at the start of a logical line:

// OK:
module ;
module module-name
import :
import <
import "
import module-name
export module ;
export module module-name
export import :
export import <
export import "
export import module-name

// Error:
int i; module ;

Microsoft Specific

In Microsoft C++, the tokens import and module are always identifiers and never keywords when they're used as arguments to a macro.

Example

#define foo(...) __VA_ARGS__
foo(
import // Always an identifier, never a keyword
)

End Microsoft Specific

See also

Overview of modules in C++
Import the C++ standard library using modules