import-export-module.md

title	ms.date	f1_keywords	helpviewer_keywords	description
module, import, export	02/14/2022	module_cpp import_cpp export_cpp	modules [C++] modules [C++], import modules [C++], export	Use import and export declarations to access and to publish types and functions defined in the specified module.

module, import, export

The module, import, and export declarations are available in C++20 and require the /experimental:module compiler switch along with /std:c++20 or later (such as /std:c++latest). 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 must have extension .ixx:

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
}

Non-exported names aren't visible to code that imports the module:

//MyProgram.cpp

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.core;
import std.regex;
import ModuleB;

// 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++