import-export-module.md

title	module, import, export
ms.date	07/15/2019
f1_keywords	module_cpp import_cpp export_cpp
helpviewer_keywords	modules [C++] modules [C++], import modules [C++], export
ms.description	Use the import statement to access types and functions defined in the specified module.

module, import, export

The module, import, and export keywords are available in C++20 and require the /experimental:modules compiler switch along with /std:c++latest. For more information, see Overview of modules in C++.

module

Use the module statement at the beginning of a module implemetation file to specify that the file contents belong to the named module.

module ModuleA;

export

Use the export module statement 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 Bar
{
   export int f();
   export double d();
   double internal_f(); // not exported
}

Non-exported names are not visible to code that imports the module:

//MyProgram.cpp

import module ModuleA;

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

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

import

Use the import statement to make a module's names visible in your program. The import statement must appear after the module statement 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
{...};

See Also

Overview of modules in C++