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title /headerUnit (Use header unit IFC)
description Use the /headerUnit compiler option to specify an existing IFC header unit to import in the current compilation.
ms.date 09/13/2020
f1_keywords
/headerUnit
helpviewer_keywords
/headerUnit
Use header unit IFC

/headerUnit (Use header unit IFC)

Tells the compiler to translate #include directives for an importable header-name into an import header-name; directive, rather than use textual inclusion.

Syntax

/headerUnit header-filename=ifc-filename

Arguments

header-filename
The name of a file that the compiler resolves a header-name to. During import header-name ; the compiler resolves header-name to some file on disk. Use header-filename to specify that file. Once matched, the compiler opens the corresponding IFC named by ifc-filename for import.

ifc-filename
The name of a file that contains IFC data, prebuilt module information. To import more than one header unit, include a separate /headerUnit option for each file.

Remarks

The /headerUnit compiler option requires you enable experimental modules support by use of the /experimental:module compiler option, along with the /std:c++latest option. This option is available starting in Visual Studio 2019 version 16.8.

The compiler can't map a single header-name to multiple IFC files. While mapping multiple header-name arguments to a single IFC is possible, we don't recommend it. The contents of the IFC get imported as if it was only the header specified by header-name.

Examples

Given a project that references two header files and their header units, listed in this table:

Header file IFC file
C:\utils\util.h C:\util.h.ifc
C:\app\app.h C:\app.h.ifc

The compiler options to reference the header units for these particular header files might look like this example:

cl ... /experimental:module /translateInclude /headerUnit C:\utils\util.h=C:\util.h.ifc /headerUnit C:\app\app.h=C:\app.h.ifc

To set this compiler option in the Visual Studio development environment

  1. Open the project's Property Pages dialog box. For details, see Set C++ compiler and build properties in Visual Studio.

  2. Set the Configuration drop-down to All Configurations.

  3. Select the Configuration Properties > C/C++ > Command Line property page.

  4. Modify the Additional Options property to add the /headerUnit options and arguments. Then, choose OK or Apply to save your changes.

See also

/experimental:module (Enable module support)
/module:exportHeader (Create header units)
/module:reference (Use named module IFC)
/translateInclude (Translate include directives into import directives)\