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title /headerUnit (Use header unit IFC)
description Use the /headerUnit compiler option to associate a header file with the header unit to import in its place.
ms.date 02/01/2022
f1_keywords
/headerUnit
helpviewer_keywords
/headerUnit
Use header unit IFC
author tylermsft
ms.author twhitney

/headerUnit (Use header unit IFC)

Imports a header unit. Tells the compiler where to find the .ifc file (the binary representation of the header unit) for the specified header.

Syntax

/headerUnit header-filename=ifc-filename
/headerUnit:quote [header-filename=ifc-filename]
/headerUnit:angle [header-filename=ifc-filename]

Arguments

header-filename
During import header-name; the compiler resolves header-name to a 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 compiled header unit information. To import more than one header unit, add a separate /headerUnit option for each file.

Remarks

The /headerUnit compiler option requires /std:c++20 or later.

The /headerUnit compiler option is available in Visual Studio 2019 version 16.10 or later.

When the compiler comes across import "file"; or import <file>; this compiler option helps the compiler find the compiled header unit (.ifc) for the specified header file. The path to this file can be expressed in these ways:

  • /headerUnit looks up the compiled header unit in the current directory, or at the location specified by ifc-filename.

  • /headerUnit:quote looks up the compiled header unit file using the same rules as #include "file".

  • /headerUnit:angle looks up the compiled header unit file using the same rules as #include <file>.

The compiler can't map a single header-name to multiple .ifc files. Mapping multiple header-name arguments to a single .ifc is possible, but it isn't recommended. The contents of the .ifc are imported as if it was only the header specified by header-name.

The compiler implicitly enables the new preprocessor when this option is used. If any form of /headerUnit is specified on the command line, then /Zc:preprocessor is added to the command line by the compiler. To opt out of the implicit /Zc:preprocessor, specify: /Zc:preprocessor-

If you disable the new preprocessor, but a file you compile imports a header unit, the compiler will report an error.

Examples

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

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

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

cl ... /std:c++latest /headerUnit C:\utils\util.h=C:\util.h.ifc /headerUnit:quote app.h=app.h.ifc

To set this compiler option in the Visual Studio development environment

You normally shouldn't set this in the Visual Studio development environment. It's set by the build system.

See also

/exportHeader (Create header units)
/headerName (Create a header unit from the specified header)
/reference (Use named module IFC)
/translateInclude (Translate include directives into import directives)