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/external (External headers diagnostics)

  • Artikel

The /external compiler options let you specify compiler diagnostic behavior for certain header files. "External" headers are the natural complement of "Just my code": Header files such as system files or third-party library files that you can't or don't intend to change. Since you aren't going to change these files, you may decide it isn't useful to see diagnostic messages from the compiler about them. The /external compiler options give you control over these warnings.

The /external compiler options are available starting in Visual Studio 2017 version 15.6. In versions of Visual Studio before Visual Studio 2019 version 16.10, the /external options require you also set the /experimental:external compiler option.

Syntax

Use external header options (Not required in 16.10 and later):

/experimental:external

Specify external headers:

/external:anglebrackets
/external:env:var
/external:I path

Specify diagnostics behavior:

/external:W0
/external:W1
/external:W2
/external:W3
/external:W4
/external:templates-

Arguments

/experimental:external
Enables the external headers options. This option isn't required in Visual Studio 2019 version 16.10 and later.

/external:anglebrackets
Treats all headers included by #include <header>, where the header file is enclosed in angle brackets (< >), as external headers.

/external:I path
Defines a root directory that contains external headers. All recursive subdirectories of path are considered external, but only the path value is added to the list of directories the compiler searches for include files. The space between /external:I and path is optional. Directories that include spaces must be enclosed in double quotes. A directory may be an absolute path or a relative path.

/external:env:var
Specifies the name of an environment variable var that holds a semicolon-separated list of external header directories. It's useful for build systems that rely on environment variables such as INCLUDE, which you use to specify the list of external include files. Or, CAExcludePath, for files that shouldn't be analyzed by /analyze. For example, you can specify /external:env:INCLUDE to make every directory in INCLUDE an external header directory at once. It's the same as using /external:I to specify the individual directories, but much less verbose. There should be no space between var and /external:env:.

/external:Wn
This option sets the default warning level to n (a value from 0 to 4) for external headers. For example, /external:W0 effectively turns off warnings for external headers. If this option isn't specified, the compiler issues command line warning D9007 for other /external options. Those options are ignored, because they would have no effect.

The /external:Wn option has an effect similar to wrapping an included header in a #pragma warning directive:

#pragma warning (push, 0)
// the global warning level is now 0 here
#include <external_header>
#pragma warning (pop)

/external:templates-
Allows warnings from external headers when they occur in a template that's instantiated in your code.

Remarks

By default, the /Wn warning level you specify for your build applies to all files. The options to specify external headers only define a set of files to which you can apply a different default warning level. So, if you specify external headers, also use /external:Wn to specify an external warning level to change compiler behavior.

All the existing mechanisms to enable, disable, and suppress warnings still work in both external and non-external files. For example, a warning pragma can still override the default warning level you set for external headers.

Example: Set external warning level

This sample program has two source files, program.cpp and header_file.h. The header_file.h file is in an include_dir subdirectory of the directory containing the program.cpp file:

Source file include_dir/header_file.h:

// External header: include_dir/header_file.h

template <typename T>
struct sample_struct
{
    static const T value = -7; // W4: warning C4245: 'initializing':
    // conversion from 'int' to 'unsigned int', signed/unsigned mismatch
};

Source file program.cpp:

// User code: program.cpp
#include <header_file.h>

int main()
{
    return sample_struct<unsigned int>().value;
}

You can build the sample by using this command line:

cl /EHsc /I include_dir /W4 program.cpp

As expected, this sample generates a warning:

program.cpp
include_dir\header_file.h(6): warning C4245: 'initializing': conversion from 'int' to 'const T', signed/unsigned mismatch
        with
        [
            T=unsigned int
        ]
program.cpp(6): note: see reference to class template instantiation 'sample_struct<unsigned int>' being compiled

To treat the header file as an external file and suppress the warning, you can use this command line instead*:

cl /EHsc /I include_dir /external:anglebrackets /external:W0 /W4 program.cpp

This command line suppresses the warning inside header_file.h while preserving warnings inside program.cpp.

Warnings across the internal and external boundary

Setting a low warning level for external headers can hide some actionable warnings. In particular, it can turn off warnings emitted on template instantiations in user code. These warnings might indicate a problem in your code that only happens in instantiations for particular types. (For example, if you forgot to apply a type trait removing const or &.) To avoid silencing warnings inside templates defined in external headers, you can use the /external:templates- option. The compiler considers both the effective warning level in the file that defines the template, and the warning level where template instantiation occurs. Warnings emitted inside an external template appear if the template is instantiated within non-external code. For example, this command line re-enables warnings from template sources in the sample code*:

cl /EHsc /I include_dir /external:anglebrackets /external:W0 /external:templates- /W4 program.cpp

The C4245 warning appears again in the output, even though the template code is inside an external header.

Enable, disable, or suppress warnings

All the existing mechanisms to enable, disable, and suppress warnings still work in external headers. When a warning appears because you use the /external:templates- option, you can still suppress the warning at the point of instantiation. For example, to explicitly suppress the warning in the sample that reappears because of /external:templates-, use a warning pragma directive:

int main()
{
    #pragma warning( suppress : 4245)
    return sample_struct<unsigned int>().value;
}

Library writers can use the same mechanisms to enforce certain warnings, or all the warnings at certain level, if they feel those warnings should never be silenced by /external:Wn. For example, this version of the header file forces warning C4245 to report an error:

// External header: include_dir/header_file.h

#pragma warning( push, 4 )
#pragma warning( error : 4245 )

template <typename T>
struct sample_struct
{
    static const T value = -7; // W4: warning C4245: 'initializing': conversion from 'int'
                               // to 'unsigned int', signed/unsigned mismatch
};

#pragma warning( pop )

With this change to the library header, the author of the library ensures that the global warning level in this header is 4, no matter what gets specified in /external:Wn. Now all level 4 and above warnings are reported. The library author can also force certain warnings to be errors, disabled, suppressed, or emitted only once in the header. The /external options don't override that deliberate choice.

system_header pragma

#pragma system_header is an intrusive marker that allows library writers to mark certain headers as external. A file containing #pragma system_header is considered external from the point of the pragma to the end of the file, as if it were specified as external on the command line. The compiler emits any diagnostics after the pragma at the warning level specified by /external:Wn. For more information, see system_header pragma.

Limitations

Some warnings emitted by the compiler's back-end code generation aren't affected by the /external options. These warnings usually start with C47XX, though not all C47XX warnings are back-end warnings. You can still disable these warnings individually by using /wd47XX. Code analysis warnings are also unaffected, since they don't have warning levels.

To set this compiler option in the Visual Studio development environment

In Visual Studio 2019 version 16.10 and later:

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

  2. Select the Configuration Properties > VC++ Directories property page.

  3. Set the External Include Directories property to specify the IDE equivalent of the /external:I path option for each semicolon-delimited path.

  4. Select the Configuration Properties > C/C++ > External Includes property page.

  5. Set properties:

    • Set Treat Files Included with Angle Brackets as External to Yes to set the /external:anglebrackets option.

    • External Header Warning Level allows you to set the /external:Wn option. If this value is set to Inherit Project Warning Level or the default, other /external options are ignored.

    • Set Template Diagnostics in External Headers to Yes to set the /external:templates- option.

  6. Choose OK or Apply to save your changes.

In versions of Visual Studio before Visual Studio 2019 version 16.10:

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

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

  3. Enter the /experimental:external option and other /external compiler options in the Additional Options box.

  4. Choose OK or Apply to save your changes.

To set this compiler option programmatically

* Add the /experimental:external option to enable the external headers options in versions of Visual Studio before Visual Studio 2019 version 16.10.

See also

MSVC compiler options
MSVC compiler command-line syntax