Extra Clang Tools 3.8 documentation

clang-modernize Usage

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clang-modernize Usage

clang-modernize [options] [<sources>...] [-- [args]]

<source#> specifies the path to the source to migrate. This path may be relative to the current directory. If no sources are provided, a compilation database provided with -p can be used to provide sources together with the include/exclude options.

By default all transformations are applied. There are two ways to enable a subset of the transformations:

  1. Explicitly, by referring to the transform options directly, see Transform-Specific Command Line Options.
  2. Implicitly, based on the compilers to support, see -for-compilers=<string>.

If both ways of specifying transforms are used only explicitly specified transformations that are supported by the given compilers will be applied.

General Command Line Options


Displays tool usage instructions and command line options.


Displays the version information of this tool.


<build-path> is the directory containing a compilation databasefile, a file named compile_commands.json, which provides compiler arguments for building each source file. CMake can generate this file by specifying -DCMAKE_EXPORT_COMPILE_COMMANDS=ON when running CMake. Ninja, since v1.2 can also generate this file with ninja -t compdb. If the compilation database cannot be used for any reason, an error is reported.

This option is ignored if -- is present.

Files in the compilation database will be transformed if no sources are provided and paths to files are explicitly included using -include or -include-from. In order to transform all files in a compilation database the following command line can be used:

clang-modernize -p=<build-path> -include=<project_root>

Use -exclude or -exclude-from to limit the scope of -include.

-- [args]

Another way to provide compiler arguments is to specify all arguments on the command line following --. Arguments provided this way are used for every source file.

If neither -- nor -p are specified a compilation database is searched for starting with the path of the first-provided source file and proceeding through parent directories. If no compilation database is found or one is found and cannot be used for any reason then -std=c++11 is used as the only compiler argument.


Some transformations may cause a change in semantics. In such cases the maximum acceptable risk level specified through the -risk command line option decides whether or not a transformation is applied.

Three different risk level options are available:

Perform only safe transformations.
-risk=reasonable (default)
Enable transformations that may change semantics.
Enable transformations that are likely to change semantics.

The meaning of risk is handled differently for each transform. See transform documentation for details.


After applying the final transform to a file, parse the file to ensure the last transform did not introduce syntax errors. Syntax errors introduced by earlier transforms are already caught when subsequent transforms parse the file.


Displays a summary of the number of changes each transform made or could have made to each source file immediately after each transform is applied. Accepted changes are those actually made. Rejected changes are those that could have been made if the acceptable risk level were higher. Deferred changes are those that might be possible but they might conflict with other accepted changes. Re-applying the transform will resolve deferred changes.


Select transforms targeting the intersection of language features supported by the given compilers.

Four compilers are supported. The transforms are enabled according to this table:

Transforms clang gcc icc mscv
AddOverride (1) 3.0 4.7 14 8
LoopConvert 3.0 4.6 13 11
PassByValue 3.0 4.6 13 11
ReplaceAutoPtr 3.0 4.6 13 11
UseAuto 2.9 4.4 12 10
UseNullptr 3.0 4.6 12.1 10

(1): if -override-macros is provided it’s assumed that the macros are C++11 aware and the transform is enabled without regard to the supported compilers.

The structure of the argument to the -for-compilers option is <compiler>-<major ver>[.<minor ver>] where <compiler> is one of the compilers from the above table.

Some examples:

  1. To support Clang >= 3.0, gcc >= 4.6 and MSVC >= 11:

    clang-modernize -for-compilers=clang-3.0,gcc-4.6,msvc-11 <args..>

    Enables LoopConvert, ReplaceAutoPtr, UseAuto, UseNullptr.

  2. To support icc >= 12 while using a C++11-aware macro for the override virtual specifier:

    clang-modernize -for-compilers=icc-12 -override-macros <args..>

    Enables AddOverride and UseAuto.


If your version of Clang depends on the GCC headers (e.g: when libc++ is not used), then you probably want to add the GCC version to the targeted platforms as well.


Turns on performance measurement and output functionality. The time it takes to apply each transform is recorded by the migrator and written in JSON format to a uniquely named file in the given <directory>. All sources processed by a single Modernizer process are written to the same output file. If <directory> is not provided the default is ./migrate_perf/.

The time recorded for a transform includes parsing and creating source code replacements.


Causes the modernizer to generate replacements and serialize them to disk but not apply them. This can be useful for debugging or for manually running clang-apply-replacements. Replacements are serialized in YAML format. By default serialzied replacements are written to a temporary directory whose name is written to stderr when serialization is complete.


Choose a directory to serialize replacements to. The directory must exist.

Path Inclusion/Exclusion Options


Use this option to indicate which directories contain files that can be changed by the modernizer. Inidividual files may be specified if desired. Multiple paths can be specified as a comma-separated list. Sources mentioned explicitly on the command line are always included so this option controls which other files (e.g. headers) may be changed while transforming translation units.


Used with -include to provide finer control over which files and directories can be transformed. Individual files and files within directories specified by this option will not be transformed. Multiple paths can be specified as a comma-separated list.


Like -include but read paths from the given file. Paths should be one per line.


Like -exclude but read paths from the given file. Paths are listed one per line.

Formatting Command Line Options


Enable reformatting of code changed by transforms. Formatting is done after every transform.


Specifies how formatting should be done. The behaviour of this option is identical to the same option provided by clang-format. Refer to clang-format’s style options for more details.


When using -style=file, the default behaviour is to look for .clang-format starting in the current directory and then in ancestors. To specify a directory to find the style configuration file, use this option.


  // file.cpp
  for (std::vector<int>::const_iterator I = my_container.begin(),
                                        E = my_container.end();
       I != E; ++I) {
    std::cout << *I << std::endl;

  // No reformatting:
  //     clang-modernize -use-auto file.cpp
  for (auto I = my_container.begin(),
                                        E = my_container.end();
       I != E; ++I) {
    std::cout << *I << std::endl;

  // With reformatting enabled:
  //     clang-modernize -format -use-auto file.cpp
  for (auto I = my_container.begin(), E = my_container.end(); I != E; ++I) {
    std::cout << *I << std::endl;

Transform-Specific Command Line Options


Makes use of C++11 range-based for loops where possible. See Loop Convert Transform.


Makes use of the new C++11 keyword nullptr where possible. See Use-Nullptr Transform.


<string> is a comma-separated list of user-defined macros that behave like the NULL macro. The -use-nullptr transform will replace these macros along with NULL. See Use-Nullptr Transform.


Replace the type specifier of variable declarations with the auto type specifier. See Use-Auto Transform.


Adds the override specifier to member functions where it is appropriate. That is, the override specifier is added to member functions that override a virtual function in a base class and that don’t already have the specifier. See Add-Override Transform.


Tells the Add Override Transform to locate a macro that expands to override and use that macro instead of the override keyword directly. If no such macro is found, override is still used. This option enables projects that use such macros to maintain build compatibility with non-C++11 code.


Replace const-reference parameters by values in situations where it can be beneficial. See Pass-By-Value Transform.


Replace std::auto_ptr (deprecated in C++11) by std::unique_ptr and wrap calls to the copy constructor and assignment operator with std::move(). See Replace-AutoPtr Transform.

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