==========
Clang-Tidy
==========

.. contents::

See also:

.. toctree::
   :maxdepth: 1

   The list of clang-tidy checks <checks/list>
   Clang-tidy IDE/Editor Integrations <Integrations>
   Getting Involved <Contributing>

:program:`clang-tidy` is a clang-based C++ "linter" tool. Its purpose is to
provide an extensible framework for diagnosing and fixing typical programming
errors, like style violations, interface misuse, or bugs that can be deduced via
static analysis. :program:`clang-tidy` is modular and provides a convenient
interface for writing new checks.


Using clang-tidy
================

:program:`clang-tidy` is a `LibTooling`_-based tool, and it's easier to work
with if you set up a compile command database for your project (for an example
of how to do this, see `How To Setup Tooling For LLVM`_). You can also specify
compilation options on the command line after ``--``:

.. code-block:: console

  $ clang-tidy test.cpp -- -Imy_project/include -DMY_DEFINES ...

:program:`clang-tidy` has its own checks and can also run Clang Static Analyzer
checks. Each check has a name and the checks to run can be chosen using the
``-checks=`` option, which specifies a comma-separated list of positive and
negative (prefixed with ``-``) globs. Positive globs add subsets of checks, and
negative globs remove them. For example,

.. code-block:: console

  $ clang-tidy test.cpp -checks=-*,clang-analyzer-*,-clang-analyzer-cplusplus*

will disable all default checks (``-*``) and enable all ``clang-analyzer-*``
checks except for ``clang-analyzer-cplusplus*`` ones.

The ``-list-checks`` option lists all the enabled checks. When used without
``-checks=``, it shows checks enabled by default. Use ``-checks=*`` to see all
available checks or with any other value of ``-checks=`` to see which checks are
enabled by this value.

.. _checks-groups-table:

There are currently the following groups of checks:

====================== =========================================================
Name prefix            Description
====================== =========================================================
``abseil-``            Checks related to Abseil library.
``altera-``            Checks related to OpenCL programming for FPGAs.
``android-``           Checks related to Android.
``boost-``             Checks related to Boost library.
``bugprone-``          Checks that target bug-prone code constructs.
``cert-``              Checks related to CERT Secure Coding Guidelines.
``clang-analyzer-``    Clang Static Analyzer checks.
``concurrency-``       Checks related to concurrent programming (including
                       threads, fibers, coroutines, etc.).
``cppcoreguidelines-`` Checks related to C++ Core Guidelines.
``darwin-``            Checks related to Darwin coding conventions.
``fuchsia-``           Checks related to Fuchsia coding conventions.
``google-``            Checks related to Google coding conventions.
``hicpp-``             Checks related to High Integrity C++ Coding Standard.
``linuxkernel-``       Checks related to the Linux Kernel coding conventions.
``llvm-``              Checks related to the LLVM coding conventions.
``llvmlibc-``          Checks related to the LLVM-libc coding standards.
``misc-``              Checks that we didn't have a better category for.
``modernize-``         Checks that advocate usage of modern (currently "modern"
                       means "C++11") language constructs.
``mpi-``               Checks related to MPI (Message Passing Interface).
``objc-``              Checks related to Objective-C coding conventions.
``openmp-``            Checks related to OpenMP API.
``performance-``       Checks that target performance-related issues.
``portability-``       Checks that target portability-related issues that don't
                       relate to any particular coding style.
``readability-``       Checks that target readability-related issues that don't
                       relate to any particular coding style.
``zircon-``            Checks related to Zircon kernel coding conventions.
====================== =========================================================

Clang diagnostics are treated in a similar way as check diagnostics. Clang
diagnostics are displayed by :program:`clang-tidy` and can be filtered out using
the ``-checks=`` option. However, the ``-checks=`` option does not affect
compilation arguments, so it cannot turn on Clang warnings which are not
already turned on in the build configuration. The ``-warnings-as-errors=``
option upgrades any warnings emitted under the ``-checks=`` flag to errors (but
it does not enable any checks itself).

Clang diagnostics have check names starting with ``clang-diagnostic-``.
Diagnostics which have a corresponding warning option, are named
``clang-diagnostic-<warning-option>``, e.g. Clang warning controlled by
``-Wliteral-conversion`` will be reported with check name
``clang-diagnostic-literal-conversion``.

The ``-fix`` flag instructs :program:`clang-tidy` to fix found errors if
supported by corresponding checks.

An overview of all the command-line options:

.. code-block:: console

  $ clang-tidy --help
  USAGE: clang-tidy [options] <source0> [... <sourceN>]

  OPTIONS:

  Generic Options:

    --help                           - Display available options (--help-hidden for more)
    --help-list                      - Display list of available options (--help-list-hidden for more)
    --version                        - Display the version of this program

  clang-tidy options:

    --checks=<string>                - Comma-separated list of globs with optional '-'
                                       prefix. Globs are processed in order of
                                       appearance in the list. Globs without '-'
                                       prefix add checks with matching names to the
                                       set, globs with the '-' prefix remove checks
                                       with matching names from the set of enabled
                                       checks. This option's value is appended to the
                                       value of the 'Checks' option in .clang-tidy
                                       file, if any.
    --config=<string>                - Specifies a configuration in YAML/JSON format:
                                         -config="{Checks: '*',
                                                   CheckOptions: {x: y}}"
                                       When the value is empty, clang-tidy will
                                       attempt to find a file named .clang-tidy for
                                       each source file in its parent directories.
    --config-file=<string>           - Specify the path of .clang-tidy or custom config file:
                                        e.g. --config-file=/some/path/myTidyConfigFile
                                       This option internally works exactly the same way as
                                        --config option after reading specified config file.
                                       Use either --config-file or --config, not both.
    --dump-config                    - Dumps configuration in the YAML format to
                                       stdout. This option can be used along with a
                                       file name (and '--' if the file is outside of a
                                       project with configured compilation database).
                                       The configuration used for this file will be
                                       printed.
                                       Use along with -checks=* to include
                                       configuration of all checks.
    --enable-check-profile           - Enable per-check timing profiles, and print a
                                       report to stderr.
    --enable-module-headers-parsing  - Enables preprocessor-level module header parsing
                                       for C++20 and above, empowering specific checks
                                       to detect macro definitions within modules. This
                                       feature may cause performance and parsing issues
                                       and is therefore considered experimental.
    --exclude-header-filter=<string> - Regular expression matching the names of the
                                       headers to exclude diagnostics from. Diagnostics
                                       from the main file of each translation unit are
                                       always displayed.
                                       Must be used together with --header-filter.
                                       Can be used together with -line-filter.
                                       This option overrides the 'ExcludeHeaderFilterRegex'
                                       option in .clang-tidy file, if any.
    --explain-config                 - For each enabled check explains, where it is
                                       enabled, i.e. in clang-tidy binary, command
                                       line or a specific configuration file.
    --export-fixes=<filename>        - YAML file to store suggested fixes in. The
                                       stored fixes can be applied to the input source
                                       code with clang-apply-replacements.
    --extra-arg=<string>             - Additional argument to append to the compiler command line
    --extra-arg-before=<string>      - Additional argument to prepend to the compiler command line
    --fix                            - Apply suggested fixes. Without -fix-errors
                                       clang-tidy will bail out if any compilation
                                       errors were found.
    --fix-errors                     - Apply suggested fixes even if compilation
                                       errors were found. If compiler errors have
                                       attached fix-its, clang-tidy will apply them as
                                       well.
    --fix-notes                      - If a warning has no fix, but a single fix can
                                       be found through an associated diagnostic note,
                                       apply the fix.
                                       Specifying this flag will implicitly enable the
                                       '--fix' flag.
    --format-style=<string>          - Style for formatting code around applied fixes:
                                         - 'none' (default) turns off formatting
                                         - 'file' (literally 'file', not a placeholder)
                                           uses .clang-format file in the closest parent
                                           directory
                                         - '{ <json> }' specifies options inline, e.g.
                                           -format-style='{BasedOnStyle: llvm, IndentWidth: 8}'
                                         - 'llvm', 'google', 'webkit', 'mozilla'
                                       See clang-format documentation for the up-to-date
                                       information about formatting styles and options.
                                       This option overrides the 'FormatStyle` option in
                                       .clang-tidy file, if any.
    --header-filter=<string>         - Regular expression matching the names of the
                                       headers to output diagnostics from. Diagnostics
                                       from the main file of each translation unit are
                                       always displayed.
                                       Can be used together with -line-filter.
                                       This option overrides the 'HeaderFilterRegex'
                                       option in .clang-tidy file, if any.
    --line-filter=<string>           - List of files with line ranges to filter the
                                       warnings. Can be used together with
                                       -header-filter. The format of the list is a
                                       JSON array of objects:
                                         [
                                           {"name":"file1.cpp","lines":[[1,3],[5,7]]},
                                           {"name":"file2.h"}
                                         ]
    --list-checks                    - List all enabled checks and exit. Use with
                                       -checks=* to list all available checks.
    --load=<pluginfilename>          - Load the specified plugin
    -p <string>                      - Build path
    --quiet                          - Run clang-tidy in quiet mode. This suppresses
                                       printing statistics about ignored warnings and
                                       warnings treated as errors if the respective
                                       options are specified.
    --store-check-profile=<prefix>   - By default reports are printed in tabulated
                                       format to stderr. When this option is passed,
                                       these per-TU profiles are instead stored as JSON.
    --system-headers                 - Display the errors from system headers.
                                       This option overrides the 'SystemHeaders' option
                                       in .clang-tidy file, if any.
    --use-color                      - Use colors in diagnostics. If not set, colors
                                       will be used if the terminal connected to
                                       standard output supports colors.
                                       This option overrides the 'UseColor' option in
                                       .clang-tidy file, if any.
    --verify-config                  - Check the config files to ensure each check and
                                       option is recognized.
    --vfsoverlay=<filename>          - Overlay the virtual filesystem described by file
                                       over the real file system.
    --warnings-as-errors=<string>    - Upgrades warnings to errors. Same format as
                                       '-checks'.
                                       This option's value is appended to the value of
                                       the 'WarningsAsErrors' option in .clang-tidy
                                       file, if any.

  -p <build-path> is used to read a compile command database.

    For example, it can be a CMake build directory in which a file named
    compile_commands.json exists (use -DCMAKE_EXPORT_COMPILE_COMMANDS=ON
    CMake option to get this output). When no build path is specified,
    a search for compile_commands.json will be attempted through all
    parent paths of the first input file . See:
    https://clang.llvm.org/docs/HowToSetupToolingForLLVM.html for an
    example of setting up Clang Tooling on a source tree.

  <source0> ... specify the paths of source files. These paths are
    looked up in the compile command database. If the path of a file is
    absolute, it needs to point into CMake's source tree. If the path is
    relative, the current working directory needs to be in the CMake
    source tree and the file must be in a subdirectory of the current
    working directory. "./" prefixes in the relative files will be
    automatically removed, but the rest of a relative path must be a
    suffix of a path in the compile command database.


  Configuration files:
    clang-tidy attempts to read configuration for each source file from a
    .clang-tidy file located in the closest parent directory of the source
    file. The .clang-tidy file is specified in YAML format. If any configuration
    options have a corresponding command-line option, command-line option takes
    precedence.

    The following configuration options may be used in a .clang-tidy file:

    CheckOptions                 - List of key-value pairs defining check-specific
                                   options. Example:
                                     CheckOptions:
                                       some-check.SomeOption: 'some value'
    Checks                       - Same as '--checks'. Additionally, the list of
                                   globs can be specified as a list instead of a
                                   string.
    ExcludeHeaderFilterRegex     - Same as '--exclude-header-filter'.
    ExtraArgs                    - Same as '--extra-args'.
    ExtraArgsBefore              - Same as '--extra-args-before'.
    FormatStyle                  - Same as '--format-style'.
    HeaderFileExtensions         - File extensions to consider to determine if a
                                   given diagnostic is located in a header file.
    HeaderFilterRegex            - Same as '--header-filter-regex'.
    ImplementationFileExtensions - File extensions to consider to determine if a
                                   given diagnostic is located in an
                                   implementation file.
    InheritParentConfig          - If this option is true in a config file, the
                                   configuration file in the parent directory
                                   (if any exists) will be taken and the current
                                   config file will be applied on top of the
                                   parent one.
    SystemHeaders                - Same as '--system-headers'.
    UseColor                     - Same as '--use-color'.
    User                         - Specifies the name or e-mail of the user
                                   running clang-tidy. This option is used, for
                                   example, to place the correct user name in
                                   TODO() comments in the relevant check.
    WarningsAsErrors             - Same as '--warnings-as-errors'.

    The effective configuration can be inspected using --dump-config:

      $ clang-tidy --dump-config
      ---
      Checks:              '-*,some-check'
      WarningsAsErrors:    ''
      HeaderFileExtensions:         ['', 'h','hh','hpp','hxx']
      ImplementationFileExtensions: ['c','cc','cpp','cxx']
      HeaderFilterRegex:   ''
      FormatStyle:         none
      InheritParentConfig: true
      User:                user
      CheckOptions:
        some-check.SomeOption: 'some value'
      ...

.. _clang-tidy-nolint:

Suppressing Undesired Diagnostics
=================================

:program:`clang-tidy` diagnostics are intended to call out code that does not
adhere to a coding standard, or is otherwise problematic in some way. However,
if the code is known to be correct, it may be useful to silence the warning.
Some clang-tidy checks provide a check-specific way to silence the diagnostics,
e.g. `bugprone-use-after-move <checks/bugprone/use-after-move.html>`_ can be
silenced by re-initializing the variable after it has been moved out,
`bugprone-string-integer-assignment
<checks/bugprone/string-integer-assignment.html>`_ can be suppressed by
explicitly casting the integer to ``char``,
`readability-implicit-bool-conversion
<checks/readability/implicit-bool-conversion.html>`_ can also be suppressed by
using explicit casts, etc.

If a specific suppression mechanism is not available for a certain warning, or
its use is not desired for some reason, :program:`clang-tidy` has a generic
mechanism to suppress diagnostics using ``NOLINT``, ``NOLINTNEXTLINE``, and
``NOLINTBEGIN`` ... ``NOLINTEND`` comments.

The ``NOLINT`` comment instructs :program:`clang-tidy` to ignore warnings on the
*same line* (it doesn't apply to a function, a block of code or any other
language construct; it applies to the line of code it is on). If introducing the
comment on the same line would change the formatting in an undesired way, the
``NOLINTNEXTLINE`` comment allows suppressing clang-tidy warnings on the *next
line*. The ``NOLINTBEGIN`` and ``NOLINTEND`` comments allow suppressing
clang-tidy warnings on *multiple lines* (affecting all lines between the two
comments).

All comments can be followed by an optional list of check names in parentheses
(see below for the formal syntax). The list of check names supports globbing,
with the same format and semantics as for enabling checks. Note: negative globs
are ignored here, as they would effectively re-activate the warning.

For example:

.. code-block:: c++

  class Foo {
    // Suppress all the diagnostics for the line
    Foo(int param); // NOLINT

    // Consider explaining the motivation to suppress the warning
    Foo(char param); // NOLINT: Allow implicit conversion from `char`, because <some valid reason>

    // Silence only the specified checks for the line
    Foo(double param); // NOLINT(google-explicit-constructor, google-runtime-int)

    // Silence all checks from the `google` module
    Foo(bool param); // NOLINT(google*)

    // Silence all checks ending with `-avoid-c-arrays`
    int array[10]; // NOLINT(*-avoid-c-arrays)

    // Silence only the specified diagnostics for the next line
    // NOLINTNEXTLINE(google-explicit-constructor, google-runtime-int)
    Foo(bool param);

    // Silence all checks from the `google` module for the next line
    // NOLINTNEXTLINE(google*)
    Foo(bool param);

    // Silence all checks ending with `-avoid-c-arrays` for the next line
    // NOLINTNEXTLINE(*-avoid-c-arrays)
    int array[10];

    // Silence only the specified checks for all lines between the BEGIN and END
    // NOLINTBEGIN(google-explicit-constructor, google-runtime-int)
    Foo(short param);
    Foo(long param);
    // NOLINTEND(google-explicit-constructor, google-runtime-int)

    // Silence all checks from the `google` module for all lines between the BEGIN and END
    // NOLINTBEGIN(google*)
    Foo(bool param);
    // NOLINTEND(google*)

    // Silence all checks ending with `-avoid-c-arrays` for all lines between the BEGIN and END
    // NOLINTBEGIN(*-avoid-c-arrays)
    int array[10];
    // NOLINTEND(*-avoid-c-arrays)
  };

The formal syntax of ``NOLINT``, ``NOLINTNEXTLINE``, and ``NOLINTBEGIN`` ...
``NOLINTEND`` is the following:

.. parsed-literal::

  lint-comment:
    lint-command
    lint-command lint-args

  lint-args:
    **(** check-name-list **)**

  check-name-list:
    *check-name*
    check-name-list **,** *check-name*

  lint-command:
    **NOLINT**
    **NOLINTNEXTLINE**
    **NOLINTBEGIN**
    **NOLINTEND**

Note that whitespaces between
``NOLINT``/``NOLINTNEXTLINE``/``NOLINTBEGIN``/``NOLINTEND`` and the opening
parenthesis are not allowed (in this case the comment will be treated just as
``NOLINT``/``NOLINTNEXTLINE``/``NOLINTBEGIN``/``NOLINTEND``), whereas in the
check names list (inside the parentheses), whitespaces can be used and will be
ignored.

All ``NOLINTBEGIN`` comments must be paired by an equal number of ``NOLINTEND``
comments. Moreover, a pair of comments must have matching arguments -- for
example, ``NOLINTBEGIN(check-name)`` can be paired with
``NOLINTEND(check-name)`` but not with ``NOLINTEND`` `(zero arguments)`.
:program:`clang-tidy` will generate a ``clang-tidy-nolint`` error diagnostic if
any ``NOLINTBEGIN``/``NOLINTEND`` comment violates these requirements.

.. _LibTooling: https://clang.llvm.org/docs/LibTooling.html
.. _How To Setup Tooling For LLVM: https://clang.llvm.org/docs/HowToSetupToolingForLLVM.html