==========
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,
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.
``android-``           Checks related to Android.
``boost-``             Checks related to Boost library.
``bugprone-``          Checks that target bugprone code constructs.
``cert-``              Checks related to CERT Secure Coding Guidelines.
``cppcoreguidelines-`` Checks related to C++ Core Guidelines.
``clang-analyzer-``    Clang Static Analyzer checks.
``fuchsia-``           Checks related to Fuchsia coding conventions.
``google-``            Checks related to Google coding conventions.
``hicpp-``             Checks related to High Integrity C++ Coding Standard.
``llvm-``              Checks related to the LLVM coding conventions.
``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.
``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
``-checks=`` option. However, the ``-checks=`` option does not affect
compilation arguments, so it can not turn on Clang warnings which are not
already turned on in 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: [{key: x,
                                                                value: 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.
    -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.
    -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.
    -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 'HeaderFilter' 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.
    -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.
    -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. If any configuration options have a corresponding command-line
    option, command-line option takes precedence. The effective
    configuration can be inspected using -dump-config:

      $ clang-tidy -dump-config
      ---
      Checks:          '-*,some-check'
      WarningsAsErrors: ''
      HeaderFilterRegex: ''
      FormatStyle:     none
      User:            user
      CheckOptions:
        - key:             some-check.SomeOption
          value:           'some value'
      ...

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`` or ``NOLINTNEXTLINE``
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 in the same line would change the formatting in undesired way, the
``NOLINTNEXTLINE`` comment allows to suppress clang-tidy warnings on the *next
line*.

Both comments can be followed by an optional list of check names in parentheses
(see below for the formal syntax).

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

The formal syntax of ``NOLINT``/``NOLINTNEXTLINE`` 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**

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

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