Clang-Tidy

See also:

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. clang-tidy is modular and provides a convenient interface for writing new checks.

Using clang-tidy

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 --:

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

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,

$ 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.

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 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 clang-tidy to fix found errors if supported by corresponding checks.

An overview of all the command-line options:

$ 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

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 can be silenced by re-initializing the variable after it has been moved out, bugprone-string-integer-assignment can be suppressed by explicitly casting the integer to char, readability-implicit-bool-conversion 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, clang-tidy has a generic mechanism to suppress diagnostics using NOLINT or NOLINTNEXTLINE comments.

The NOLINT comment instructs 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:

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:

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.