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Getting Involved
:program:`clang-tidy` has several own checks and can run Clang static analyzer
checks, but its power is in the ability to easily write custom checks.
Checks are organized in modules, which can be linked into :program:`clang-tidy`
with minimal or no code changes in :program:`clang-tidy`.
Checks can plug into the analysis on the preprocessor level using PPCallbacks
or on the AST level using AST Matchers. When an error is found, checks can
report them in a way similar to how Clang diagnostics work. A fix-it hint can be
attached to a diagnostic message.
The interface provided by :program:`clang-tidy` makes it easy to write useful
and precise checks in just a few lines of code. If you have an idea for a good
check, the rest of this document explains how to do this.

There are a few tools particularly useful when developing clang-tidy checks:


add_new_check.py is a script to automate the process of adding a new
check, it will create the check, update the CMake file and create a test;

rename_check.py does what the script name suggests, renames an existing
check;

:program:`clang-query` is invaluable for interactive prototyping of AST
matchers and exploration of the Clang AST;

clang-check with the -ast-dump (and optionally -ast-dump-filter)
provides a convenient way to dump AST of a C++ program.


If CMake is configured with CLANG_ENABLE_STATIC_ANALYZER,
:program:`clang-tidy` will not be built with support for the
clang-analyzer-* checks or the mpi-* checks.

Choosing the Right Place for your Check
If you have an idea of a check, you should decide whether it should be
implemented as a:


Clang diagnostic: if the check is generic enough, targets code patterns that
most probably are bugs (rather than style or readability issues), can be
implemented effectively and with extremely low false positive rate, it may
make a good Clang diagnostic.

Clang static analyzer check: if the check requires some sort of control flow
analysis, it should probably be implemented as a static analyzer check.

clang-tidy check is a good choice for linter-style checks, checks that are
related to a certain coding style, checks that address code readability, etc.


Preparing your Workspace
If you are new to LLVM development, you should read the Getting Started with
the LLVM System, Using Clang Tools and How To Setup Clang Tooling For
LLVM documents to check out and build LLVM, Clang and Clang Extra Tools with
CMake.
Once you are done, change to the llvm/clang-tools-extra directory, and
let's start!

The Directory Structure
:program:`clang-tidy` source code resides in the
llvm/clang-tools-extra directory and is structured as follows:
clang-tidy/                       # Clang-tidy core.
|-- ClangTidy.h                   # Interfaces for users.
|-- ClangTidyCheck.h              # Interfaces for checks.
|-- ClangTidyModule.h             # Interface for clang-tidy modules.
|-- ClangTidyModuleRegistry.h     # Interface for registering of modules.
   ...
|-- google/                       # Google clang-tidy module.
|-+
  |-- GoogleTidyModule.cpp
  |-- GoogleTidyModule.h
        ...
|-- llvm/                         # LLVM clang-tidy module.
|-+
  |-- LLVMTidyModule.cpp
  |-- LLVMTidyModule.h
        ...
|-- objc/                         # Objective-C clang-tidy module.
|-+
  |-- ObjCTidyModule.cpp
  |-- ObjCTidyModule.h
        ...
|-- tool/                         # Sources of the clang-tidy binary.
        ...
test/clang-tidy/                  # Integration tests.
    ...
unittests/clang-tidy/             # Unit tests.
|-- ClangTidyTest.h
|-- GoogleModuleTest.cpp
|-- LLVMModuleTest.cpp
|-- ObjCModuleTest.cpp
    ...


Writing a clang-tidy Check
So you have an idea of a useful check for :program:`clang-tidy`.
First, if you're not familiar with LLVM development, read through the Getting
Started with LLVM document for instructions on setting up your workflow and
the LLVM Coding Standards document to familiarize yourself with the coding
style used in the project. For code reviews we mostly use LLVM Phabricator.
Next, you need to decide which module the check belongs to. Modules
are located in subdirectories of clang-tidy/
and contain checks targeting a certain aspect of code quality (performance,
readability, etc.), certain coding style or standard (Google, LLVM, CERT, etc.)
or a widely used API (e.g. MPI). Their names are same as user-facing check
groups names described :ref:`above <checks-groups-table>`.
After choosing the module and the name for the check, run the
clang-tidy/add_new_check.py script to create the skeleton of the check and
plug it to :program:`clang-tidy`. It's the recommended way of adding new checks.
If we want to create a readability-awesome-function-names, we would run:
$ clang-tidy/add_new_check.py readability awesome-function-names


The add_new_check.py script will:


create the class for your check inside the specified module's directory and
register it in the module and in the build system;
create a lit test file in the test/clang-tidy/ directory;
create a documentation file and include it into the
docs/clang-tidy/checks/list.rst.


Let's see in more detail at the check class definition:
...

#include "../ClangTidyCheck.h"

namespace clang {
namespace tidy {
namespace readability {

...
class AwesomeFunctionNamesCheck : public ClangTidyCheck {
public:
  AwesomeFunctionNamesCheck(StringRef Name, ClangTidyContext *Context)
      : ClangTidyCheck(Name, Context) {}
  void registerMatchers(ast_matchers::MatchFinder *Finder) override;
  void check(const ast_matchers::MatchFinder::MatchResult &Result) override;
};

} // namespace readability
} // namespace tidy
} // namespace clang

...

Constructor of the check receives the Name and Context parameters, and
must forward them to the ClangTidyCheck constructor.
In our case the check needs to operate on the AST level and it overrides the
registerMatchers and check methods. If we wanted to analyze code on the
preprocessor level, we'd need instead to override the registerPPCallbacks
method.
In the registerMatchers method we create an AST Matcher (see AST Matchers
for more information) that will find the pattern in the AST that we want to
inspect. The results of the matching are passed to the check method, which
can further inspect them and report diagnostics.
using namespace ast_matchers;

void AwesomeFunctionNamesCheck::registerMatchers(MatchFinder *Finder) {
  Finder->addMatcher(functionDecl().bind("x"), this);
}

void AwesomeFunctionNamesCheck::check(const MatchFinder::MatchResult &Result) {
  const auto *MatchedDecl = Result.Nodes.getNodeAs<FunctionDecl>("x");
  if (MatchedDecl->getName().startswith("awesome_"))
    return;
  diag(MatchedDecl->getLocation(), "function %0 is insufficiently awesome")
      << MatchedDecl
      << FixItHint::CreateInsertion(MatchedDecl->getLocation(), "awesome_");
}

(If you want to see an example of a useful check, look at
clang-tidy/google/ExplicitConstructorCheck.h
and clang-tidy/google/ExplicitConstructorCheck.cpp).

Registering your Check
(The add_new_check.py takes care of registering the check in an existing
module. If you want to create a new module or know the details, read on.)
The check should be registered in the corresponding module with a distinct name:
class MyModule : public ClangTidyModule {
 public:
  void addCheckFactories(ClangTidyCheckFactories &CheckFactories) override {
    CheckFactories.registerCheck<ExplicitConstructorCheck>(
        "my-explicit-constructor");
  }
};

Now we need to register the module in the ClangTidyModuleRegistry using a
statically initialized variable:
static ClangTidyModuleRegistry::Add<MyModule> X("my-module",
                                                "Adds my lint checks.");

When using LLVM build system, we need to use the following hack to ensure the
module is linked into the :program:`clang-tidy` binary:
Add this near the ClangTidyModuleRegistry::Add<MyModule> variable:
// This anchor is used to force the linker to link in the generated object file
// and thus register the MyModule.
volatile int MyModuleAnchorSource = 0;

And this to the main translation unit of the :program:`clang-tidy` binary (or
the binary you link the clang-tidy library in)
clang-tidy/tool/ClangTidyMain.cpp:
// This anchor is used to force the linker to link the MyModule.
extern volatile int MyModuleAnchorSource;
static int MyModuleAnchorDestination = MyModuleAnchorSource;


Configuring Checks
If a check needs configuration options, it can access check-specific options
using the Options.get<Type>("SomeOption", DefaultValue) call in the check
constructor. In this case the check should also override the
ClangTidyCheck::storeOptions method to make the options provided by the
check discoverable. This method lets :program:`clang-tidy` know which options
the check implements and what the current values are (e.g. for the
-dump-config command line option).
class MyCheck : public ClangTidyCheck {
  const unsigned SomeOption1;
  const std::string SomeOption2;

public:
  MyCheck(StringRef Name, ClangTidyContext *Context)
    : ClangTidyCheck(Name, Context),
      SomeOption(Options.get("SomeOption1", -1U)),
      SomeOption(Options.get("SomeOption2", "some default")) {}

  void storeOptions(ClangTidyOptions::OptionMap &Opts) override {
    Options.store(Opts, "SomeOption1", SomeOption1);
    Options.store(Opts, "SomeOption2", SomeOption2);
  }
  ...

Assuming the check is registered with the name "my-check", the option can then
be set in a .clang-tidy file in the following way:
CheckOptions:
  - key: my-check.SomeOption1
    value: 123
  - key: my-check.SomeOption2
    value: 'some other value'

If you need to specify check options on a command line, you can use the inline
YAML format:
$ clang-tidy -config="{CheckOptions: [{key: a, value: b}, {key: x, value: y}]}" ...


Testing Checks
To run tests for :program:`clang-tidy` use the command:
$ ninja check-clang-tools

:program:`clang-tidy` checks can be tested using either unit tests or
lit tests. Unit tests may be more convenient to test complex replacements
with strict checks. Lit tests allow using partial text matching and regular
expressions which makes them more suitable for writing compact tests for
diagnostic messages.
The check_clang_tidy.py script provides an easy way to test both
diagnostic messages and fix-its. It filters out CHECK lines from the test
file, runs :program:`clang-tidy` and verifies messages and fixes with two
separate FileCheck invocations: once with FileCheck's directive
prefix set to CHECK-MESSAGES, validating the diagnostic messages,
and once with the directive prefix set to CHECK-FIXES, running
against the fixed code (i.e., the code after generated fix-its are
applied). In particular, CHECK-FIXES: can be used to check
that code was not modified by fix-its, by checking that it is present
unchanged in the fixed code. The full set of FileCheck directives
is available (e.g., CHECK-MESSAGES-SAME:, CHECK-MESSAGES-NOT:), though
typically the basic CHECK forms (CHECK-MESSAGES and CHECK-FIXES)
are sufficient for clang-tidy tests. Note that the FileCheck
documentation mostly assumes the default prefix (CHECK), and hence
describes the directive as CHECK:, CHECK-SAME:, CHECK-NOT:, etc.
Replace CHECK by either CHECK-FIXES or CHECK-MESSAGES for
clang-tidy tests.
An additional check enabled by check_clang_tidy.py ensures that
if CHECK-MESSAGES: is used in a file then every warning or error
must have an associated CHECK in that file. Or, you can use CHECK-NOTES:
instead, if you want to also ensure that all the notes are checked.
To use the check_clang_tidy.py script, put a .cpp file with the
appropriate RUN line in the test/clang-tidy directory. Use
CHECK-MESSAGES: and CHECK-FIXES: lines to write checks against
diagnostic messages and fixed code.
It's advised to make the checks as specific as possible to avoid checks matching
to incorrect parts of the input. Use [[@LINE+X]]/[[@LINE-X]]
substitutions and distinct function and variable names in the test code.
Here's an example of a test using the check_clang_tidy.py script (the full
source code is at test/clang-tidy/google-readability-casting.cpp):
// RUN: %check_clang_tidy %s google-readability-casting %t

void f(int a) {
  int b = (int)a;
  // CHECK-MESSAGES: :[[@LINE-1]]:11: warning: redundant cast to the same type [google-readability-casting]
  // CHECK-FIXES: int b = a;
}

To check more than one scenario in the same test file use
-check-suffix=SUFFIX-NAME on check_clang_tidy.py command line or
-check-suffixes=SUFFIX-NAME-1,SUFFIX-NAME-2,....
With -check-suffix[es]=SUFFIX-NAME you need to replace your CHECK-*
directives with CHECK-MESSAGES-SUFFIX-NAME and CHECK-FIXES-SUFFIX-NAME.
Here's an example:
// RUN: %check_clang_tidy -check-suffix=USING-A %s misc-unused-using-decls %t -- -- -DUSING_A
// RUN: %check_clang_tidy -check-suffix=USING-B %s misc-unused-using-decls %t -- -- -DUSING_B
// RUN: %check_clang_tidy %s misc-unused-using-decls %t
...
// CHECK-MESSAGES-USING-A: :[[@LINE-8]]:10: warning: using decl 'A' {{.*}}
// CHECK-MESSAGES-USING-B: :[[@LINE-7]]:10: warning: using decl 'B' {{.*}}
// CHECK-MESSAGES: :[[@LINE-6]]:10: warning: using decl 'C' {{.*}}
// CHECK-FIXES-USING-A-NOT: using a::A;$
// CHECK-FIXES-USING-B-NOT: using a::B;$
// CHECK-FIXES-NOT: using a::C;$

There are many dark corners in the C++ language, and it may be difficult to make
your check work perfectly in all cases, especially if it issues fix-it hints. The
most frequent pitfalls are macros and templates:

code written in a macro body/template definition may have a different meaning
depending on the macro expansion/template instantiation;
multiple macro expansions/template instantiations may result in the same code
being inspected by the check multiple times (possibly, with different
meanings, see 1), and the same warning (or a slightly different one) may be
issued by the check multiple times; :program:`clang-tidy` will deduplicate
_identical_ warnings, but if the warnings are slightly different, all of them
will be shown to the user (and used for applying fixes, if any);
making replacements to a macro body/template definition may be fine for some
macro expansions/template instantiations, but easily break some other
expansions/instantiations.


Running clang-tidy on LLVM
To test a check it's best to try it out on a larger code base. LLVM and Clang
are the natural targets as you already have the source code around. The most
convenient way to run :program:`clang-tidy` is with a compile command database;
CMake can automatically generate one, for a description of how to enable it see
How To Setup Clang Tooling For LLVM. Once compile_commands.json is in
place and a working version of :program:`clang-tidy` is in PATH the entire
code base can be analyzed with clang-tidy/tool/run-clang-tidy.py. The script
executes :program:`clang-tidy` with the default set of checks on every
translation unit in the compile command database and displays the resulting
warnings and errors. The script provides multiple configuration flags.

The default set of checks can be overridden using the -checks argument,
taking the identical format as :program:`clang-tidy` does. For example
-checks=-*,modernize-use-override will run the modernize-use-override
check only.
To restrict the files examined you can provide one or more regex arguments
that the file names are matched against.
run-clang-tidy.py clang-tidy/.*Check\.cpp will only analyze clang-tidy
checks. It may also be necessary to restrict the header files warnings are
displayed from using the -header-filter flag. It has the same behavior
as the corresponding :program:`clang-tidy` flag.
To apply suggested fixes -fix can be passed as an argument. This gathers
all changes in a temporary directory and applies them. Passing -format
will run clang-format over changed lines.


On checks profiling
:program:`clang-tidy` can collect per-check profiling info, and output it
for each processed source file (translation unit).
To enable profiling info collection, use the -enable-check-profile argument.
The timings will be output to stderr as a table. Example output:
$ clang-tidy -enable-check-profile -checks=-*,readability-function-size source.cpp
===-------------------------------------------------------------------------===
                          clang-tidy checks profiling
===-------------------------------------------------------------------------===
  Total Execution Time: 1.0282 seconds (1.0258 wall clock)

   ---User Time---   --System Time--   --User+System--   ---Wall Time---  --- Name ---
   0.9136 (100.0%)   0.1146 (100.0%)   1.0282 (100.0%)   1.0258 (100.0%)  readability-function-size
   0.9136 (100.0%)   0.1146 (100.0%)   1.0282 (100.0%)   1.0258 (100.0%)  Total

It can also store that data as JSON files for further processing. Example output:
$ clang-tidy -enable-check-profile -store-check-profile=.  -checks=-*,readability-function-size source.cpp
$ # Note that there won't be timings table printed to the console.
$ ls /tmp/out/
20180516161318717446360-source.cpp.json
$ cat 20180516161318717446360-source.cpp.json
{
"file": "/path/to/source.cpp",
"timestamp": "2018-05-16 16:13:18.717446360",
"profile": {
  "time.clang-tidy.readability-function-size.wall": 1.0421266555786133e+00,
  "time.clang-tidy.readability-function-size.user": 9.2088400000005421e-01,
  "time.clang-tidy.readability-function-size.sys": 1.2418899999999974e-01
}
}

There is only one argument that controls profile storage:


-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.
If the prefix is not an absolute path, it is considered to be relative to the
directory from where you have run :program:`clang-tidy`. All . and ..
patterns in the path are collapsed, and symlinks are resolved.
Example:
Let's suppose you have a source file named example.cpp, located in the
/source directory. Only the input filename is used, not the full path
to the source file. Additionally, it is prefixed with the current timestamp.

If you specify -store-check-profile=/tmp, then the profile will be saved
to /tmp/<ISO8601-like timestamp>-example.cpp.json

If you run :program:`clang-tidy` from within /foo directory, and specify
-store-check-profile=., then the profile will still be saved to
/foo/<ISO8601-like timestamp>-example.cpp.json