Clang-Tidy, part 1: Modernize your source code using C++11/C++14 Automated refactoring of your source code using powerful open-source tooling

This blog series will introduce the clang-tidy utility from the Clang/LLVM project and show how to use it to automatically refactor C++ source code and integrate with your build system, as well as how to use the tool on other platforms than Unices.

Motivation: The joy of legacy code bases

C++11 added a significant amount of new C++ language features which to date still aren’t used to their full extent. The most visual ones are for sure auto , override , Lambda expressions, range-based for , uniform initialization syntax, you name it. While C++11 is now several years old already, there’s still lots of code bases which don’t use any of the new language features, be it by policy from management, or by pure laziness to avoid the porting effort from the developer side. Clang-Tidy from the LLVM compiler infrastructure project is here to at least overcome the latter, to allow automatic refactoring of your source code so it uses the new language features.

Use override, anyone?

Now, what happens if you already maintain a fairly large code base which is still compiles under C++03 mode but you do want to embrace using C++11 in the future, with all the helpful features it has? You’ll likely have lots and lots of code similar to this:

struct Base { virtual void reimplementMe(int a) {} }; struct Derived : public Base { virtual void reimplementMe(int a) {} };

So far you of course always re-implemented the correct base-class method, because you’ve extensively tested your code via unit tests! Of course you did. Thus the code is fine but you would now like to move on. You’d like to add the override specifier to every single re-implemented method in your code base, but of course without hiring a trainee which goes through the code line-by-line and adds them manually.

Just to stress that adding override indeed serves a purpose, we’ve just recently fixed a bug in Qt 3D where we were not overloading the correct base-class method. With the specifier added earlier, we would have noticed instantly, after recompilation.

We’ll take the missing-override example further to explain basic usage of clang-tidy.

Clang-Tidy to the rescue!

Clang-Tidy is a clang-based C++ linter tool which provides a shell executable called clang-tidy as the main entry point. It is an extensible framework for diagnosing typical programming errors, or style issues — generally anything which can be detected during static analysis of the code. The real benefit of the tool is that it additionally allows to automatically refactor the source code by applying fixits each individual issue may provide. It is heavily plugin-based and comes with a useful set of plugins out of the box, which we’re going to discuss in the next paragraph.

Setup

When running Linux, clang-tidy is usually easy to get via your distribution’s package manager. On Ubuntu Linux for instance, installing it is as easy as running the following commands:

% sudo apt-get install clang-tidy

We’ll be discussing installing the tool on other platforms than Linux in one of the upcoming blog posts.

Note: We recommend you to always install the latest version (at the time of writing, the version based on Clang/LLVM 3.9 is recommended), as the number of available plugins/checkers varies greatly from version to version and grows constantly.

Introduction

Note: In this blog post, clang-tidy-3.9 was used

A typical invocation of the command-line tool looks like this:

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

Executing it like this, the tool will print a bunch of warnings and notes (if applicable), in exactly the same way Clang/GCC provide diagnostics, too.

Clang-Tidy is a useful static analysis tool on its own with lots of different available checkers, this, however, is not the focus of this blog post. We’d rather want to leverage the tool’s powerful refactoring capabilities to modernize our source code.

Listing available checkers

Running the tool without any specific command-line parameters will run the default set of checkers enabled by the utility. Let’s check what other checkers it has to offer (by passing –checks=’*’ to see them all), and specifically grep for the ones with modernize in their names. Those checkers advocate usage of modern language constructs:

$ clang-tidy --list-checks -checks='*' | grep "modernize" modernize-avoid-bind modernize-deprecated-headers modernize-loop-convert modernize-make-shared modernize-make-unique modernize-pass-by-value modernize-raw-string-literal modernize-redundant-void-arg modernize-replace-auto-ptr modernize-shrink-to-fit modernize-use-auto modernize-use-bool-literals modernize-use-default modernize-use-emplace modernize-use-nullptr modernize-use-override modernize-use-using

Impressive list of options already, isn’t it? Clang-Tidy indeed ships some interesting checkers out of the box (as of Clang/LLVM 3.9), with the list growing constantly from release to release.

The names of the checkers are pretty much self-explanatory (e.g. modernize-use-auto will embrace using auto where applicable), but if you want to explore what each of them means, please consult the list of checkers on the clang-tidy homepage:

To show how the tool is being used let’s focus on the modernize-use-override checker, as it’s the most applicable and most uncontroversial checker.

Refactoring a single file

Our override example again:

struct Base { virtual void reimplementMe(int a) {} }; struct Derived : public Base { virtual void reimplementMe(int a) {} };

Running clang-tidy on the example (this time with the modernize-use-override checker enabled):

% clang-tidy-3.9 -checks='modernize-use-override' test.cpp -- -std=c++11 1 warning generated. /home/kfunk/test.cpp:5:18: warning: prefer using 'override' or (rarely) 'final' instead of 'virtual' [modernize-use-override] virtual void reimplementMe(int a) {} ^ override

Alright. So it noticed that Derived::reimplementMe(int) overrides a base-class method but is lacking the override specifier! Now we could add that manually… or just let the tool do it for us by passing -fix!

Running it on the example (with modernize-use-override checker & fix-its enabled):

% clang-tidy-3.9 -checks='modernize-use-override' -fix test.cpp -- -std=c++11 1 warning generated. /home/kfunk/test.cpp:5:18: warning: prefer using 'override' or (rarely) 'final' instead of 'virtual' [modernize-use-override] virtual void reimplementMe(int a) {} ^ override /home/kfunk/test.cpp:5:5: note: FIX-IT applied suggested code changes virtual void reimplementMe(int a) {} ^ /home/kfunk/test.cpp:5:38: note: FIX-IT applied suggested code changes virtual void reimplementMe(int a) {} ^ clang-tidy applied 2 of 2 suggested fixes.

Clang-tidy applied the fix-it, and inserted override after the method declaration in line 5. Done!

A couple of notes

There are a few things worth mentioning:

Not all checkers of clang-tidy actually carry fix-its, but the ones starting with modernize all do.

You can use fix-its from multiple checkers at the same time (consider -checks=’modernize-use-override,modernize-use-auto’ -fix)

Running clang-tidy invokes the complete Clang compiler frontend, thus will need some time to complete

Refactoring results from clang-tidy are perfectly accurate, due to the fact it’s backed by a fully-fledged C++ parser

Refactoring a complete project (CMake-based)

So far we’ve run clang-tidy on a single, standalone file only. What happens if you have a more complex project setup, with lots of files, all with custom compile flags? Clang-tidy always operates on a single file, or rather, translation unit. We can help the tool to figure out the correct compile flags for each translation unit we compile in our project. The most convenient way to run it is with a compile command database. CMake can automatically generate one, and once a compile_commands.json is in place and a working version of clang-tidy is in PATH the entire code base can be analyzed with the run-clang-tidy.py script (usually shipped as part of the installation). If not you can simply download it here.

Note: It is highly recommended to use run-clang-tidy.py to run clang-tidy on a whole project, since it’ll run the tool multiple times in parallel and makes sure concurrent executions don’t interfere with each other (e.g. by avoiding modifying a shared header in parallel and in turn generating broken code).

Generating a compile_commands.json file

For generating the compile_commands.json file in a CMake-based project, just run:

% cd my-cmake-based-project % mkdir build % cd build % cmake -DCMAKE_EXPORT_COMPILE_COMMANDS=ON ..

Use script to run clang-tidy

Now for running the tool with the default checks on each translation unit in the project, simply invoke the run-clang-tidy script inside the directory with the compile_commands.json file:

% run-clang-tidy.py

As seen before, this will not modify anything so far, as we’ve run clang-tidy with just the default checks enabled. To e.g. run the modernize-use-override check on all translations units and actually refactor all your code, this invocation is needed:

% run-clang-tidy.py -header-filter='.*' -checks='-*,modernize-use-override' -fix

That’s it. clang-tidy will now be invoked on each translation unit in your project and will add overrides where missing. The parameter -header-filter=’.*’ makes sure clang-tidy actually refactors code in the headers being consumed in the translation unit. The parameter checks=’-*,…’ makes sure all default checks are disabled.

Note that the fixes are only applied in once run-clang-tidy has finished! The script will only record the changes to-be-performed and applies them all at once at the end.

Running other checkers

Again, the modernize-use-override is just an example, clang-tidy has lots of other checkers which are useful. Another super useful one is the modernize-use-nullptr checker, which transforms 0 , or e.g. NULL literals into modern C++11 nullptr version. To refactor all uses of the old-style literals in your project, simply run:

% run-clang-tidy.py -header-filter='.*' -checks='-*,modernize-use-nullptr' -fix

It’s usually a good idea to perform one checker after another, committing intermediate results (think of “Port towards C++11 nullptr”, “Port towards C++11 override”, …) into your version-control system.

Some real world examples

I’ve personally used clang-tidy on a lot of different projects already, with positive results. Remember, this tool has perfect knowledge of your code (as it is in fact using the Clang compiler frontend) and thus will refactor your code without ever introducing broken code.

Examples:

This patch for instance ports all of KDE’s Frameworks libraries towards C++11 nullptr, by touching around 9000 different code locations

This patch ports the KDE Marble code base to C++11 override, by touching around 2300 different code locations

Conclusion

Clang-Tidy is a powerful tool which makes porting your legacy code base towards C++11 a matter of running a one-liner. It comes with a great set of default checkers and the list of additional ones grows constantly. The modernize- checkers can be used to modernize/refactor your source code to use new C++ language features.

In the next part of this series we will discuss how to use clang-tidy project-wide with other build systems.