This is a text-only version of the following page on https://raymii.org:
---
Title       : 	Run one specific clang-tidy check on your entire codebase
Author      : 	Remy van Elst
Date        : 	05-04-2021
URL         : 	https://raymii.org/s/snippets/Run_one_specific_clang-tidy_check_on_your_codebase.html
Format      : 	Markdown/HTML
---



Recently I did a major refactor on a piece of code that involved thousands of
lines of code which were in one  way or another related to string handling.
All of the code handled `char*` (C style character pointer arrays) and the
concept of `const` or ownership was literally unknown in that part of the
codebase. The refactored code uses `std::string`'s, but due to the legacy
nature, a large number of methods returned `nullptr`'s instead of empty
strings (like `""`). I understand why this was done, but finding all those
instances and the fact it only gives a runtime error was a bit of a bummer. 


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Luckily `clang-tidy` is here to save the day. In my IDE, CLion, it gives a
warning when you return a `nullptr`. It however does that only in the file
you're currently editing, and since we're talking millions of files, I wasn't
going to open them by hand. You can run `clang-tidy` easily on one file, and
it's not hard to run it on an entire codebase as well, using  the script
`run-clang-tidy.py`, provided in their packages.

This snippet shows you how to run [one specific][4] `clang-tidy` check, in my
case, `bugprone-string-constructor`, on a (cmake and C++) codebase.

Here's the clang-tidy message in CLion:

![screenshot][3]

### Example code with undefined behaviour

This is an example piece of code demonstrating the behavior:

	#include <string>
	#include <iostream>

	class Example {
	public:
	    std::string getName() { return nullptr; }
	};

	int main() {
	    Example ex;
	    std::cout << "Example: " << ex.getName() << std::endl;
	    return 0;
	}

If you try to run the above code example, you'll get a runtime error:

	terminate called after throwing an instance of 'std::logic_error'
  		what():  basic_string::_M_construct null not valid


Opinions on `nullptr` and `std::string` [differ][1] [depending][2] on who you
ask, but as of now it's not possible to construct a `std::string` with a
`nullptr`. 

### Run clang-tidy on you entire codebase

Make sure you have `clang-tidy` installed:

	apt install clang-tidy

Navigate into your project folder:

	cd my/cpp/project

If you haven't already, create a build folder (`mkdir build; cd build`) and
run `cmake` with an extra flag to create the compilation database for
`clang-tidy`:

	cmake .. -DCMAKE_EXPORT_COMPILE_COMMANDS=ON -DCMAKE_BUILD_TYPE=Debug 

In the build folder, run `run-clang-tidy`. It might be a different command
(`run-clang-tidy.py` or `run-clang-tidy-VERSIONNUMBER`) depending on your
distro's packaging preference.

	run-clang-tidy -extra-arg=-Wno-unknown-warning-option -checks='-*,bugprone-string-constructor' 2>&1 | tee -a clang-tidy-result

This will take a while, when the command is finished, you can look at the
results, or in the file `clang-tidy-result`. In my case it gave specific
filenames and line numbers where it found the undefined behavior.

The `-extra-arg` was required due to some other compiler extension flag for
our code, you can probably omit that.

The `-checks='-*'` disables all checks, the next
`,bugprone-string-constructor` enables [only the specific string check][4] I
want to run. You can add more specific checks, separate them by a comma. An
example with just 2 checks enabled:

	-checks='-*,bugprone-string-constructor,bugprone-string-integer-assignment'

An up to date list of `clang-tidy` checks can be [found on the LLVM website][5].



[1]: https://web.archive.org/web/20180302201006/https://stackoverflow.com/questions/49058133/why-doesnt-stdstring-take-a-null-pointer
[2]: https://web.archive.org/web/20200412122443/https://stackoverflow.com/questions/10771864/assign-a-nullptr-to-a-stdstring-is-safe
[3]: /s/inc/img/nullptr-string.png
[4]: https://web.archive.org/web/20210406070324/https://clang.llvm.org/extra/clang-tidy/checks/bugprone-string-constructor.html
[5]: https://clang.llvm.org/extra/clang-tidy/



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