# UHD Coding Standards

Note: This file applies to all code within the UHD repository, not just for
code that is actually part of the UHD library.

## Preamble

To quote R. W. Emerson: "A foolish consistency is the hobgoblin of little minds,
adored by little statesmen and philosophers and divines". Ignoring the little
statesmen for a minute, these coding standards are here to make our life
*easier*, not simply add additional rules. They are meant as additional guidance
for developers, and are not meant to be interpreted as law.

So, ultimately, it is up to the developer to decide how much these guidelines
should be heeded when writing code, and up to reviewers how much they are
relevant to new submissions.
That said, a consistent codebase is easier to maintain, read, understand, and
extend. Choosing personal preferences over these coding guidelines is not a
helpful move for the team and future maintainability of the UHD codebase.

## General Coding Guidelines

* Never submit code with trailing whitespace.
* Code layout: We use 4 spaces for indentation levels, and never tabs.
* Code is read more often than it's written. Code readability is thus something
  worth optimizing for.
* Try and keep lines short (79 characters is nice, maximum is 90/100 characaters
  for C/C++ and Python, respectively), unless readability suffers. We
  often have to do fun things like SSH into machines and edit code in a
  terminal, and do side-by-side views of code on small-ish screens, so this is
  actually pretty helpful.
* Go crazy with log messages. Trace-level log messages in particular can be
  used copiously and freely (unless in rare cases where they can interfere with
  performance). Note that in C++, we have the option of fully compiling out
  trace-level messages (and even higher levels).

## C++-specific Guidelines

* All C++ code must be formatted according to the .clang-format file in the root
  of the project.
* If in doubt, consult the [C++ Core Guidelines][CppCoreGuidelines]. If the
  guidelines have an answer, and it works for you, just pick that.
* Use Doxygen doc-blocks copiously.
* All things equal, prefer standard C++ constructs over Boost constructs (see
  also Boost guidelines).
* Given the option, prefer C++ lambdas over std::bind, and just don't use
  boost::bind.
* `size_t` is the correct container for all indexing of C++ structures (such
  as vectors). But keep in mind that the size of `size_t` is
  *platform-dependent*!
* Use size-specific types whenever interacting with hardware (`int32_t`, etc.).
  It's very easy to get bitten by incorrect sizes.
* Include include files in the following order: Local headers, other
  UHD headers, 3rd-party library headers, Boost headers, standard headers.
  The rationale is to include from most to least specific. This is the best way
  to catch missing includes (if you were to include the standard header first,
  it would be available to all include files that come later. If they need that
  standard header too, they should be including it themselves). Note that
  clang-format will do this for you.
  Example:

```cpp
#include "x300_specific.hpp"
#include <uhd/utils/log.hpp>
#include <libusb/foo.hpp>
#include <boost/shared_ptr.hpp>
#include <mutex>
```

* Feel free to use modern C/C++ features even if they were not used before.
  Make sure they work with the compilers and dependencies which are set for the
  version of UHD the commit will be made upon. Our continuous integration system
  will be able to confirm this.
  For interesting new features, use 'anchor commits', which describe clearly
  which new feature was used. They can be used as a reference for other
  developers. Example:

```
commit 9fe731cc371efee7f0051186697e611571c5b41b
Author: Andrej Rode <andrej.rode@ettus.com>
Date:   Tue Nov 22 16:19:38 2016 -0800

    utils: uhd_find_devices display one device for each unique serial found
    
    Note: This also is the first precedent for the usage of the 'auto' keyword
    in UHD. Commits past this one will also be able to use 'auto'.
    
    Reviewed-By: Martin Braun <martin.braun@ettus.com>
```

* Prefer `.at()` over `[]` for maps and vectors. Keep in mind that `[]` will
  invoke a default constructor of the value type, whereas `.at()` will throw
  an exception if the index doesn't exist -- which is usually the desired
  behaviour.

* Documenting methods is highly encouraged. To make the documentation well
  digestible for Doxygen, use the Qt style and add an exclamation mark (`!`)
  after the opening C-style comment block. Use `\brief` to mark a brief
  introductory line, then add an empty line (starting with `*`), then
  continue with the detailed description. Add `\param` for each parameter
  and `\return` for describing the return value. For single-line method
  documentation, `//!` is okay. Example

```cpp
/*! \brief Summary line. 
 *
 * more details 
 * \param variable Explanation of variable (if applicable)
 * \return Explanation of return value (if applicable)
 */
bool example_method(uint32_t variable) {};

//! Short description of a simple method.
void example_method2() {};
```


## Boost-specific Guidelines

* Avoid Boost where possible.
* Don't use Boost's sleep functions. Prefer `std::chrono` functions.
* When using `boost::assign::list_of` or `boost::assign::map_list_of`, make
  sure to explicitly cast to the appropriate container (even better: Avoid
  `boost::assign`, maybe use std initializer lists where possible):

```cpp
std::vector<std::string> foo =
    boost::assign::list_of<std::string> // Explicitly list the type (in this
        ("string1")                     // case, std::string). Then list all
        ("string2")                     // the items (string1, string2, etc).
        ("etc")
    .to_container(foo); // Finally, call conversion routine explicitly.
// Same for maps:
std::map<std::string, std::string> bar =
    boost::assign::map_list_of<std::string, std::string>
        ("a", "b")
        ("c", "d")
        ("etc", "etc")
    .to_container(bar);
```


## Python-specific Guidelines

* Starting with UHD 4.0, Python 2 is no longer supported, and we don't need to
  accommodate for it any longer. Prefer Python 3 constructs.
* We follow the [NI Python Style Guidelines][ni-python]. Install
  `ni-python-styleguide` to help following the recommendations (e.g., by running
  `pip install ni-python-styleguide`. The tool is available
  [on GitHub](https://github.com/ni/python-styleguide).

## CMake-specific Guidelines

* CMake commands are written in lowercase.

## Revision Control Hygiene

* In this repository, we almost always use fast-forward merges, and no merge
  commits.
* Prefix all commit message subject lines with the section of code they apply
  to, and use the imperative mood (Example: "x300: Fix overflow at full moon").
  Try and keep the subject line to 50 characters, but make 72 characters a hard
  limit.
* Follow up in greater detail in the body of the commmit message. The body is
  separated from the subject line with one blank line. Consider the body of the
  git commit an email to the future reader of this changeset, so don't be
  sparse in the commit body, and use it to explain the *what* and *why* of this
  commit (the "how" part should be obvious from the code change). Lines should
  be limited to 72 characters.
* Avoid refactoring, whitespace cleanup, or fixing code to match coding
  guidelines in the same commit as modifying behaviour. Prefer dedicated
  cleanup commits.
* Remember that we ship git repositories, not just code. This means every
  commit is part of our product and should be treated as such.

## Useful tooling

### Autoformatters

- Before checking in C/C++/Python code, make sure it was reformatted with an
  autoformatter. For C/C++, that's `clang-format`. For Python, use `black` and
  `isort`, or install `ni-python-styleguide` which incorporates all of those
  tools.

### Linters

There are a variety of linters available, all of which provide valuable insight:

- `flake8` is a very, very nitpicky linter for Python. It is integrated into
  `ni-python-styleguide` with some additional configuration, so we recommend
  using the latter.
- `clang-tidy` can do some static analysis on C and C++ files.
- For HDL code, Verilator and iverilog can be integrated as syntax error checkers
  in some editors, but keep in mind they often can't find available modules, and
  not always support SystemVerilog.

### Git hooks

The easiest way to use git hooks is to use `pre-commit`:

- Install `pre-commit` (e.g., `pip install pre-commit`, or `apt install pre-commit`).
- In your UHD repository, install the pre-commit hooks by running
  `pre-commit install`
- Manually install `ni-python-styleguide` (`pip install ni-python-styleguide`)

Before attempting to commit changes, the files will be automatically scanned for
style/formatting issues.

## FPGA Guidelines

FPGA guidelines are stored in a separate file. See [CODING.md][fpga-coding] in
the FPGA repository.

[CppCoreGuideLines]: https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md
[PEP8]: https://www.python.org/dev/peps/pep-0008/
[Pep257]: https://www.python.org/dev/peps/pep-0257/
[fpga-coding]: https://github.com/EttusResearch/uhd/blob/master/fpga/CODING.md
[ni-python]: https://ni.github.io/python-styleguide/