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CPP Code Style
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Developer Overview

Gluten CPP Core Guidelines

This is a set of CPP core guidelines for Gluten. The aim is to make the codebase simpler, more efficient, more maintainable by promoting consistency and according to best practices.

Philosophy

Philosophical rules are generally not measurable. However, they are valuable. For Gluten CPP coding, there are a few Philosophical rules as the following.

  • Write in ISO Standard C++.
  • Standard API first, the CPP programming APIs are priority to system calls.
  • Write code consistently. It's good for understanding and maintaining.
  • Keep simple, making code clear and easy to read.
  • Optimize code for reader, not for writer. Thus, more time will be spent reading code than writing it.
  • Make it work, and then make it better or faster.
  • Don't import any complexity if possible. Collaborate with minimal knowledge consensus.

Code Formatting

Many aspects of C++ coding style will be covered by clang-format, such as spacing, line width, indentation and ordering (for includes, using directives and etc).

  • Always ensure your code is compatible with clang-format-15 for Velox backend.
  • dev/formatcppcode.sh is provided for formatting Velox CPP code.

To format cmake files like CMakeLists.txt & *.cmake, cmake-format is required to be installed. Here is an example.

apt install python3-pip -y
pip3 install --user cmake-format
cmake-format --first-comment-is-literal True --in-place cpp/velox/CMakeLists.txt

Naming Conventions

  • Use PascalCase for types (class, struct, enum, type alias, type template parameter) and file name.
  • Use camelCase for function, member and local variable, and non-type template parameter.
  • Use camelCase_ for private and protected member variable.
  • Use snake_case for namespace name and build target.
  • Use UPPER_SNAKE_CASE for macro.
  • Use kPascalCase for static constant and enumerator.

Designs

  • No over design.
  • No negation of negation, isValid is better than isNotInvalid.
  • Avoid corner case, and common case first.
  • Express ideas directly, don't let me think.
  • Make a check for the arguments in the interface between modules, and don't make a check in the inner implementation, use assert in the private implementation instead of too much safe check.

Source File & Header File

  • All header files must have a single-inclusion guard using #pragma once
  • Always use .h as header file suffix, not .hpp.
  • Always use .cc as source file suffix, neither .cpp nor .cxx.
  • One file should contain one main class, and the file name should be consistent with the main class name.
    • Obvious exception: files used for defining various misc functions.
  • If a header file has a corresponding source file, they should have the same file name with different suffix, such as a.h vs a.cc.
  • If a function is declared in the file a.h, ensure it's defined in the corrosponding source file a.cc, do not define it in other files.
  • No deep source directory for CPP files, not do it as JAVA.
  • Include header files should satisfy the following rules.
    • Include the necessary header files, which means the source file (.cc) containing the only one line #include "test.h" can be compiled successfully without including any other header files.
    • Do not include any unnecessary header files, the more including, the slower compiling.
    • In one word, no more, no less, just as needed.

Class

  • Base class name doesn't end with Base, use Backend instead of BackendBase.
  • Ensure one class does one thing, and follows the single responsibility principle.
  • No big class, No huge class, No too much interfaces.
  • Distinguish interface from implementation, make implementations private.
  • When designing a class hierarchy, distinguish between interface inheritance and implementation inheritance.
    • Ensure that public inheritance represent the relation of is-a.
    • Ensure that private inheritance represent the relation of implements-with.
  • Don't make a function virtual without reason.
  • Ensure the polymorphic base class has a virtual deconstructor.
  • Use override to make overriding explicit and to make the compiler work.
  • Use const to mark the member function read-only as far as possible.
  • When you try to define a copy constructor or a operator= for a class, remember the Rule of three/five/zero.

Function

  • Make functions short and simple.
  • Calling a meaningful function is more readable than writing too many statements in place, but the performance-sensitive code path is an exception.
  • Give the function a good name, how to check whether the function name is good or not.
    • When you read it loudly, you feel smooth.
    • The information can be represented by arguments should not be encoded into the function name. such as. use get(size_t index) instead of getByIndex.
  • A function should focus on a single logic operation.
  • A function should do as the name meaning.
    • do everything converd by the function name
    • don't do anything not convered by the function name

Variable

  • Make variable names simple and meaningful.
  • Don't group all your variables at the top of the scope, it's an outdated habit.
  • Declare variables as close to the usage point as possible.

Constant

  • Prefer const variables to using preprocessor (#define) to define constant values.

Macro

  • Macros downgrade readability, break mind, and affect debug.
  • Macros have side effects.
  • Use macros cautiously and carefully.
  • Consider using const variables or inline functions to replace macros.
  • Consider defining macros with the wrap of do {...} while (0)
  • Avoid using 3rd party library macros directly.

Namespace

  • Don't using namespace xxx in header files. Instead, you can do this in source files. But it's still not encouraged.
  • Place all Gluten CPP codes under namespace gluten because one level namespace is enough. No nested namespace. Nested namespaces bring mess.
  • The anonymous namespace is recommended for defining file level classes, functions and variables. It's used to place file scoped static functions and variables.

Resource Management

  • Use handles and RAII to manage resources automatically.
  • Immediately give the result of an explicit resource allocation to a manager object.
  • Prefer scoped objects and stack objects.
  • Use raw pointers to denote individual objects.
  • Use pointer + size_t to denote array objects if you don't want to use containers.
  • A raw pointer (a T*) is non-owning.
  • A raw reference (a T&) is non-owning.
  • Understand the difference of unique_ptr, shared_ptr, weak_ptr.
    • unique_ptr represents ownership, but not share ownership. unique_ptr is equivalent to RAII, release the resource when the object is destructed.
    • shared_ptr represents shared ownership by use-count. It is more expensive that unqiue_ptr.
    • weak_ptr models temporary ownership. It is useful in breaking reference cycles formed by objects managed by shared_ptr.
  • Use unique_ptr or shared_ptr to represent ownership.
  • Prefer unique_ptr over shared_ptr unless you need to share ownership.
  • Use make_unique to make unique_ptrs.
  • Use make_shared to make shared_ptrs.
  • Take smart pointers as parameters only to explicitly express lifetime semantics.
  • For general use, take T* or T& arguments rather than smart pointers.

Exception

  • The exception specifications are changing always. The difference between various CPP standards is big, so we should use exception cautiously in Gluten.
  • Prefer return code to throwing exceptions.
  • Prefer compile-time checking to run-time checking.
  • Encapsulate messy constructors, rather than spreading through the code.

Code Comment

  • Add necessary comments. The comment is not the more the better, also not the less the better.
  • Good comment makes obscure code easily understood. It's unnecessary to add comments for quite obvious code.

References