Kokkos Coding Standards¶

Source Code Formatting¶

File Headers¶

Every source file should have the Kokkos SPDX file header with our license identifier and copyright notice.

The header block must appear at the very top of the file:

// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
// SPDX-FileCopyrightText: Copyright Contributors to the Kokkos project

// The rest of the file content follows here.

Header Guard¶

The header file’s guard should reflect the all-caps file name and path, using an underscore instead of path separator and extension marker.

The logic behind this convention is to ensure global uniqueness that extends not only across the entire Kokkos repository, but also into the full Kokkos ecosystem and within user applications that include Kokkos headers. This is achieved through two key steps:

Project Prefix: Starting all guards with PROJECT_NAME_ to ensure the macro does not conflict with external libraries or system headers.
Path and Name Derivation: Converting the full file path and name (e.g., impl/Kokkos_GarbageCollector.hpp) to uppercase, replacing path separators (/) and the extension marker (.) with underscores (_).

For example, the guard for impl/Kokkos_GarbageCollector.hpp should be something like KOKKOS_IMPL_GARBAGE_COLLECTOR_HPP. Or the guard for a HIP backend-specific implementation file HIP/Kokkos_HIP_BorrowChecker.hpp should be KOKKOS_HIP_BORROW_CHECKER_HPP.

Comment Formatting¶

In general, prefer C++-style comments (// for normal comments, /// for doxygen documentation comments).

Style Issues¶

Don’t use `inline` when defining a function within the class definition¶

C++ implicitly treats any member function defined within the class body as inline. Adding the inline keyword, or using KOKKOS_INLINE_FUNCTION, adds unnecessary syntactic noise without changing the compiler’s behavior.

Don’t:

class Foo {
public:
  // Redundant: already implicitly inline
  inline void bar() { /* ... */ }

  // Redundant: KOKKOS_INLINE_FUNCTION expands to 'inline'
  KOKKOS_INLINE_FUNCTION void baz() { /* ... */ }
};

Do:

class Foo {
public:
  // Clean: standard C++ handles inlining
  void bar() { /* ... */ }

  // Correct: Provides __host__ __device__ tags; inlining is implicit
  KOKKOS_FUNCTION void baz() { /* ... */ }
};