Local Conventions For New Code

Use this guide when writing new declarations anywhere in the tree. It records the forms review asks for. It does not authorize rewriting code the task is not otherwise changing; rule 5 in the repository-root AGENTS.md governs untouched lines. Eigen predates most of these forms, so a tree-wide count is not the convention: new code uses the current form, and a file being edited heavily should come out uniform rather than half converted.

Declarations

  • Trait and evaluator constants are static constexpr members, not enum blocks; enum constants are being phased out. Give each the type it is used as: Flags is unsigned int by convention, predicates are bool.
  • Prefer using to typedef, nullptr to NULL, = default and default member initializers to empty constructor bodies that assign each member.
  • Use numext:: math functions rather than std:: in library code, and Eigen‘s metaprogramming aliases (bool_constant, void_t, remove_all_t; see Eigen/src/Core/util/Meta.h) rather than spelling out the standard forms. internal::is_arithmetic is not a spelling of std::is_arithmetic: it is deliberately specialized for packet and Eigen scalar types and differs on long double during GPU compilation, so use it only when Eigen’s extended arithmetic category is specifically intended.
  • Put SFINAE in a defaulted template parameter rather than the return type. When an overload set needs the negative case too, constrain both overloads: an exact-match overload next to an unconstrained one can bind a converted temporary and return a dangling reference.
  • An in-class definition is already implicitly inline; a bare inline there is noise. Use EIGEN_STRONG_INLINE or EIGEN_ALWAYS_INLINE when inlining matters, and nothing otherwise.
  • Spell names out (scratch, not scr) and name traits for the property they assert.

The C++14 baseline

Supported headers compile as C++14, which rules out forms that review suggestions often reach for:

  • if constexpr is C++17: use EIGEN_IF_CONSTEXPR(...) wherever the condition is compile-time constant. Note the condition must still be valid C++14 either way — the macro lowers to a plain if there.
  • Designated initializers are C++20: use aggregate assignment with /*name=*/ comments. Fields derived from other fields of the same object must be computed from locals first; the braced temporary cannot read fields it is about to set.
  • std::span, CTAD, fold expressions, constinit, and later library additions are unavailable outside guarded backends with a documented newer requirement (the SYCL configurations force C++17, for example).

Comments

The comment rules in the repository-root AGENTS.md are enforced in review and are the most repeated style finding here. Before publishing a diff, reread each added comment and delete the ones that narrate code or restate an identifier. Keep the ones recording mathematics, invariants, compatibility constraints, provenance, or the reason a slower or unusual form is deliberate — stated at the construct, not in the merge request.

REUSE metadata for new files

Every new source file needs accurate REUSE metadata. Original Eigen code normally uses MPL-2.0; prefer the collective form when an agent cannot truthfully attribute an individual author:

// SPDX-FileCopyrightText: The Eigen Authors
// SPDX-License-Identifier: MPL-2.0

Use the language's comment syntax. Documentation or assets that should not carry inline tags must be covered precisely in REUSE.toml; do not add a broad annotation that hides unrelated files. Compatible adapted material may require a different license expression and attribution, which must be preserved rather than relabeled as MPL-2.0.