Use this guide when writing new declarations anywhere in the tree. It records the forms reviewers ask for. It does not authorize rewriting lines your task leaves alone; rule 5 in the repository-root AGENTS.md covers those. Eigen predates most of these forms, so the most common form in the tree is not the convention. Write new code in the current form. A file that you edit heavily should end up uniform rather than half converted.
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. In C++14 the in-class declaration is not a definition. If code odr-uses such a member of a class template, the member also needs a namespace-scope definition, template <...> constexpr T Cls<...>::kName; (arch/Default/Half.h has the form). Without it, the code links at -O2 but fails to link at -O0. Binding the member to a const T& parameter, such as numext::mini's, odr-uses it, and so does taking its address. Pass a prvalue (+kName, Index(kName)) or add the definition. The test suite builds optimized, so it will not catch the omission.using to typedef, nullptr to NULL, = default and default member initializers to empty constructor bodies that assign each member. The using rule applies everywhere, test/ and contrib/ included. The sweep that converted Eigen/src skipped those directories, so most aliases near new code there are still typedef. Copying the neighbors reproduces the form the sweep removed. Do not rely on CI to catch it: clang-tidy's modernize-use-using check does not report function-local typedefs, a gap recorded in scripts/check_style.py.kCamelCase is an accepted spelling for static constexpr and static constants, alongside the older snake_case and SCREAMING_CASE forms. It is not a review finding.numext:: math functions rather than std:: in library code. Use 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.inline; a bare inline there is noise. Use EIGEN_STRONG_INLINE or EIGEN_ALWAYS_INLINE when inlining matters, and nothing otherwise.EIGEN_STATIC_ASSERT_* helper that names them (_VECTOR_ONLY, _SAME_MATRIX_SIZE, ...) or with EIGEN_STATIC_ASSERT(cond, TOKEN). These honor EIGEN_NO_STATIC_ASSERT and a user-provided EIGEN_STATIC_ASSERT override. Write unconditional implementation invariants as static_assert(cond, "what must hold"); a plain static_assert bypasses those configuration mechanisms.EIGEN_DEPRECATED or EIGEN_DEPRECATED_WITH_REASON("use ..."), keep it working by forwarding to the replacement, and name the replacement in its Doxygen block.scratch, not scr) and name traits for the property they assert.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. Under C++14 the macro expands to a plain if, so the code must still be valid C++14./*name=*/ comments. When a field is derived from other fields of the same object, compute it 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).Code added to a hot inner loop makes the enclosing function bigger. The bigger function can fall out of the instruction cache even when an EIGEN_PREDICT_FALSE guard keeps the new code from running. Watch for this when you add a check or a fallback to such a loop. If a benchmark shows the cost, one way to recover it is to move the cold path into an EIGEN_DONT_INLINE helper.
Reviewers enforce the comment rules in the repository-root AGENTS.md, and breaking them is the most common 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. Put them at the code they explain, not in the merge request.
Prefer the most precise notation that fits. A recurrence, an error bound, an invariant written as an expression, or two lines of pseudo-code usually carry more than a paragraph and are read faster by this audience:
// Bad: the relative error in summing n elements this way is bounded by roughly twice the // machine epsilon multiplied by the quantity log base two of n over B, plus B, where B is // the number of elements summed sequentially in each leaf of the tree. // Good: tree summation, relative error <= ~2*eps*(log2(n/B) + B) for leaf size B.
An exact bound, invariant, or identity beats prose that conveys it only in part (m kept in [1, 2) says more than “balanced form”). A comment that names a theorem also states it. Reuse the symbols of the surrounding file and the cited reference rather than inventing new ones for a single sentence. Notation that restates what the code shows is the same defect as prose that does. Prose remains the tool for a reason: why this form and not the obvious one. Comments are plain text, so write expressions the way the rest of the tree does, not in a markup language that does not render.
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. Material adapted from a compatibly licensed source can require a different license expression and attribution. Preserve them rather than relabeling the material as MPL-2.0.