D2mcpp Authoring

d2mcpp (D2X) Authoring Conventions

d2mcpp teaches Modern C++ core language features through a tightly coupled quartet: Book (mdBook) + Code (exercises-as-tests) + Solution + Auto-checker (d2x checker / native mcpp test), all of it bilingual (zh + en). A "lesson" is never one file — it is a coordinated set across several directories. The cardinal sin here is producing a half-set (a chapter with no exercise, a zh file with no en counterpart, an exercise missing from solutions/). This skill exists so every lesson lands complete and consistent.

Read references/anatomy.md for the full directory map and the exact list of files every lesson must touch. Use the ready-made scaffolds in assets/ rather than reconstructing format from memory.

The design principle (decide this BEFORE writing)

The project is organized by standard (cpp11/ cpp14/ cpp17/ cpp20/ cpp23/). Honor that axis:

• Teach each feature as the standard that introduced it actually delivered it — the historical state, compiled at its introduction standard. The compiler-driven (dslings) format is at its best when the learner feels the C++11-era restriction first (e.g. constexpr single-return, std::is_pod, copy-elision behavior). Folding every later refinement back into the introduction chapter erases that, and empties out the later-standard sections.
• Independent later capabilities become their own chapter under the later standard's section — this is what fills cpp14/17/20. Examples: fold expressions → cpp17, using enum → cpp20, designated initializers → cpp20, if constexpr → cpp17, decltype(auto) → cpp14, consteval/constinit → cpp20.
• Pure refinements / rule-tweaks / deprecations stay in the introduction chapter as a short "特性演进 / Feature Evolution" note with a cross-link forward — they are not chapter-worthy on their own. Examples: auto x{1} deduction change, std::is_pod deprecation pointer, guaranteed copy elision remark, P0136 inheriting-constructor semantics fix.
• All exercises compile as c++23 — the cppNN/ directory denotes when the feature was introduced, not the compile flags. Teach the introduction-era shape through content and assertions (an untested teaching point disappears silently). If one exercise needs special flags, declare a per-glob entry in the member's src/<std>/mcpp.toml [build].flags — never a bare TODO hack.

Background and the full per-feature evolution analysis live in .agents/docs/2026-06-08-cpp11-feature-evolution-and-cxx20-baseline.md. Consult it when deciding where a feature's later improvements should go.

The decision rule, compactly:

The unit of work: one lesson = these files, all bilingual

When adding a feature numbered NN with slug topic (e.g. 06-scoped-enums):

1. book/src/<std>/NN-topic.md — zh chapter
2. book/en/src/<std>/NN-topic.md — en chapter (translate prose, keep code identical)
3. src/<std>/tests/NN-topic/K.cpp — one or more exercises (K = 0,1,2…), zh comments
4. src/en/<std>/tests/NN-topic/K.cpp — en exercises (translate comments only)
5. solutions/tests/<std>/NN-topic/K.cpp — reference solution per exercise (zh/en 共用)
6. (nothing to register — exercises are tests, discovered by directory convention)
7. Add the chapter line to both book/src/SUMMARY.md and book/en/src/SUMMARY.md
8. Add a changelog entry to both book/src/changelog.md and book/en/src/changelog.md

Missing any of these = an incomplete lesson. references/anatomy.md has the exact registration snippets and SUMMARY/changelog line formats.

Naming and numbering

• Chapter/exercise prefix is a two-digit sequence NN within the standard section; slug is kebab-case and matches book + tests + solutions.
• Exercises split into 0.cpp, 1.cpp, … by sub-topic inside the chapter directory src/<std>/tests/NN-topic/; a single-exercise chapter is just 0.cpp.
• The d2x checker <name> name is the slug (and <slug>-1, <slug>-2 for later exercises) — it omits the NN- prefix. Keep checker names, file names, and book references mutually consistent.

Book chapter format

Use assets/chapter.zh.md / assets/chapter.en.md. Required structure, in order:

1. Language-switch header <div align=right> 🌎 [中文] | [English] + link defs.
   • zh links: [中文]: ./NN-topic.html and [English]: ../en/<std>/NN-topic.html
   • en links: [中文]: ../../<std>/NN-topic.html and [English]: ./NN-topic.html
2. # 标题 - english name (zh) / # English Title (en), then a one-paragraph intro that states the feature was introduced in C++NN and what problem it solves.
3. Resource table | Book | Video | Code | X | — cppreference link + markdown permalink + video link + first practice-code permalink.
4. Bold motivation blocks: **为什么引入?**, **和……的区别?**.
5. Numbered Chinese-numeral sections, in order: ## 一、基础用法和场景, ## 二、真实案例 - STL 中的 <feature>, ## 三、注意事项, ## 四、练习代码, ## 五、其他.
6. ## 二、真实案例 corroborates the feature with real, verbatim STL code quoted from the in-repo msvc-stl/ basis — see "The 真实案例 section" below.
7. ## 四、练习代码 lists every exercise topic of the chapter (-0, -1, …, matching the dslings file count) with dslings links as an index, but the checker block shows only the single entry command d2x checker <name> — d2x auto- advances through the remaining exercises, so do NOT list one command per exercise. Then a ### 练习交流讨论 link to the chapter's forum thread. Each exercise's own d2x checker <name>-K name and a Tips line specific to that exercise belong in that exercise file's header, not in the book.
8. ## 五、其他 is the fixed footer (forum / repo / video list / xlings links).
9. If the feature evolved later, add a ## 特性演进 / Feature Evolution block before ## 五、其他 — introduced standard + bullet list of later refinements + forward links to the later-standard chapters.

Keep prose tone consistent with existing chapters: explanatory, example-driven, short code blocks with inline comments showing the failure and the fix.

The 真实案例 / Real-World Case section (msvc-stl basis)

## 二、真实案例 exists to prove the feature earns its place by showing real production code, quoted verbatim — never invented, never paraphrased. The single source of truth is the in-repo vendored MSVC STL basis at the top-level msvc-stl/ (the complete upstream stl/ subtree; msvc-stl/SOURCE.md records the pinned upstream commit and how to refresh it). Rules:

• Quote from msvc-stl/stl/inc/<header> (or src/) byte-for-byte; mark any elision with (有删节) / (abridged). Do not reconstruct from memory.
• Keep internal macros (_NODISCARD, _CONSTEXPR17, _STD, …) as written and add one line telling the reader they are library-internal and ignorable.
• Pick a usage that echoes a syntax point from ## 一 (e.g. std::begin's auto … -> decltype(...) for the auto/decltype chapter), so the section reads as "you use it this way → the standard library does too".
• Scope-lock to the chapter's standard: a cpp11 chapter quotes C++11-era usage only; do not show later forms (e.g. decltype(auto)).
• Link to the local msvc-stl/ path, not the upstream GitHub repo, so the citation stays pinned to the vendored snapshot.
• Every chapter should carry this section. If a feature genuinely has no representative usage in the STL implementation, omit it and state why in the PR.

Exercise format (exercises are tests)

Use assets/exercise.cpp. Essentials:

• Header comment block: d2mcpp URL, license: Apache-2.0, file: path, Exercise/练习: line (<std> | NN - topic | 中文小标题), Tips/提示:, Docs/文档: (cppreference), and the Auto-Checker/自动检测命令: with d2x checker <name>.
• import std; + import d2x; — no #include unless the lesson needs a macro from a header (e.g. NULL teaching needs <cstddef>; put the include before the imports with a comment saying why).
• main() seeded with intentional errors the learner fixes, each flagged by a numbered inline comment (// 1.…, // 2.…) telling them what to do.
• Conventions (the d2x library, import d2x;, no macros):
  • d2x::check(cond, "原文") / d2x::check_eq(a, b, "a == b") — runtime checkpoints; the third argument is the expression text shown to learners (c++23 has no reflection to capture it automatically — always pass it). Learners must make checks pass by fixing code, not by deleting them.
  • D2X_YOUR_ANSWER — the fill-in placeholder. It is not defined anywhere; the compiler error pointing at it is the feature. Never define it.
  • d2x::wait() — separates exercises; learner deletes it to advance. Put exactly one at the end of each exercise's checkpoints.
  • d2x::dont_delete_this(expr) — guards expressions that must not be removed.
• A pure observation exercise may be zero-dependency plain C++ (no imports) — judged by exit code alone, copy-pasteable into Compiler Explorer.
• The en exercise translates only the comment prose; the code stays byte-for-byte identical to the zh exercise so checker behavior matches.

Reference solution format

Use assets/solution.cpp. It is the corrected exercise with:

• A solution header: reference solution for: <exercise path>, plus the note that it is for CI/maintainers only, not a tutorial entry, and a pointer back to the exercise file.
• d2x::wait() removed; all checkpoints passing; unused locals silenced with (void)var; rather than left to warn.
• Same imports and structure as the exercise.

Definition of done

Before claiming a lesson complete, verify the whole set exists and is wired up, and that the exercise/solution actually build & check. See references/anatomy.md for the verification commands (d2x checker <name> and the mcpp build) — run them; do not assert success without the output.