Writing Documentation for a C++ Project: From A to Z

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Introduction: Why Documentation is Not a Luxury, but a Necessity

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Every C++ developer has at least once encountered a situation where someone else's (or their own) code turns into a dark forest after a month. The high complexity of the language, the abundance of templates, smart pointers, and inheritance make C++ projects especially vulnerable to a lack of documentation. Without it, a project risks becoming "legacy" even before its first release.

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In this article, we will break down the complete cycle of creating documentation for a C++ project: from choosing tools to publication. You will learn how to use Doxygen and Markdown to turn raw code into a coherent, readable, and maintainable system.

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1. Planning and Choosing Tools

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1.1. Define Your Audience and Goals

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Before writing a single line, answer three questions:

Who will read the documentation? (colleagues, clients, open-source community)
What is the reader's level of C++ proficiency? (junior, middle, senior)
What should be documented? (only the public API or internal logic)

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For a C++ project, the standard set includes:

README.md — the entry point for new developers
API Reference — automatically generated documentation for classes and functions
User Guide — examples of building, configuration, and usage
CHANGELOG — version history

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1.2. Tools for C++

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The de facto standard is Doxygen. It supports C++, C, Python, and other languages, generating HTML, LaTeX, PDF. Alternatives:

Sphinx with Breathe — if the project uses Python wrappers
MkDocs — for quickly generating a site from Markdown
Doxygen + Doxybook2 — a hybrid approach

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We will use Doxygen, as it is maximally adapted for C++.

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2. Commenting Code: Rules and Examples

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2.1. Basic Doxygen Syntax

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Doxygen supports two comment styles:

Javadoc style — /** ... */
Qt style — /*! ... */

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Example of documenting a class and method:

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/**\n * @brief Smart pointer with reference counting.\n *\n * Implements basic std::shared_ptr functionality.\n * Is not thread-safe.\n *\n * @tparam T Type of the stored object\n */\ntemplate <typename T>\nclass SharedPtr {\npublic:\n    /**\n     * @brief Creates a pointer from a raw object.\n     * @param ptr Pointer to the allocated object (can be nullptr).\n     * @warning After calling this function, ownership transfers to SharedPtr.\n     */\n    explicit SharedPtr(T* ptr = nullptr);\n    \n    /**\n     * @brief Get the number of references to the object.\n     * @return Current number of owners.\n     */\n    size_t use_count() const noexcept;\n};

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