Working with Markdown in C++: md4c, cmark, and cpp-markdown Libraries

Working with Markdown in C++: Overview of Libraries and Usage Examples

Markdown is a lightweight markup language widely used for formatting text in documentation, README files, blogs, and content management systems. In the C++ ecosystem, there are several mature libraries that allow you to parse Markdown and convert it to HTML or other formats. In this article, we will look at three popular tools: md4c, cmark, and cpp-markdown, and provide practical examples of their use.

1. The md4c Library

md4c is a lightweight Markdown parser written in pure C (with C++-compatible headers). It supports the CommonMark specification and GitHub Flavored Markdown (GFM) extensions, including tables, strikethrough, and autolinks. The library is known for its high performance and minimal memory consumption.

Key features:

• Compliance with the CommonMark 0.30 standard.
• Support for GFM extensions (tables, task lists, strikethrough).
• API for step-by-step parsing (SAX-like) and HTML generation.
• No external dependencies.

Example of using md4c to convert Markdown to HTML:

#include <md4c-html.h>#include <iostream>#include <string>

// Callback for writing HTML outputstatic void append_html(const MD_CHAR* text, MD_SIZE size, void* userdata) {    std::string* output = static_cast<std::string*>(userdata);    output->append(text, size);}

int main() {    std::string markdown = "# Hello, **World**!

This is a *test*.";    std::string html;        MD_HTML_FLAGS flags = MD_HTML_FLAG_SKIP_UTF8_BOM;    int result = md_html(markdown.c_str(), markdown.length(),                         append_html, &html, flags, 0);        if (result == 0) {        std::cout << html << std::endl;    } else {        std::cerr << "Parsing failed" << std::endl;    }    return 0;}

2. The cmark Library

cmark (formerly cmark-gfm) is the reference implementation of the CommonMark parser from the community. It is also written in C and provides a C++ wrapper. The library is actively maintained and is used in projects such as GitHub, GitLab, and Pandoc.

Key features:

• Full support for CommonMark (including the latest version of the specification).
• Built-in support for GitHub Flavored Markdown (tables, formulas, mentions).
• Generation of HTML, LaTeX, man, XML, and other formats.
• API for programmatic control of the AST (Abstract Syntax Tree).

Example of using cmark for parsing and outputting HTML:

#include <cmark-gfm.h>#include <iostream>

int main() {    const char* markdown = "## To-Do List

- [x] Buy milk- [ ] Call a friend";        // Parsing with GFM support    cmark_node* doc = cmark_parse_document(markdown, strlen(markdown), CMARK_OPT_DEFAULT);        // Conversion to HTML    char* html = cmark_render_html(doc, CMARK_OPT_DEFAULT, NULL);    std::cout << html << std::endl;        // Freeing memory    free(html);    cmark_node_free(doc);        return 0;}

3. The cpp-markdown Library

cpp-markdown is a C++ library written from scratch using the STL and modern C++11/14 features. It is focused on ease of integration and convenience of use in C++ projects. Unlike md4c and cmark, it does not require external C dependencies.

Features:

• Pure C++ implementation (templates, RAII, exceptions).
• Support for basic Markdown syntax (headings, lists, links, images).
• Extensible architecture (you can add your own parsing rules).
• Sim