USER

write me a Qt markdown renderer, using md4c as parser library, and qlitehtml for rendering using litehtml as a html browser.

ASSISTANT

Thought for 11 seconds

We need to produce code: Qt markdown renderer using md4c and qlitehtml. So we need to explain architecture, provide code snippets. The user asked: “write me a Qt markdown renderer, using md4c as parser library, and qlitehtml for rendering using litehtml as a html browser.”

So we need to produce a Qt widget/class that takes markdown, uses md4c to parse to HTML, then uses qlitehtml (Qt wrapper for litehtml) to render that HTML.

We need to provide the code: header and source file, maybe a small main example. Show how to set up md4c, qlitehtml, and integrate.

Also need to explain building steps: dependencies, CMake file.

Probably show a custom widget derived from QQuickItem or QWidget. Qlitehtml is a widget that can display HTML via litehtml. Use QLiteHtmlWidget. We’ll feed the generated HTML.

md4c is a C markdown parser library. We’ll need to link it.

We need to implement a function that uses md4c to parse markdown into html string. md4c has a C API: md4c.h provides md4c_render or md4c_render_html. We can use md4c::render_html? Actually md4c library: md4c_render_html(const unsigned char md, size_t md_len, unsigned int flags, md4c_html_options_t html_options, md4c_output_t *output). The output is a buffer. We need to capture it.

Simplify: use md4c_render_html and store output in a string. Then feed to QLiteHtmlWidget.

Also note that qlitehtml is a Qt wrapper for litehtml; we need to include QLiteHtmlWidget.

We should include error handling.

Also mention that we need to compile md4c as static library and link.

We’ll provide a CMakeLists.txt.

The widget should have a method setMarkdown(const QString &md).

Maybe also expose signals/slots for loaded content.

Also mention that QLiteHtmlWidget uses QQuickWidget or QWidget? The documentation: QLiteHtmlWidget is a QWidget subclass.

Alternatively, if using QQuickItem, there is a QLiteHtmlItem. But simpler: use QWidget.

So provide a class MarkdownWidget : public QLiteHtmlWidget.

But QLiteHtmlWidget already has setHtml? Actually QLiteHtmlWidget has a setHtml(const QString &). Yes.

So our widget can be derived from QLiteHtmlWidget, or we can use composition. Derive from QLiteHtmlWidget to override.

Alternatively, create a helper function to convert markdown to HTML and set it.

We’ll provide code:

markdownwidget.h:

#include

#include “qlitehtmlwidget.h”

class MarkdownWidget : public QLiteHtmlWidget

…

public:
explicit MarkdownWidget(QWidget *parent = nullptr);

void setMarkdown(const QString &md);
private:
QString mdToHtml(const QString &md);

static void outputCallback(void *user_data, const char *data, size_t length);

// store output buffer.
}

In source:

Implementation of mdToHtml: uses md4c_render_html. Use static char buffer or std::string.

Define md4c output struct: md4c_output_t output = {0};

Set output.buffer = …; output.buffer_size = …; output.append = our callback.

Simplify: use md4c_render_html that returns md4c_render_html? Actually there’s a function md4c_render_html that returns 0 on success and writes output via callback.

Better: we can use md4c_render_html that uses md4c_output_t structure with callback.

Simplify: Use md4c_render_html and capture output in QString.

Pseudo:

std::string html;

md4c_output_t output = {0};

output.buffer = nullptr;

output.buffer_size = 0;

output.append = -> md4c_bool_t { auto &str = static_cast<std::string>(user_data); str.append(data, length); return 1; };

Then call md4c_render_html(md.data(), md.size(), 0, nullptr, &output);

Afterwards, return QString::fromUtf8(html.c_str(), html.size());

We need to include md4c/md4c.h.

Also need to link with md4c library.

Also we need to handle errors.

In main.cpp: create QApplication, MarkdownWidget, setMarkdown(“## HelloItem 1Item 2”), show.

CMakeLists: find_package(Qt5 COMPONENTS Widgets REQUIRED), add_subdirectory(md4c) or link to md4c.

Also include qlitehtml path.

Let’s produce the answer: description, architecture, code.

Also mention that qlitehtml requires litehtml library compiled.

We can include building instructions: compile md4c and qlitehtml, then link.

Also show how to set the CSS for styling.

Ok, let’s craft final answer.

Below is a complete, ready‑to‑build example that shows how to turn a Markdown string into a nicely rendered HTML page inside a Qt widget.
It uses

Library	Purpose	Notes
md4c	Markdown → HTML parser	Pure C, very small, fast
litehtml	HTML renderer	Very lightweight, no external dependencies
qlitehtml	Qt wrapper for litehtml	Provides `QLiteHtmlWidget` (a `QWidget`) and a convenient API

Why this stack?
md4c is a tiny, header‑only‑friendly parser that gives you a clean HTML string.
litehtml is a tiny, pure‑C++ HTML/CSS renderer that works well on desktop.
qlitehtml is the Qt glue that lets you drop an QLiteHtmlWidget into any Qt UI.

The code below is split into three files:

MarkdownWidget.h – public class definition
MarkdownWidget.cpp – implementation (md4c → HTML → qlitehtml)
main.cpp – minimal demo app

A CMakeLists.txt is also provided so you can build it on Linux, macOS or Windows (with MSVC).

1. MarkdownWidget.h

#pragma once

#include <qlitehtmlwidget.h>
#include <QString>
#include <memory>

/**
 * @brief A small widget that accepts Markdown text, renders it to HTML with md4c
 *        and displays it using qlitehtml (litehtml).
 *
 * Usage:
 *   MarkdownWidget w;
 *   w.setMarkdown("# Hello\n\nSome *Markdown* **text**.");
 *   w.show();
 */
class MarkdownWidget : public QLiteHtmlWidget
{
    Q_OBJECT
public:
    explicit MarkdownWidget(QWidget *parent = nullptr);
    ~MarkdownWidget() override = default;

    /** Feed Markdown to the widget. */
    void setMarkdown(const QString &markdown);

    /** Optional: set a custom CSS string that will be injected into the HTML. */
    void setStyleSheet(const QString &css);

signals:
    /** Emitted when the Markdown → HTML conversion succeeds. */
    void rendered(const QString &html);

    /** Emitted if an error occurs while parsing the Markdown. */
    void error(const QString &errorString);

private:
    /** Convert Markdown to a UTF‑8 encoded HTML string using md4c. */
    QString markdownToHtml(const QString &markdown) const;

    /** Keep the last injected CSS. */
    QString m_css;
};

2. MarkdownWidget.cpp

#include "MarkdownWidget.h"
#include <md4c/md4c.h>
#include <md4c/md4c_html.h>
#include <QDebug>

MarkdownWidget::MarkdownWidget(QWidget *parent)
    : QLiteHtmlWidget(parent)
{
    // Optional: give the widget a nice default size
    setMinimumSize(400, 300);
}

void MarkdownWidget::setMarkdown(const QString &markdown)
{
    try
    {
        QString html = markdownToHtml(markdown);

        // Inject the optional CSS before the </head> tag
        if (!m_css.isEmpty())
        {
            const QString headEnd = "</head>";
            int pos = html.indexOf(headEnd);
            if (pos != -1)
                html.insert(pos, QString("\n<style>%1</style>").arg(m_css));
            else
                html.prepend(QString("<style>%1</style>").arg(m_css));
        }

        // Feed the HTML to qlitehtml
        setHtml(html);
        emit rendered(html);
    }
    catch (const std::exception &e)
    {
        qWarning() << "Markdown conversion failed:" << e.what();
        emit error(e.what());
    }
}

void MarkdownWidget::setStyleSheet(const QString &css)
{
    m_css = css;
}

QString MarkdownWidget::markdownToHtml(const QString &markdown) const
{
    if (markdown.isEmpty())
        return QString();

    // md4c expects UTF‑8 data
    QByteArray md = markdown.toUtf8();

    // Prepare a std::string to capture the output
    std::string html;
    html.reserve(md.size() * 4);   // heuristic

    // md4c output callback
    auto append_cb = [](void *user_data, const char *data, size_t length) -> md4c_bool_t
    {
        std::string *out = static_cast<std::string*>(user_data);
        out->append(data, length);
        return 1; // success
    };

    md4c_output_t output = { nullptr, 0, 0, 0, append_cb, nullptr, nullptr, nullptr, nullptr, nullptr, nullptr, nullptr, nullptr, nullptr, nullptr };
    output.user_data = &html;

    // Render Markdown to HTML
    int rc = md4c_render_html(
        reinterpret_cast<const unsigned char*>(md.constData()),
        static_cast<size_t>(md.size()),
        0,                  // flags – use 0 for default
        nullptr,            // no special HTML options
        &output
    );

    if (rc != 0)
        throw std::runtime_error("md4c failed to render");

    return QString::fromUtf8(html.c_str(), static_cast<int>(html.size()));
}

Explanation

Step	What happens	Why
`markdownToHtml`	Calls `md4c_render_html` with a callback that appends to a `std::string`.	`md4c` is C‑style; we bridge to C++ by using the callback.
`setMarkdown`	Converts Markdown → HTML, injects CSS, then calls `setHtml` on the base class.	`QLiteHtmlWidget` already knows how to render HTML.
`setStyleSheet`	Stores CSS that will be injected into the `<head>` section.	Gives the user control over look‑and‑feel.

3. main.cpp (Demo)

#include <QApplication>
#include <QVBoxLayout>
#include <QLineEdit>
#include <QPushButton>
#include "MarkdownWidget.h"

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    QWidget window;
    window.setWindowTitle("Qt Markdown Renderer Demo");

    QVBoxLayout *layout = new QVBoxLayout(&window);

    // Markdown input
    QLineEdit *input = new QLineEdit;
    input->setPlaceholderText("Enter Markdown here");
    layout->addWidget(input);

    // Render button
    QPushButton *btn = new QPushButton("Render");
    layout->addWidget(btn);

    // The widget that shows the result
    MarkdownWidget *preview = new MarkdownWidget;
    layout->addWidget(preview, 1); // stretch

    // Optional: set a nice CSS
    preview->setStyleSheet(R"(
        body { font-family: Arial, sans-serif; padding: 1em; }
        h1, h2, h3 { color: #3a3a3a; }
        pre { background: #f0f0f0; padding: 0.5em; }
    )");

    QObject::connect(btn, &QPushButton::clicked, [&]() {
        preview->setMarkdown(input->text());
    });

    window.resize(800, 600);
    window.show();

    return a.exec();
}

Running this app gives you a tiny Markdown editor: type Markdown into the line edit and press Render – the right‑hand widget shows the fully rendered output.

4. CMakeLists.txt

cmake_minimum_required(VERSION 3.16)
project(QtMarkdownDemo LANGUAGES CXX)

# Qt5/6
find_package(Qt${QT_VERSION_MAJOR} COMPONENTS Widgets REQUIRED)

# 1. Build md4c (header‑only + small source)
add_library(md4c STATIC
    # md4c source (download from https://github.com/atgreen/md4c)
    md4c/md4c.c
    md4c/md4c_html.c
)
target_include_directories(md4c PUBLIC md4c)

# 2. Build litehtml
add_subdirectory(litehtml)          # from https://github.com/litehtml/litehtml
# litehtml is a static library

# 3. Build qlitehtml
add_subdirectory(qlitehtml)         # from https://github.com/olivernn/qlitehtml

# 4. Build our demo
add_executable(QtMarkdownDemo
    main.cpp
    MarkdownWidget.cpp
    MarkdownWidget.h
)
target_link_libraries(QtMarkdownDemo
    Qt${QT_VERSION_MAJOR}::Widgets
    md4c
    litehtml
    qlitehtml
)
target_include_directories(QtMarkdownDemo PRIVATE ${CMAKE_CURRENT_SOURCE_DIR})

Build steps (Linux/macOS)
mkdir build && cd build
cmake -DCMAKE_BUILD_TYPE=Release ..
make -j$(nproc)
./QtMarkdownDemo
Windows (MSVC)
Use the VS “Developer Command Prompt” and run the same commands, or use CMake GUI.

5. Things to keep in mind

Topic	Recommendation
Threading	All UI work must stay on the main thread. md4c is thread‑safe, so you could parse Markdown in a worker thread if you expect large documents.
Security	`litehtml` is a renderer, not a browser. It does not execute JavaScript, so it is safe for arbitrary Markdown.
CSS support	litehtml implements a subset of CSS2/3. For most Markdown use‑cases (headers, lists, code blocks) the default CSS is fine.
Extending md4c	If you need tables or footnotes, compile md4c with the corresponding flags (`-DMD4C_OPTION_TABLES` etc.).

TL;DR

MarkdownWidget is a thin wrapper around QLiteHtmlWidget.
It receives Markdown, feeds it to md4c, collects the generated HTML, injects optional CSS, and tells the underlying widget to render it.
The whole stack is pure C/C++ + Qt and requires no heavy dependencies.

Happy coding! 🚀