Event Horizon of Thought

cpp standard and compiler

2026-10-15T00:00:00Z

To use update-alternatives to manage your g++ versions, you must first ensure a newer compiler (like GCC 11) is installed, register both versions with the system, and then select the new default. Here is the step-by-step process to set it up and verify it with a test compilation.

GCC to Default C++ Standard Mapping

G++ Version	Default C++ Standard Flag	Base ISO Standard
G++ 16	`-std=gnu++20`	C++20
G++ 11 to 15	`-std=gnu++17`	C++17
G++ 6 to 10	`-std=gnu++14`	C++14
G++ 5 and older	`-std=gnu++98`	C++98 / C++03

Install g++

Step 1: Install a newer compiler and register both versions

Run the following commands to install g++-11 and add both your current g++ and the new g++-11 to the update-alternatives system.

# 1. Add the toolchain test repository
sudo add-apt-repository ppa:ubuntu-toolchain-r/test -y

# 2. Update your package lists to pull in the new repository data
sudo apt update

# 3. Install GCC 11
sudo apt update && sudo apt install g++-11 -y

# 4. Register your current system g++ (typically pointing to g++-9 on Ubuntu 20.04)
# We assign it a priority of 50
sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-9 50

# 5. Register the new g++-11 with a higher priority of 100
sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-11 100

# 6. Register the new g++-16 with a higher priority of 150
sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-16 150

(Note: If your original compiler is not version 9, replace /usr/bin/g++-9 above with your actual version, which you can find by running ls /usr/bin/g++*).

Step 2: Choose your default version

If you ever want to manually toggle back and forth between them later, run this command:

sudo update-alternatives --config g++

You will see a menu like this.

Type the selection number for g++-11 and press Enter:
text
  Selection    Path              Priority   Status
------------------------------------------------------------
* 0            /usr/bin/g++-11    100       auto mode
  1            /usr/bin/g++-9     50        manual mode
  2            /usr/bin/g++-11    100       manual mode

Press <enter> to keep the current choice[*], or type selection number:

Step 3: Verify by checking the macro and compiling

Now, let's verify that your system defaults to the new version and can natively compile C++20 without changing any flags.

Check the new default macro standard

Run your macro query again. It should now output 201703L (C++17) as the bare default:

echo | g++ -dM -E -x c++ - | grep __cplusplus

• 202002L indicates C++20
• 201703L indicates C++17
• 201402L indicates C++14
• 201103L indicates C++11

Test a C++20 compilation using the standard flag

Because g++-11 recognizes the finalized standard, your previous one-liner with -std=c++20 will now compile and execute flawlessly:

echo -e '#include <iostream>\nint main() {\n    std::cout << "Hello from C++20!" << std::endl;\n    return 0;\n}' > main.cpp && g++ -std=c++20 main.cpp -o program && ./program

Test a C++20 compilation using c++20 feature:

echo -e '#include <iostream>\n#include <concepts>\n\ntemplate<typename T>\nconcept Numeric = std::integral<T> || std::floating_point<T>;\n\nvoid print_val(Numeric auto x) {\n    std::cout << "Native C++20 Concept Works! Value: " << x << std::endl;\n}\n\nint main() {\n    print_val(2026);\n    return 0;\n}' > main.cpp && g++ main.cpp -o program && ./program

Secret2

2026-10-15T00:00:00Z

Input password 1234

Markdown showcase

2026-10-14T00:00:00Z

A tour of the syntax this blog understands: CommonMark plus the GitHub-flavoured extensions MD4C implements.

Inline formatting

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Lists

First
Second
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Third

Task lists
with checkboxes

Code

const md4c = await loadMd4c();
console.log(md4c.toHtml('# Hello'));

Tables

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Wiki links	`WIKILINKS`	yes
Underline	`UNDERLINE`	no

Quotes and admonitions

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note

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warning

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Mathematics

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\hat{f}(\xi) = \int_{-\infty}^{\infty} f(x)\, e^{-2\pi i x \xi}\, dx

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Footnotes

MD4C supports footnotes¹, and links between posts like Welcome.

That's the tour.

Rendered at the bottom of the post, with a link back.↩

How it works

2026-10-13T00:00:00Z

This blog has four layers, shown below from storage to screen.

MD4C as WebAssembly

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int md4c_wasm_to_html(const char* input, unsigned input_size,
                      unsigned parser_flags, unsigned renderer_flags);

A file system made of storagesdk

The native md2html reads and writes files with stdio. A browser has no files, so wasm/js/md4c-fs.js provides readFile, readdir, renderFile and friends on top of storagesdk. In the browser it uses a read-only adapter that fetches files from a plain public URL, such as a Cloudflare R2 bucket.

Path	Contents
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`posts/<slug>.md`	Markdown source with front matter

Read-only for everyone but the owner

Visitors only ever see the bucket's public URL, which accepts reads and nothing else. The owner publishes from their own machine with scripts/publish.mjs, which writes through any storagesdk adapter (R2, S3, a local folder, ...) using credentials only they have. Rendered HTML is also cleaned by DOMPurify, under a strict Content-Security-Policy.

The blog itself

A few hundred lines of plain JavaScript: a hash router, a post list and a post view. Its appearance — title, slogan, theme, colours, background image — is configured in the site section of index.json.¹

Every post is rendered when you open it. MD4C is fast enough that this usually takes well under a millisecond.↩

Welcome

2026-10-12T00:00:00Z

Welcome! Every page you see was rendered from Markdown on your own machine by MD4C, compiled to WebAssembly. There is no server code at all: the posts are plain files in read-only storage.

What you can do

Read the sample posts
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Anyone can read; only the blog owner, who holds the storage credentials, can publish. To learn how the pieces fit together, read How it works. To see what the renderer supports, open the Markdown showcase.

tip

You can edit your document on the local machine then upload to draft storage like r2 or webdav.