C++ Insights Reaches Hacker News Front Page With a Compiler's-Eye View of C++

A Clang tool that shows what the compiler writes for you

C++ Insights, an open-source project maintained by Andreas Fertig, has landed on the front page of Hacker News, drawing fresh attention to a utility that rewrites ordinary C++ into the code a compiler effectively sees. Built on Clang, the tool performs a source-to-source transformation: it takes a source file and emits an expanded version in which implicit language machinery — generated constructors, hidden casts, operator calls — is made explicit.

The project's GitHub documentation illustrates the idea with a minimal example. A pair of classes named Base and Derived, plus a few lines in main that copy, assign and upcast a Derived object, compile with nothing remarkable happening. Run through C++ Insights, the same program shows what Clang quietly adds: the compiler-generated default constructor, copy constructor and assignment operator for each class, rendered as commented-out declarations; a copy initialization rewritten as an explicit constructor call; assignment spelled out as a call to operator=; and the reference-to-base conversion expressed as a static_cast.

From lambdas to structured bindings

Beyond special member functions, the tool expands constructs that modern C++ deliberately hides. According to the project's README, C++ Insights transforms lambdas, range-based for-loops and structured bindings into their compiler-internal equivalents, marks where operators are invoked and where the compiler inserts casts, and deduces the types behind auto and decltype. The stated aim is that the output should itself be compilable, although the author notes this is not achievable in every case.

Born from teaching

Fertig writes that the project grew out of research he began in 2017, while preparing material on features introduced in C++11, C++14 and C++17. He found existing options wanting: Clang can dump its AST, and tools such as Compiler Explorer show generated assembly, but neither output is expressed in the language programmers actually write. For teaching students in particular, he found neither approach satisfying, so he started building a Clang-based tool that could, for instance, rewrite a range-based for-loop into its desugared form. The scope grew from there.

Availability and integrations

C++ Insights runs in the browser at cppinsights.io, so trying it requires no local setup. For local use, the binary is invoked as insights <YOUR_CPP_FILE> -- -std=c++17, with Clang flags passed after the double dash. The project can be built either inside the LLVM source tree as an external project or standalone against an existing Clang installation, with CMake options covering static linking, code coverage and similar concerns. A helper script, getinclude.py, collects system include paths from a chosen compiler — g++ by default — to work around include paths that end up hard-coded in the binary at build time.

The README also lists integrations including plugins for Vim, Neovim and Visual Studio Code, a Homebrew formula for macOS, a separately maintained Docker container, and shell completion scripts for bash, zsh and fish.

Caveats

Fertig is candid about the tool's limits. He says he does not claim to get everything right, that work on features from newer standards such as C++20 was ongoing, and that results ultimately depend on Clang's own understanding of the AST. Developers comparing behavior against GCC or MSVC should keep that Clang foundation in mind.

Why it matters

C++ is unusual among widely used languages in how much work the compiler does silently. The rule of zero encourages classes whose special member functions are all implicit, and a large share of the modern feature set — lambdas, structured bindings, range-based for-loops — is syntactic sugar over more verbose machinery. Compiler Explorer shows the end of the pipeline in assembly, and AST dumps show an internal representation, but C++ Insights occupies a useful middle ground: output written in the same C++ the reader already knows. That makes it valuable both for newcomers learning what a construct really means and for experienced developers reasoning about performance or subtle overload resolution. Its reappearance on Hacker News's front page suggests the appetite for such teaching aids remains strong.