C++ Insights Documentation

C++ Insights - See your source code with the eyes of a compiler.

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C++ Insights is a Clang-based tool that does a source-to-source transformation. The goal of C++ Insights is to make things visible that normally and intentionally happen behind the scenes. It's about the magic the compiler does for us to make things work.

Take this piece of code for example:

class Base {
class Derived : public Base {
int main() {
Derived d;
Derived d2 = d;
d2 = d;
Base& b = d;

Nothing special, and of course, it compiles. This is the compiler's view on it:

class Base
// inline constexpr Base() noexcept = default;
// inline constexpr Base(const Base &) noexcept = default;
// inline constexpr Base & operator=(const Base &) noexcept = default;
class Derived : public Base
// inline constexpr Derived() noexcept = default;
// inline constexpr Derived(const Derived &) noexcept = default;
// inline constexpr Derived & operator=(const Derived &) noexcept = default;
int main()
Derived d;
Derived d2 = Derived(d);
Base & b = static_cast<Base&>(d);
return 0;

You can see all the compiler-provided special member functions and the downcast from Derived to Base.


C++ Insights is a Clang-based tool that does a source-to-source transformation. The goal of C++ Insights is to make things visible that normally and intentionally happen behind the scenes. It's about the magic the compiler does for us to make things work. Or looking through the classes of a compiler.

In 2017, I started looking into some new things we got with C++11, C++14, and C++17. Amazing things like lambdas, range-based for-loops, and structured bindings. I put it together in a talk. You can find the slides and a video online.

However, all that research and some of my training and teaching got me to start thinking about how it would be if we could see with the eyes of the compiler. Sure, there is an AST dump, at least for Clang. We can see what code the compiler generates from a C++ source snippet with tools like Compiler Explorer. However, what we see is assembler. Neither the AST nor the Compiler Explorer output is in the language I write code. Hence, I'm not very familiar with this output. Plus, when teaching students C++, showing an AST and explaining that it is all there was not quite satisfying for me.

I started to write a Clang-based tool that can transform a range-based for-loop into the compiler-internal version. Then, I did the same for structured bindings and lambdas. In the end, I did much more than initially planned. It shows where operators are invoked and places in which the compiler does some casting. C++ Insights can deduce the type behind auto or decltype. The goal is to produce compilable code. However, this is not possible in all places.

You can see, for example, the transformation of a lambda, range-based for-loop, or auto. Of course, you can transform any other C++ snippet.

See yourself. C++ Insights is available online: cppinsights.io.

Still, there is work to do.

I do not claim to get all the things right. I'm also working on supporting features from new standards, like C++20, at the moment. Please remember that C++ Insights is based on Clang and its understanding of the AST.

I did a couple of talks about C++ Insights since I released C++ Insights. For example, at C++ now. Here are the slides and the video.


C++ Insights can be built inside the Clang source tree or outside.

Building on Windows

See Readme_Windows.md

Building on Arch Linux

To build with extra/clang use the following extra flags: -DINSIGHTS_USE_SYSTEM_INCLUDES=off -DCLANG_LINK_CLANG_DYLIB=on -DLLVM_LINK_LLVM_DYLIB=on

See https://github.com/andreasfertig/cppinsights/issues/186 for an explanation of why INSIGHTS_USE_SYSTEM_INCLUDES needs to be turned off.

extra/clang and extra/llvm provide /usr/lib/{libclangAST.so,libLLVM*.a,libLLVM.so}. libclangAST.so needs libLLVM.so and there would be a conflict if libLLVM*.a (instead of libLLVM.so) are linked. See https://bugs.archlinux.org/task/60512

Building outside Clang

You need to have a Clang installation in the search path.

git clone https://github.com/andreasfertig/cppinsights.git
mkdir build && cd build
cmake -G"Ninja" ../cppinsights

The resulting binary (insights) can be found in the build folder.

Building inside Clang

For building it inside the Clang source tree, assuming you have your source tree already prepared under llvm-project:

cd llvm-project/clang-tools-extra/
git clone https://github.com/andreasfertig/cppinsights.git
echo "add_subdirectory(cppinsights)" >> CMakeLists.txt

To activate the C++ Insights build you have to set -DLLVM_ENABLE_PROJECTS="clang;clang-tools-extra" for cmake:

cmake -DCMAKE_BUILD_TYPE=Release -DLLVM_ENABLE_PROJECTS="clang;clang-tools-extra" -G "Unix Makefiles" ../llvm-project

Then, build Clang as you normally do.

cmake options

There are a couple of options that can be enabled with cmake:

Option Description Default
INSIGHTS_STRIP Strip insight after build ON
INSIGHTS_STATIC Use static linking OFF
INSIGHTS_COVERAGE Enable code coverage OFF
INSIGHTS_USE_LIBCPP Use libc++ for tests OFF
DEBUG Enable debug OFF

Use it with <a target="_blank" href="https://www.cevelop.com">Cevelop</a>

git clone https://github.com/andreasfertig/cppinsights.git
mkdir build_eclipse
cd build_eclipse
cmake -G"Eclipse CDT4 - Unix Makefiles" ../cppinsights/

Then, in Cevelop Import -> General -> Existing Project into Workspace. Select build_eclipse. Enjoy editing with Cevelop.


Using C++ Insights is fairly simple:

insights <YOUR_CPP_FILE> -- -std=c++17

Things get complicated when it comes to the system-include paths. These paths are hard-coded in the binary, which seems to come from the compiler C++ Insights was built with. To help with that, check out scripts/getinclude.py. The script tries to collect the system-include paths from the compiler. Without an option, getinclude.py uses g++. You can also pass another compiler as a first argument.

Here is an example:

-isystem/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1 -isystem/usr/local/include -isystem/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/clang/7.3.0/include -isystem/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include -isystem/usr/include

The script can be used together with C++ Insights:

insights <YOUR_CPP_FILE> -- -std=c++17 `./scripts/getinclude.py`

Custom GCC installation

In case you have a custom build of the GCC compiler, for example, gcc-11.2.0, and NOT installed in the compiler in the default system path, then after building, Clang fails to find the correct libstdc++ path (GCC's STL). If you run into this situation, you can use "`--gcc-toolchain=/path/GCC-1x.x.x/installed/path`" to tell Clang/C++ Insights the location of the STL:

./cppinsights Insights.cpp -- --gcc-toolchain=${GCC_11_2_0_INSTALL_PATH} -std=c++20

Here "`${GCC_11_2_0_INSTALL_PATH}`" is the installation directory of your customized-built GCC. The option for Clang is described here.

Ready to use Docker container

There is also another GitHub project that sets up a docker container with the latest C++ Insights version in it: C++ Insights - Docker

C++ Insights @ Vim

A plugin for Vim is available at here.

C++ Insights @ VSCode

An extension for Visual Studio Code is available at the VS Code marketplace: C++ Insights - VSCode Extension.

C++ Insights @ brew

At least for macOS, you can install C++ Insights via Homebrew thanks to this formulae:

brew install cppinsights


I aim for the repository to compile with the latest version of Clang and at least the one before. The website tries to stay close to the latest release of Clang. However, due to certain issues (building Clang for Windows), the website's version is often delayed by a few months.

C++ Insights @ YouTube

I created a YouTube channel where I release a new video each month. In these videos, I use C++ Insights to show and explain certain C++ constructs, and sometimes I explain C++ Insights as well.



Get Involved


If you like to support the project, consider submitting a patch. Another alternative is to become a GitHub Sponsor or a Patreon supporter.