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

Contents

What

C++ Insights is a clang-based tool which does a source to source transformation. Its goal is it to make things visible which 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;
Base& b = d;
}

Nothing special and of course it compiles. This is the compilers view on it:

class Base {
/* public: inline constexpr Base() noexcept; */
/* public: inline ~Base(); */
/* public: inline constexpr Base(const Base &); */
/* public: inline constexpr Base(Base &&); */
};
class Derived : public Base {
/* public: inline constexpr Derived() noexcept; */
/* public: inline constexpr Derived(const Derived &); */
/* public: inline constexpr Derived(Derived &&); */
};
int main(){
Derived d;
Base& b = static_cast<Base&>(d);
}

You can see all the compiler provided functions. Also the downcast from Derived to Base.

I do not claim to get all the things right. This is just the initial version of insights I consider good enough to hand it to the public. Keep also in mind that is solely based on clang and it's understanding of the AST.

Why

C++ Insights is a clang-based tool which does a source to source transformation. Its goal is to make things visible which 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.

Some time ago 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 start thinking how it would be, if we could see with the eyes of the compiler. Sure, there is an AST-dump at least for clang. With tools like Compiler Explorer we can see what code the compiler generates from a C++ source snippet. However, what we see is assembler. Neither the AST nor the Compiler Explorer output is in the language I write code and therefore I'm most familiar with. 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 able to 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 ended up with doing a lot more as initially planned. It shows where operators are invoked, places in which the compiler does some casting. C++ Insights is able to deduce the type behind auto or decltype. The goal is to produce compilable code. However, this is not possible in all places.

Still, there is work to do.

I do not claim to get all the things right. This is just the initial version of C++ Insights I consider good enough to hand it to the public. Also, keep in mind that it is solely based on clang and my understanding of C++ and the AST.

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

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

Building

C++ Insights can be build inside the clang-source tree or outside.

Building on Windows

See Readme_Windows.md

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
ninja

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:

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

Then build clang as you normally do.

cmake options

There are a couple of options which can be enable 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 Cevelop

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.

Usage

Using C++ Insights is fairly simple:

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

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

Here is an example:

./scripts/getinclude.py
-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 ``

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

Compatibility

Currently, the most recent stable version of clang is supported as well as the current development branch.

ToDo's

See TODO.

Get Involved

Support

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