cppast

cppast

Library interface to the C++ AST — parse source files, synthesize entities, get documentation comments and generate code.

Motivation

If you're writing a tool that needs access to the C++ AST (i.e. documentation generator, reflection library, …), your only option apart from writing your own parser is to use clang. It offers three interfaces for tools, but the only one that really works for standalone applications is libclang. However, libclang has various limitations and does not expose the entire AST.

So there is no feasible option — except for this library. It was originally a part of the standardese documentation generator, but has been extracted into an independent library.

See this blog post for more information about the motiviation and design.

Features

• Exposes (almost) all C++ entities: Supports everything from functions to classes, templates to friend declarations, macros to enums;
• Exposes full information about C++ types;
• Supports and exposes documentation comments in various formats with smart entity matching;
• Supports C++11 attributes (including user-defined ones);
• AST hierarchy completely decoupled from parser: This allows synthesizing AST entities and multiple parsing backends;
• Parser based on libclang: While libclang does have its limitations and/or bugs, the implemented parser uses various workarounds/hacks to provide a parser that breaks only in rare edge cases you won't notice. See issues tagged with libclang-parser for a list;
• Simple yet customizable code generation interface.

Missing features

• Support modification of parsed entities: they're currently all immutable, need to find a decent way of implementing that
• Full support for expressions: currently only literal expressions are exposed;
• Support for statements: currently function bodies aren't parsed at all;
• Support for member specialization: members of a template can be specialized separately, this is not supported.

Example

See tool/main.cpp for a simple application of the library that prints the AST.

Documentation

TODO, refer to documentation comments in header file.

Installation

For development, we recommend to use pixi. Run pixi run build to compile the package, pixi run test to run its test suite, pixi run cppast to run the cppast tool.

The library can also be used as CMake subdirectory, download it and call add_subdirectory(path/to/cppast), then link to the cppast target and enable C++11 or higher.

The parser needs libclang and the clang++ binary, at least version 15. The clang++ binary will be found in PATH and in the same directory as the program that is being executed.

The CMake code requires llvm-config, you may need to install llvm and not just clang to get it (e.g. on ArchLinux). If llvm-config is in your path and the version is compatible, it should just work out of the box. Else you need to set the CMake variable LLVM_CONFIG_BINARY to the proper path.

If you don't have llvm-config, you need to pass the locations explictly. For that set the option LLVM_VERSION_EXPLICIT to the version you're using, LIBCLANG_LIBRARY to the location of the libclang library file, LIBCLANG_INCLUDE_DIR to the directory where the header files are located (so they can be included with clang-c/Index.h), and CLANG_BINARY to the full path of the clang++ exectuable.

The other dependencies like type_safe are installed automatically with FetchContent, if they're not installed already.

If you run into any issues with the installation, please report them.

Installation on Windows

Similar to the above instructions for cppast, there are a couple extra requirements for Windows.

The LLVM team does not currently distribute llvm-config.exe as part of the release binaries, so the only way to get it is through manual compilation or from 3rd parties. To prevent version mismatches, it's best to compile LLVM, libclang, and llvm-config.exe from source to ensure proper version matching. However, this is a non-trivial task, requiring a lot of time. The easiest way to work with LLVM and llvm-config.exe is to leverage the Chocolatey llvm package, and then compile the llvm-config.exe tool as a standalone binary.

• Install Visual Studio 2017 with the Desktop C++ development feature enabled.
• Install llvm and clang with choco install llvm
• Check the version with clang.exe --version
• Clone the LLVM project: git clone https://github.com/llvm/llvm-project
• Checkout a release version matching the version output, such as 7.0.1, with git checkout llvmorg-7.0.1
• cd llvm-project && mkdir build && cd build to prep the build environment.
• cmake -DLLVM_ENABLE_PROJECTS="clang" -DLLVM_TARGETS_TO_BUILD="X86" -G "Visual Studio 15 2017" -Thost=x64 ..\llvm
  • This will configure clang and LLVM using a 64-bit toolchain. You'll have all the necessary projects configured for building clang, if you need other LLVM tools. See the LLVM documentation and clang documentation if you only need more assistance.
• Open the LLVM.sln solution, and set the build type to be "Release".
• Build the Tools/llvm-config target.
• Copy the release binary to from build\Release\bin\llvm-config.exe to C:\Program Files\LLVM\bin\llvm-config.exe
• Open a new Powershell window and test accessiblity of llvm-config.exe, it should return with it's help message.

In your cppast based project, if you run into issues with cmake not finding libclang, set LIBCLANG_LIBRARY to be C:/Program Files/LLVM/lib in your CMakeLists.txt file.

Quick API Overview

There are three class hierarchies that represent the AST:

• cpp_entity: This is the base class for all C++ entities, i.e. declarations/definitions or things like static_assert() and function parameters;
• cpp_type: This is the base class for the C++ type hierachy. It is used in the cpp_entity hierachy, i.e. cpp_type_alias contains an underlying_type(). Derived classes are, for example, cpp_builtin_type or cpp_pointer_type;
• cpp_expression: This is the base class for all C++ expressions. It is used in the cpp_entity hierarchy, i.e. cpp_function_parameter contains a default_value() as expression. Derived classes are currently only cpp_literal_expression and cpp_unexposed_expression;

In order to parse a C++ source file, you need an implementation of parser. The library provides one, libclang_parser, but you could also write one yourself. Parsing is as simple as calling the parse() member function passing it three things:

• An object of type cpp_entity_index: This is only required to resolve cross-references in the AST (i.e. if you want to get the cpp_class referenced in the return type of a cpp_function); it does not own the entities;
• The path to the file;
• An object of a type derived from compile_config: It stores the compilation flags used for compiling the file, it needs to match the parser, i.e. use libclang_compile_config with libclang_parser;

It returns nullptr on failure and prints diagnostics using a given diagnostic_logger — note that it will only return nullptr on fatal parse errors, else it will just skip the one where the error occured. If everything went succesful, it returns a std::unique_ptr<cpp_file> which is the top-level AST entity of the current file. You can then work with it.