C and C++

OpenIDEA supports C, C++, Objective-C and Objective-C++ through clangd. Highlighting, folding and the outline work without any setup. Completion, navigation, diagnostics and refactorings come from clangd, and how good they are depends on clangd knowing your compile flags. For the other languages, see the support matrix. .h files open as C. To treat headers as C++, change the association in Settings › Editor › File Types.

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Language server

clangd starts with --background-index --header-insertion=never for C and C++, so it indexes the whole project in the background and never adds #include lines on its own. clangd finds the workspace root by looking up from the file for one of these files: compile_commands.json, compile_flags.txt, .clangd, CMakeLists.txt, meson.build or Makefile. Objective-C and Objective-C++ only use compile_commands.json, compile_flags.txt and .clangd.

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Compile flags

clangd needs the flags each file is compiled with. Without them it guesses, and you see false errors about missing headers or unknown macros. Provide one of:

• compile_commands.json: a compilation database. CMake writes it into the build directory when CMAKE_EXPORT_COMPILE_COMMANDS is on, and the CMake configurations OpenIDEA creates always pass -DCMAKE_EXPORT_COMPILE_COMMANDS=ON (see CMake). For Make-based projects, tools such as Bear can generate one.
• compile_flags.txt: one flag per line, applied to every file. Good for small projects.
• .clangd: clangd’s own configuration file, for example to point CompilationDatabase at a build directory.

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Apple’s clangd

The clangd that ships with Xcode and the Command Line Tools does not run clang-tidy. When a project has a .clang-tidy file and the IDE is using Apple’s clangd, it shows a warning with an Install button that installs the LLVM build instead.

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Setup

When clangd is missing, opening a C or C++ file shows a “clangd is not installed” notification with Install. You can also install it from Settings › Languages & Frameworks › Language Servers. A clangd or debugger already on your PATH is used as is. To use a specific binary, select it on the tool’s page in Settings › Languages & Frameworks › Language Servers. The compiler is not managed by the IDE. Single-file runs look for clang, gcc or cc (C) and clang++, g++ or c++ (C++) on PATH.

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Build, run and test

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CMake

For every CMakeLists.txt project, OpenIDEA creates run configurations:

• CMake Configure: cmake -S . -B <build dir> -DCMAKE_EXPORT_COMPILE_COMMANDS=ON (plus -DCMAKE_BUILD_TYPE=Debug for a new build directory).
• CMake Build: cmake --build <build dir>. It runs the configure step first when the build directory is not configured yet.
• CTest: ctest --test-dir <build dir> --output-on-failure, after a build.
• Build <target> and one run configuration per executable target. Running a target builds it first.

The executable targets come from CMake’s File API, so they appear after the first configure. Nested CMake projects (for example in a folder that holds several repositories) get their own set of configurations. Without profiles, the IDE uses every subdirectory of the project that contains a CMakeCache.txt, or build/ when there is none.

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CMake profiles

To control the build directories yourself, add profiles in Settings › Build, Execution, Deployment › CMake. Each profile has: Once profiles exist, only they are used. Changing the generator of an existing build directory clears its CMake cache on the next configure.

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Makefiles

A Makefile, makefile or GNUmakefile gives one make <target> configuration per target. test and check targets are marked as tests.

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Single files

A C or C++ file with a main function can be run without a build system: use Run Context Configuration (⌃⇧R / Ctrl+Shift+F10) or Debug Context Configuration (⌃⇧D on macOS). The IDE compiles the file with debug info (-g, and -std=c++17 for C++) into a temporary folder and runs it. See Run configurations and Testing for the general workflow.

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Debugging

Debugging uses a DAP adapter. C and C++ configurations default to lldb-dap. To use another one, open the run configuration and change Debug adapter:

• lldb-dap (C, C++, Swift): LLVM’s adapter, the default. The IDE does not install it.
• CodeLLDB (Rust, C, C++): the IDE can install it; if it is missing when you start debugging, a notification offers to install it.
• GDB (C, C++): GDB’s own DAP mode (gdb -i dap). It needs GDB 14 or newer; older versions are refused with a message.

Attach to Process… lists native processes too. It attaches with CodeLLDB when it is installed, otherwise with lldb-dap. See Debugging for breakpoints, stepping and the variables view.

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Arduino

.ino sketches use the Arduino Language Server, which drives clangd with Arduino’s generated compile context. The IDE does not install it: install Arduino IDE 2 (it includes the Arduino Language Server, Arduino CLI and clangd) and the board’s core. OpenIDEA finds those tools on PATH or inside Arduino IDE 2. To override the paths, set them in Settings › Languages & Frameworks › Language Servers, or set languages.arduino.cli, languages.arduino.clangd, languages.arduino.languageServer and languages.arduino.cliConfig in settings.toml. Set the board FQBN in sketch.yaml or languages.arduino.fqbn. Plain .cpp files next to a sketch are still served by clangd. There are no upload or serial monitor actions for Arduino.

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Limitations

• C and C++ code intelligence is only as good as clangd’s compile flags. Without a compile_commands.json or compile_flags.txt, expect false errors.
• The IDE does not generate a compilation database for Make, Meson or other build systems.
• CTest runs as a single configuration and shows its console output; individual tests are not listed in a test tree.
• There are no gutter run icons for C and C++ test frameworks (GoogleTest, Catch2 and so on).
• lldb-dap and GDB are not installed by the IDE.