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YouCompleteMe: a code-completion engine for Vim

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Help, Advice, Support

Looking for help, advice, or support? Having problems getting YCM to work?

First carefully read the installation instructions for your OS. We recommend you use the supplied install.py - the "full" installation guide is for rare, advanced use cases and most users should use install.py.

If the server isn't starting and you're getting a "YouCompleteMe unavailable" error, check the Troubleshooting guide.

Next, check the User Guide section on the semantic completer that you are using. For C/C++/Objective-C/Objective-C++/CUDA, you must read this section.

Finally, check the FAQ.

If, after reading the installation and user guides, and checking the FAQ, you're still having trouble, check the contacts section below for how to get in touch.

Please do NOT go to #vim on Freenode for support. Please contact the YouCompleteMe maintainers directly using the contact details below.

Vundle

Please note that the below instructions suggest using Vundle. Currently there are problems with Vundle, so here are some alternative instructions using Vim packages.

Contents

Intro

YouCompleteMe is a fast, as-you-type, fuzzy-search code completion, comprehension and refactoring engine for Vim.

It has several completion engines built-in and supports any protocol-compliant Language Server, so can work with practically any language. YouCompleteMe contains:

  • an identifier-based engine that works with every programming language,
  • a powerful clangd-based engine that provides native semantic code completion for C/C++/Objective-C/Objective-C++/CUDA (from now on referred to as "the C-family languages"),
  • a Jedi-based completion engine for Python 2 and 3,
  • an OmniSharp-Roslyn-based completion engine for C#,
  • a Gopls-based completion engine for Go,
  • a TSServer-based completion engine for JavaScript and TypeScript,
  • a rust-analyzer-based completion engine for Rust,
  • a jdt.ls-based completion engine for Java.
  • a generic Language Server Protocol implementation for any language
  • and an omnifunc-based completer that uses data from Vim's omnicomplete system to provide semantic completions for many other languages (Ruby, PHP, etc.).

YouCompleteMe GIF completion demo

Here's an explanation of what happened in the last GIF demo above.

First, realize that no keyboard shortcuts had to be pressed to get the list of completion candidates at any point in the demo. The user just types and the suggestions pop up by themselves. If the user doesn't find the completion suggestions relevant and/or just wants to type, they can do so; the completion engine will not interfere.

When the user sees a useful completion string being offered, they press the TAB key to accept it. This inserts the completion string. Repeated presses of the TAB key cycle through the offered completions.

If the offered completions are not relevant enough, the user can continue typing to further filter out unwanted completions.

A critical thing to notice is that the completion filtering is NOT based on the input being a string prefix of the completion (but that works too). The input needs to be a subsequence match of a completion. This is a fancy way of saying that any input characters need to be present in a completion string in the order in which they appear in the input. So abc is a subsequence of xaybgc, but not of xbyxaxxc. After the filter, a complicated sorting system ranks the completion strings so that the most relevant ones rise to the top of the menu (so you usually need to press TAB just once).

All of the above works with any programming language because of the identifier-based completion engine. It collects all of the identifiers in the current file and other files you visit (and your tags files) and searches them when you type (identifiers are put into per-filetype groups).

The demo also shows the semantic engine in use. When the user presses ., -> or :: while typing in insert mode (for C++; different triggers are used for other languages), the semantic engine is triggered (it can also be triggered with a keyboard shortcut; see the rest of the docs).

The last thing that you can see in the demo is YCM's diagnostic display features (the little red X that shows up in the left gutter; inspired by Syntastic) if you are editing a C-family file. As the completer engine compiles your file and detects warnings or errors, they will be presented in various ways. You don't need to save your file or press any keyboard shortcut to trigger this, it "just happens" in the background.

And that's not all...

YCM might be the only Vim completion engine with the correct Unicode support. Though we do assume UTF-8 everywhere.

YouCompleteMe GIF unicode demo

YCM also provides semantic IDE-like features in a number of languages, including:

For example, here's a demo of signature help:

Signature Help Early Demo

Below we can see YCM being able to do a few things:

  • Retrieve references across files
  • Go to declaration/definition
  • Expand auto in C++
  • Fix some common errors, and provide refactorings, with FixIt
  • Not shown in the GIF are GoToImplementation and GoToType for servers that support it.

YouCompleteMe GIF subcommands demo

And here's some documentation being shown in a hover popup, automatically and manually:

hover demo

Features vary by file type, so make sure to check out the file type feature summary and the full list of completer subcommands to find out what's available for your favourite languages.

You'll also find that YCM has filepath completers (try typing ./ in a file) and a completer that integrates with UltiSnips.

Installation

Requirements

Runtime Min Version Recommended Version (full support) Python
Vim 8.2.3995 9.0.214 3.8
Neovim 0.5 Vim 9.0.214 3.8

Supported Vim Versions

Our policy is to support the Vim version that's in the latest LTS of Ubuntu. That's currently Ubuntu 22.04 which contains vim-nox at v8.2.3995.

Vim must have a working Python 3 runtime.

For Neovim users, our policy is to require the latest released version. Currently, Neovim 0.5.0 is required. Please note that some features are not available in Neovim, and Neovim is not officially supported.

Supported Python runtime

YCM has two components: A server and a client. Both the server and client require Python 3.8 or later 3.x release.

For the Vim client, Vim must be, compiled with --enable-shared (or --enable-framework on macOS). You can check if this is working with :py3 import sys; print( sys.version). It should say something like 3.8.2 (...).

For Neovim, you must have a python 3.8 runtime and the Neovim python extensions. See Neovim's :help provider-python for how to set that up.

For the server, you must run the install.py script with a python 3.8 (or later) runtime. Anaconda etc. are not supported. YCM will remember the runtime you used to run install.py and will use that when launching the server, so if you usually use anaconda, then make sure to use the full path to a real cpython3, e.g. /usr/bin/python3 install.py --all etc.

Our policy is to support the python3 version that's available in the latest Ubuntu LTS (similar to our Vim version policy). We don't increase the Python runtime version without a reason, though. Typically, we do this when the current python version we're using goes out of support. At that time we will typically pick a version that will be supported for a number of years.

Supported Compilers

In order to provide the best possible performance and stability, ycmd has updated its code to C++17. This requires a version bump of the minimum supported compilers. The new requirements are:

Compiler Current Min
GCC 8
Clang 7
MSVC 15.7 (VS 2017)

YCM requires CMake 3.13 or greater. If your CMake is too old, you may be able to simply pip install --user cmake to get a really new version.

Individual completer requirements

When enabling language support for a particular language, there may be runtime requirements, such as needing a very recent Java Development Kit for Java support. In general, YCM is not in control of the required versions for the downstream compilers, though we do our best to signal where we know them.

macOS

Quick start, installing all completers

  • Install YCM plugin via Vundle
  • Install CMake, MacVim and Python 3; Note that the pre-installed macOS system Vim is not supported (due to it having broken Python integration).
$ brew install cmake python go nodejs
  • Install mono from Mono Project (NOTE: on Intel Macs you can also brew install mono. On arm Macs, you may require Rosetta)

  • For Java support you must install a JDK, one way to do this is with Homebrew:

$ brew install java
$ sudo ln -sfn $(brew --prefix java)/libexec/openjdk.jdk /Library/Java/JavaVirtualMachines/openjdk.jdk
  • Pre-installed macOS system Vim does not support Python 3. So you need to install either a Vim that supports Python 3 OR MacVim with Homebrew:

    • Option 1: Installing a Vim that supports Python 3
    brew install vim
    
    brew install macvim
    
  • Compile YCM.

    • For Intel and arm64 Macs, the bundled libclang/clangd work:

      cd ~/.vim/bundle/YouCompleteMe
      python3 install.py --all
      
    • If you have troubles with finding system frameworks or C++ standard library, try using the homebrew llvm:

      brew install llvm
      cd ~/.vim/bundle/YouCompleteMe
      python3 install.py --system-libclang --all
      

      And edit your vimrc to add the following line to use the Homebrew llvm's clangd:

      " Use homebrew's clangd
      let g:ycm_clangd_binary_path = trim(system('brew --prefix llvm')).'/bin/clangd'
  • For using an arbitrary LSP server, check the relevant section

Explanation for the quick start

These instructions (using install.py) are the quickest way to install YouCompleteMe, however they may not work for everyone. If the following instructions don't work for you, check out the full installation guide.

A supported Vim version with Python 3 is required. MacVim is a good option, even if you only use the terminal. YCM won't work with the pre-installed Vim from Apple as its Python support is broken. If you don't already use a Vim that supports Python 3 or MacVim, install it with Homebrew. Install CMake as well:

brew install vim cmake     

OR

brew install macvim cmake

Install YouCompleteMe with Vundle.

Remember: YCM is a plugin with a compiled component. If you update YCM using Vundle and the ycm_core library APIs have changed (happens rarely), YCM will notify you to recompile it. You should then rerun the install process.

NOTE: If you want C-family completion, you MUST have the latest Xcode installed along with the latest Command Line Tools (they are installed automatically when you run clang for the first time, or manually by running xcode-select --install)

Compiling YCM with semantic support for C-family languages through clangd:

cd ~/.vim/bundle/YouCompleteMe
./install.py --clangd-completer

Compiling YCM without semantic support for C-family languages:

cd ~/.vim/bundle/YouCompleteMe
./install.py

The following additional language support options are available:

  • C# support: install by downloading the Mono macOS package and add --cs-completer when calling install.py.
  • Go support: install Go and add --go-completer when calling install.py.
  • JavaScript and TypeScript support: install Node.js 18+ and npm and add --ts-completer when calling install.py.
  • Rust support: add --rust-completer when calling install.py.
  • Java support: install JDK 17 and add --java-completer when calling install.py.

To simply compile with everything enabled, there's a --all flag. So, to install with all language features, ensure xbuild, go, node and npm tools are installed and in your PATH, then simply run:

cd ~/.vim/bundle/YouCompleteMe
./install.py --all

That's it. You're done. Refer to the User Guide section on how to use YCM. Don't forget that if you want the C-family semantic completion engine to work, you will need to provide the compilation flags for your project to YCM. It's all in the User Guide.

YCM comes with sane defaults for its options, but you still may want to take a look at what's available for configuration. There are a few interesting options that are conservatively turned off by default that you may want to turn on.

Linux 64-bit

The following assume you're using Ubuntu 22.04.

Quick start, installing all completers

  • Install YCM plugin via Vundle
  • Install CMake, Vim and Python
apt install build-essential cmake vim-nox python3-dev
  • Install mono-complete, go, node, java, and npm
sudo mkdir -p /etc/apt/keyrings
curl -fsSL https://deb.nodesource.com/gpgkey/nodesource-repo.gpg.key | sudo gpg --dearmor -o /etc/apt/keyrings/nodesource.gpg
echo "deb [signed-by=/etc/apt/keyrings/nodesource.gpg] https://deb.nodesource.com/node_current.x nodistro main" | sudo tee /etc/apt/sources.list.d/nodesource.list
apt install mono-complete golang nodejs openjdk-17-jdk openjdk-17-jre npm
  • Compile YCM
cd ~/.vim/bundle/YouCompleteMe
python3 install.py --all

Explanation for the quick start

These instructions (using install.py) are the quickest way to install YouCompleteMe, however they may not work for everyone. If the following instructions don't work for you, check out the full installation guide.

Make sure you have a supported version of Vim with Python 3 support and a supported compiler. The latest LTS of Ubuntu is the minimum platform for simple installation. For earlier releases or other distributions, you may have to do some work to acquire the dependencies.

If your Vim version is too old, you may need to compile Vim from source (don't worry, it's easy).

Install YouCompleteMe with Vundle.

Remember: YCM is a plugin with a compiled component. If you update YCM using Vundle and the ycm_core library APIs have changed (which happens rarely), YCM will notify you to recompile it. You should then rerun the installation process.

Install development tools, CMake, and Python headers:

  • Fedora-like distributions:
sudo dnf install cmake gcc-c++ make python3-devel
  • Ubuntu LTS:
sudo apt install build-essential cmake3 python3-dev

Compiling YCM with semantic support for C-family languages through clangd:

cd ~/.vim/bundle/YouCompleteMe
python3 install.py --clangd-completer

Compiling YCM without semantic support for C-family languages:

cd ~/.vim/bundle/YouCompleteMe
python3 install.py

The following additional language support options are available:

  • C# support: install Mono and add --cs-completer when calling install.py.
  • Go support: install Go and add --go-completer when calling install.py.
  • JavaScript and TypeScript support: install Node.js 18+ and npm and add --ts-completer when calling install.py.
  • Rust support: add --rust-completer when calling install.py.
  • Java support: install JDK 17 and add --java-completer when calling install.py.

To simply compile with everything enabled, there's a --all flag. So, to install with all language features, ensure xbuild, go, node, and npm tools are installed and in your PATH, then simply run:

cd ~/.vim/bundle/YouCompleteMe
python3 install.py --all

That's it. You're done. Refer to the User Guide section on how to use YCM. Don't forget that if you want the C-family semantic completion engine to work, you will need to provide the compilation flags for your project to YCM. It's all in the User Guide.

YCM comes with sane defaults for its options, but you still may want to take a look at what's available for configuration. There are a few interesting options that are conservatively turned off by default that you may want to turn on.

Windows

Quick start, installing all completers

cd YouCompleteMe
python3 install.py --all

Explanation for the quick start

These instructions (using install.py) are the quickest way to install YouCompleteMe, however they may not work for everyone. If the following instructions don't work for you, check out the full installation guide.

Important: we assume that you are using the cmd.exe command prompt and that you know how to add an executable to the PATH environment variable.

Make sure you have a supported Vim version with Python 3 support. You can check the version and which Python is supported by typing :version inside Vim. Look at the features included: +python3/dyn for Python 3. Take note of the Vim architecture, i.e. 32 or 64-bit. It will be important when choosing the Python installer. We recommend using a 64-bit client. Daily updated installers of 32-bit and 64-bit Vim with Python 3 support are available.

Add the following line to your vimrc if not already present.:

set encoding=utf-8

This option is required by YCM. Note that it does not prevent you from editing a file in another encoding than UTF-8. You can do that by specifying the ++enc argument to the :e command.

Install YouCompleteMe with Vundle.

Remember: YCM is a plugin with a compiled component. If you update YCM using Vundle and the ycm_core library APIs have changed (happens rarely), YCM will notify you to recompile it. You should then rerun the install process.

Download and install the following software:

  • Python 3. Be sure to pick the version corresponding to your Vim architecture. It is Windows x86 for a 32-bit Vim and Windows x86-64 for a 64-bit Vim. We recommend installing Python 3. Additionally, the version of Python you install must match up exactly with the version of Python that Vim is looking for. Type :version and look at the bottom of the page at the list of compiler flags. Look for flags that look similar to -DDYNAMIC_PYTHON3_DLL=\"python36.dll\". This indicates that Vim is looking for Python 3.6. You'll need one or the other installed, matching the version number exactly.
  • CMake. Add CMake executable to the PATH environment variable.
  • Build Tools for Visual Studio 2019. During setup, select C++ build tools in Workloads.

Compiling YCM with semantic support for C-family languages through clangd:

cd %USERPROFILE%/vimfiles/bundle/YouCompleteMe
python install.py --clangd-completer

Compiling YCM without semantic support for C-family languages:

cd %USERPROFILE%/vimfiles/bundle/YouCompleteMe
python install.py

The following additional language support options are available:

  • C# support: add --cs-completer when calling install.py. Be sure that the build utility msbuild is in your PATH.
  • Go support: install Go and add --go-completer when calling install.py.
  • JavaScript and TypeScript support: install Node.js 18+ and npm and add --ts-completer when calling install.py.
  • Rust support: add --rust-completer when calling install.py.
  • Java support: install JDK 17 and add --java-completer when calling install.py.

To simply compile with everything enabled, there's a --all flag. So, to install with all language features, ensure msbuild, go, node and npm tools are installed and in your PATH, then simply run:

cd %USERPROFILE%/vimfiles/bundle/YouCompleteMe
python install.py --all

You can specify the Microsoft Visual C++ (MSVC) version using the --msvc option. YCM officially supports MSVC 15 (2017), MSVC 16 (Visual Studio 2019) and MSVC 17 (Visual Studio 17 2022).

That's it. You're done. Refer to the User Guide section on how to use YCM. Don't forget that if you want the C-family semantic completion engine to work, you will need to provide the compilation flags for your project to YCM. It's all in the User Guide.

YCM comes with sane defaults for its options, but you still may want to take a look at what's available for configuration. There are a few interesting options that are conservatively turned off by default that you may want to turn on.

Full Installation Guide

The full installation guide has been moved to the wiki.

Quick Feature Summary

General (all languages)

  • Super-fast identifier completer including tags files and syntax elements
  • Intelligent suggestion ranking and filtering
  • File and path suggestions
  • Suggestions from Vim's omnifunc
  • UltiSnips snippet suggestions

C-family languages (C, C++, Objective C, Objective C++, CUDA)

  • Semantic auto-completion with automatic fixes
  • Signature help
  • Real-time diagnostic display
  • Go to include/declaration/definition (GoTo, etc.)
  • Go to alternate file (e.g. associated header GoToAlternateFile)
  • Find Symbol (GoToSymbol), with interactive search
  • Document outline (GoToDocumentOutline), with interactive search
  • View documentation comments for identifiers (GetDoc)
  • Type information for identifiers (GetType)
  • Automatically fix certain errors (FixIt)
  • Perform refactoring (FixIt)
  • Reference finding (GoToReferences)
  • Renaming symbols (RefactorRename <new name>)
  • Code formatting (Format)
  • Semantic highlighting
  • Inlay hints

C♯

  • Semantic auto-completion
  • Signature help
  • Real-time diagnostic display
  • Go to declaration/definition (GoTo, etc.)
  • Go to implementation (GoToImplementation)
  • Find Symbol (GoToSymbol), with interactive search
  • View documentation comments for identifiers (GetDoc)
  • Type information for identifiers (GetType)
  • Automatically fix certain errors (FixIt)
  • Perform refactoring (FixIt)
  • Management of OmniSharp-Roslyn server instance
  • Renaming symbols (RefactorRename <new name>)
  • Code formatting (Format)

Python

  • Semantic auto-completion
  • Signature help
  • Go to definition (GoTo)
  • Find Symbol (GoToSymbol), with interactive search
  • Reference finding (GoToReferences)
  • View documentation comments for identifiers (GetDoc)
  • Type information for identifiers (GetType)
  • Renaming symbols (RefactorRename <new name>)

Go

  • Semantic auto-completion
  • Signature help
  • Real-time diagnostic display
  • Go to declaration/definition (GoTo, etc.)
  • Go to type definition (GoToType)
  • Go to implementation (GoToImplementation)
  • Document outline (GoToDocumentOutline), with interactive search
  • Automatically fix certain errors (FixIt)
  • Perform refactoring (FixIt)
  • View documentation comments for identifiers (GetDoc)
  • Type information for identifiers (GetType)
  • Code formatting (Format)
  • Management of gopls server instance

JavaScript and TypeScript

  • Semantic auto-completion with automatic import insertion
  • Signature help
  • Real-time diagnostic display
  • Go to definition (GoTo, GoToDefinition, and GoToDeclaration are identical)
  • Go to type definition (GoToType)
  • Go to implementation (GoToImplementation)
  • Find Symbol (GoToSymbol), with interactive search
  • Reference finding (GoToReferences)
  • View documentation comments for identifiers (GetDoc)
  • Type information for identifiers (GetType)
  • Automatically fix certain errors and perform refactoring (FixIt)
  • Perform refactoring (FixIt)
  • Renaming symbols (RefactorRename <new name>)
  • Code formatting (Format)
  • Organize imports (OrganizeImports)
  • Management of TSServer server instance
  • Inlay hints

Rust

  • Semantic auto-completion
  • Real-time diagnostic display
  • Go to declaration/definition (GoTo, etc.)
  • Go to implementation (GoToImplementation)
  • Reference finding (GoToReferences)
  • Document outline (GoToDocumentOutline), with interactive search
  • View documentation comments for identifiers (GetDoc)
  • Automatically fix certain errors (FixIt)
  • Perform refactoring (FixIt)
  • Type information for identifiers (GetType)
  • Renaming symbols (RefactorRename <new name>)
  • Code formatting (Format)
  • Management of rust-analyzer server instance
  • Semantic highlighting
  • Inlay hints

Java

  • Semantic auto-completion with automatic import insertion
  • Signature help
  • Real-time diagnostic display
  • Go to definition (GoTo, GoToDefinition, and GoToDeclaration are identical)
  • Go to type definition (GoToType)
  • Go to implementation (GoToImplementation)
  • Find Symbol (GoToSymbol), with interactive search
  • Reference finding (GoToReferences)
  • Document outline (GoToDocumentOutline), with interactive search
  • View documentation comments for identifiers (GetDoc)
  • Type information for identifiers (GetType)
  • Automatically fix certain errors including code generation (FixIt)
  • Renaming symbols (RefactorRename <new name>)
  • Code formatting (Format)
  • Organize imports (OrganizeImports)
  • Detection of Java projects
  • Execute custom server command (ExecuteCommand <args>)
  • Management of jdt.ls server instance
  • Semantic highlighting

User Guide

General Usage

If the offered completions are too broad, keep typing characters; YCM will continue refining the offered completions based on your input.

Filtering is "smart-case" and "smart-diacritic" sensitive; if you are typing only lowercase letters, then it's case-insensitive. If your input contains uppercase letters, then the uppercase letters in your query must match uppercase letters in the completion strings (the lowercase letters still match both). On top of that, a letter with no diacritic marks will match that letter with or without marks:

matches foo fôo fOo fÔo
foo ✔️ ✔️ ✔️ ✔️
fôo ✔️ ✔️
fOo ✔️ ✔️
fÔo ✔️

Use the TAB key to accept a completion and continue pressing TAB to cycle through the completions. Use Shift-TAB to cycle backward. Note that if you're using console Vim (that is, not gvim or MacVim) then it's likely that the Shift-TAB binding will not work because the console will not pass it to Vim. You can remap the keys; see the Options section below.

Knowing a little bit about how YCM works internally will prevent confusion. YCM has several completion engines: an identifier-based completer that collects all of the identifiers in the current file and other files you visit (and your tags files) and searches them when you type (identifiers are put into per-filetype groups).

There are also several semantic engines in YCM. There are libclang-based and clangd-based completers that provide semantic completion for C-family languages. There's a Jedi-based completer for semantic completion for Python. There's also an omnifunc-based completer that uses data from Vim's omnicomplete system to provide semantic completions when no native completer exists for that language in YCM.

There are also other completion engines, like the UltiSnips completer and the filepath completer.

YCM automatically detects which completion engine would be the best in any situation. On occasion, it queries several of them at once, merges the outputs and presents the results to you.

Client-Server Architecture

YCM has a client-server architecture; the Vim part of YCM is only a thin client that talks to the ycmd HTTP+JSON server that has the vast majority of YCM logic and functionality. The server is started and stopped automatically as you start and stop Vim.

Completion String Ranking

The subsequence filter removes any completions that do not match the input, but then the sorting system kicks in. It's actually very complicated and uses lots of factors, but suffice it to say that "word boundary" (WB) subsequence character matches are "worth" more than non-WB matches. In effect, this means that given an input of "gua", the completion "getUserAccount" would be ranked higher in the list than the "Fooguxa" completion (both of which are subsequence matches). Word-boundary characters are all capital characters, characters preceded by an underscore, and the first letter character in the completion string.

Signature Help

Valid signatures are displayed in a second popup menu and the current signature is highlighted along with the current argument.

Signature help is triggered in insert mode automatically when g:ycm_auto_trigger is enabled and is not supported when it is not enabled.

The signatures popup is hidden when there are no matching signatures or when you leave insert mode. If you want to manually control when it is visible, you can map something to <plug>YCMToggleSignatureHelp (see below).

For more details on this feature and a few demos, check out the PR that proposed it.

Dismiss signature help

The signature help popup sometimes gets in the way. You can toggle its visibility with a mapping. YCM provides the "Plug" mapping <Plug>(YCMToggleSignatureHelp) for this.

For example, to hide/show the signature help popup by pressing Ctrl+l in insert mode: imap <silent> <C-l> <Plug>(YCMToggleSignatureHelp).

NOTE: No default mapping is provided because insert mappings are very difficult to create without breaking or overriding some existing functionality. Ctrl-l is not a suggestion, just an example.

Semantic highlighting

NOTE: This feature is highly experimental and offered in the hope that it is useful. It shall not be considered stable; if you find issues with it, feel free to report them, however.

Semantic highlighting is the process where the buffer text is coloured according to the underlying semantic type of the word, rather than classic syntax highlighting based on regular expressions. This can be powerful additional data that we can process very quickly.

This feature is only supported in Vim.

For example, here is a function with classic highlighting:

highliting-classic

And here is the same function with semantic highlighting:

highliting-semantic

As you can see, the function calls, macros, etc. are correctly identified.

This can be enabled globally with let g:ycm_enable_semantic_highlighting=1 or per buffer, by setting b:ycm_enable_semantic_highlighting.

Customising the highlight groups

YCM uses text properties (see :help text-prop-intro) for semantic highlighting. In order to customise the coloring, you can define the text properties that are used.

If you define a text property named YCM_HL_<token type>, then it will be used in place of the defaults. The <token type> is defined as the Language Server Protocol semantic token type, defined in the LSP Spec.

Some servers also use custom values. In this case, YCM prints a warning including the token type name that you can customise.

For example, to render parameter tokens using the Normal highlight group, you can do this:

call prop_type_add( 'YCM_HL_parameter', { 'highlight': 'Normal' } )

More generally, this pattern can be useful for customising the groups:

let MY_YCM_HIGHLIGHT_GROUP = {
      \   'typeParameter': 'PreProc',
      \   'parameter': 'Normal',
      \   'variable': 'Normal',
      \   'property': 'Normal',
      \   'enumMember': 'Normal',
      \   'event': 'Special',
      \   'member': 'Normal',
      \   'method': 'Normal',
      \   'class': 'Special',
      \   'namespace': 'Special',
      \ }

for tokenType in keys( MY_YCM_HIGHLIGHT_GROUP )
  call prop_type_add( 'YCM_HL_' . tokenType,
                    \ { 'highlight': MY_YCM_HIGHLIGHT_GROUP[ tokenType ] } )
endfor

Inlay hints

NOTE: Highly experimental feature, requiring Vim 9.0.214 or later (not supported in NeoVim).

When g:ycm_enable_inlay_hints (globally) or b:ycm_enable_inlay_hints (for a specific buffer) is set to 1, then YCM will insert inlay hints as supported by the language semantic engine.

An inlay hint is text that is rendered on the screen that is not part of the buffer and is often used to mark up the type or name of arguments, parameters, etc. which help the developer understand the semantics of the code.

Here are some examples:

  • C

c-inlay

  • TypeScript

ts-inlay

  • Go

go-inlay

Highlight groups

By default, YCM renders the inlay hints with the NonText highlight group. To override this, define the YcmInlayHint highlight yourself, e.g. in your .vimrc:

hi link YcmInlayHint Comment

Similar to semantic highlighting above, you can override specific highlighting for different inlay hint types by defining text properties named after the kind of inlay hint, for example:

call prop_type_add( 'YCM_INLAY_Type', #{ highlight: 'Comment' } )

The list of inlay hint kinds can be found in python/ycm/inlay_hints.py

Options

  • g:ycm_enable_inlay_hints or b:ycm_enable_inlay_hints - enable/disable globally or for local buffer
  • g:ycm_clear_inlay_hints_in_insert_mode - set to 1 to remove all inlay hints when entering insert mode and reinstate them when leaving insert mode

Toggling

Inlay hints can add a lot of text to the screen and may be distracting. You can toggle them on/off instantly, by mapping something to <Plug>(YCMToggleInlayHints), for example:

nnoremap <silent> <localleader>h <Plug>(YCMToggleInlayHints)

No default mapping is provided for this due to the personal nature of mappings.

General Semantic Completion

You can use Ctrl+Space to trigger the completion suggestions anywhere, even without a string prefix. This is useful to see which top-level functions are available for use.

C-family Semantic Completion

NOTE: YCM originally used the libclang based engine for C-family, but users should migrate to clangd, as it provides more features and better performance. Users who rely on override_filename in their .ycm_extra_conf.py will need to stay on the old libclang engine. Instructions on how to stay on the old engine are available on the wiki.

Some of the features of clangd:

  • Project wide indexing: Clangd has both dynamic and static index support. The dynamic index stores up-to-date symbols coming from any files you are currently editing, whereas static index contains project-wide symbol information. This symbol information is used for code completion and code navigation. Whereas libclang is limited to the current translation unit(TU).
  • Code navigation: Clangd provides all the GoTo requests libclang provides and it improves those using the above-mentioned index information to contain project-wide information rather than just the current TU.
  • Rename: Clangd can perform semantic rename operations on the current file, whereas libclang doesn't support such functionality.
  • Code Completion: Clangd can perform code completions at a lower latency than libclang; also, it has information about all the symbols in your project so it can suggest items outside your current TU and also provides proper #include insertions for those items.
  • Signature help: Clangd provides signature help so that you can see the names and types of arguments when calling functions.
  • Format Code: Clangd provides code formatting either for the selected lines or the whole file, whereas libclang doesn't have such functionality.
  • Performance: Clangd has faster re-parse and code completion times compared to libclang.

Installation

On supported architectures, the install.py script will download a suitable clangd (--clangd-completer) or libclang (--clang-completer) for you. Supported architectures are:

  • Linux glibc >= 2.31 (Intel, armv7-a, aarch64) - built on ubuntu 22.04
  • MacOS >=10.15 (Intel, arm64)
    • For Intel, compatibility per clang.llvm.org downloads
    • For arm64, macOS 10.15+
  • Windows (Intel) - compatibility per clang.llvm.org downloads

clangd:

Typically, clangd is installed by the YCM installer (either with --all or with --clangd-completer). This downloads a pre-built clangd binary for your architecture. If your OS or architecture is not supported or is too old, you can install a compatible clangd and use g:ycm_clangd_binary_path to point to it.

libclang:

libclang can be enabled also with --all or --clang-completer. As with clangd, YCM will try and download a version of libclang that is suitable for your environment, but again if your environment can't be supported, you can build or acquire libclang for yourself and specify it when building, as:

$ EXTRA_CMAKE_ARGS='-DPATH_TO_LLVM_ROOT=/path/to/your/llvm' ./install.py --clang-completer --system-libclang

Please note that if using custom clangd or libclang it must match the version that YCM requires. Currently YCM requires clang 17.0.1.

Compile flags

In order to perform semantic analysis such as code completion, GoTo, and diagnostics, YouCompleteMe uses clangd, which makes use of clang compiler, sometimes also referred to as LLVM. Like any compiler, clang also requires a set of compile flags in order to parse your code. Simply put: If clang can't parse your code, YouCompleteMe can't provide semantic analysis.

There are 2 methods that can be used to provide compile flags to clang:

Option 1: Use a compilation database

The easiest way to get YCM to compile your code is to use a compilation database. A compilation database is usually generated by your build system (e.g. CMake) and contains the compiler invocation for each compilation unit in your project.

For information on how to generate a compilation database, see the clang documentation. In short:

  • If using CMake, add -DCMAKE_EXPORT_COMPILE_COMMANDS=ON when configuring (or add set( CMAKE_EXPORT_COMPILE_COMMANDS ON ) to CMakeLists.txt) and copy or symlink the generated database to the root of your project.
  • If using Ninja, check out the compdb tool (-t compdb) in its docs.
  • If using GNU make, check out compiledb or Bear.
  • For other build systems, check out .ycm_extra_conf.py below.

If no .ycm_extra_conf.py is found, YouCompleteMe automatically tries to load a compilation database if there is one.

YCM looks for a file named compile_commands.json in the directory of the opened file or in any directory above it in the hierarchy (recursively); when the file is found before a local .ycm_extra_conf.py, YouCompleteMe stops searching the directories and lets clangd take over and handle the flags.

Option 2: Provide the flags manually

If you don't have a compilation database or aren't able to generate one, you have to tell YouCompleteMe how to compile your code some other way.

Every C-family project is different. It is not possible for YCM to guess what compiler flags to supply for your project. Fortunately, YCM provides a mechanism for you to generate the flags for a particular file with arbitrary complexity. This is achieved by requiring you to provide a Python module that implements a trivial function that, given the file name as an argument, returns a list of compiler flags to use to compile that file.

YCM looks for a .ycm_extra_conf.py file in the directory of the opened file or in any directory above it in the hierarchy (recursively); when the file is found, it is loaded (only once!) as a Python module. YCM calls a Settings method in that module which should provide it with the information necessary to compile the current file. You can also provide a path to a global configuration file with the g:ycm_global_ycm_extra_conf option, which will be used as a fallback. To prevent the execution of malicious code from a file you didn't write YCM will ask you once per .ycm_extra_conf.py if it is safe to load. This can be disabled and you can white-/blacklist files. See the g:ycm_confirm_extra_conf and g:ycm_extra_conf_globlist options respectively.

This system was designed this way so that the user can perform any arbitrary sequence of operations to produce a list of compilation flags YCM should hand to Clang.

NOTE: It is highly recommended to include -x <language> flag to libclang. This is so that the correct language is detected, particularly for header files. Common values are -x c for C, -x c++ for C++, -x objc for Objective-C, and -x cuda for CUDA.

To give you an impression, if your C++ project is trivial, and your usual compilation command is: g++ -Wall -Wextra -Werror -o FILE.o FILE.cc, then the following .ycm_extra_conf.py is enough to get semantic analysis from YouCompleteMe:

def Settings( **kwargs ):
  return {
    'flags': [ '-x', 'c++', '-Wall', '-Wextra', '-Werror' ],
  }

As you can see from the trivial example, YCM calls the Settings method which returns a dictionary with a single element 'flags'. This element is a list of compiler flags to pass to libclang for the current file. The absolute path of that file is accessible under the filename key of the kwargs dictionary. That's it! This is actually enough for most projects, but for complex projects it is not uncommon to integrate directly with an existing build system using the full power of the Python language.

For a more elaborate example, see ycmd's own .ycm_extra_conf.py. You should be able to use it as a starting point. Don't just copy/paste that file somewhere and expect things to magically work; your project needs different flags. Hint: just replace the strings in the flags variable with compilation flags necessary for your project. That should be enough for 99% of projects.

You could also consider using YCM-Generator to generate the ycm_extra_conf.py file.

Errors during compilation

If Clang encounters errors when compiling the header files that your file includes, then it's probably going to take a long time to get completions. When the completion menu finally appears, it's going to have a large number of unrelated completion strings (type/function names that are not actually members). This is because Clang fails to build a precompiled preamble for your file if there are any errors in the included headers and that preamble is key to getting fast completions.

Call the :YcmDiags command to see if any errors or warnings were detected in your file.

Java Semantic Completion

Java Quick Start

  1. Ensure that you have enabled the Java completer. See the installation guide for details.

  2. Create a project file (gradle or maven) file in the root directory of your Java project, by following the instructions below.

  3. (Optional) Configure the LSP server. The jdt.ls configuration options can be found in their codebase.

  4. If you previously used Eclim or Syntastic for Java, disable them for Java.

  5. Edit a Java file from your project.

Java Project Files

In order to provide semantic analysis, the Java completion engine requires knowledge of your project structure. In particular, it needs to know the class path to use, when compiling your code. Fortunately jdt.ls supports eclipse project files, maven projects and gradle projects.

NOTE: Our recommendation is to use either Maven or Gradle projects.

Diagnostic display - Syntastic

The native support for Java includes YCM's native real-time diagnostics display. This can conflict with other diagnostics plugins like Syntastic, so when enabling Java support, please manually disable Syntastic Java diagnostics.

Add the following to your vimrc:

let g:syntastic_java_checkers = []

Diagnostic display - Eclim

The native support for Java includes YCM's native real-time diagnostics display. This can conflict with other diagnostics plugins like Eclim, so when enabling Java support, please manually disable Eclim Java diagnostics.

Add the following to your vimrc:

let g:EclimFileTypeValidate = 0

NOTE: We recommend disabling Eclim entirely when editing Java with YCM's native Java support. This can be done temporarily with :EclimDisable.

Eclipse Projects

Eclipse-style projects require two files: .project and .classpath.

If your project already has these files due to previously being set up within Eclipse, then no setup is required. jdt.ls should load the project just fine (it's basically eclipse after all).

However, if not, it is possible (easy in fact) to craft them manually, though it is not recommended. You're better off using Gradle or Maven (see below).

A simple eclipse style project example can be found in the ycmd test directory. Normally all that is required is to copy these files to the root of your project and to edit the .classpath to add additional libraries, such as:

  <classpathentry kind="lib" path="/path/to/external/jar" />
  <classpathentry kind="lib" path="/path/to/external/java/source" />

It may also be necessary to change the directory in which your source files are located (paths are relative to the .project file itself):

  <classpathentry kind="src" output="target/classes" path="path/to/src/" />

NOTE: The eclipse project and classpath files are not a public interface and it is highly recommended to use Maven or Gradle project definitions if you don't already use Eclipse to manage your projects.

Maven Projects

Maven needs a file named pom.xml in the root of the project. Once again a simple pom.xml can be found in the ycmd source.

The format of pom.xml files is way beyond the scope of this document, but we do recommend using the various tools that can generate them for you, if you're not familiar with them already.

Gradle Projects

Gradle projects require a build.gradle. Again, there is a trivial example in ycmd's tests.

The format of build.gradle files are way beyond the scope of this document, but we do recommend using the various tools that can generate them for you if you're not familiar with them already.

Some users have experienced issues with their jdt.ls when using the Groovy language for their build.gradle. As such, try using Kotlin instead.

Troubleshooting

If you're not getting completions or diagnostics, check the server health:

  • The Java completion engine takes a while to start up and parse your project. You should be able to see its progress in the command line, and :YcmDebugInfo. Ensure that the following lines are present:
--   jdt.ls Java Language Server running
--   jdt.ls Java Language Server Startup Status: Ready
  • If the above lines don't appear after a few minutes, check the jdt.ls and ycmd log files using :YcmToggleLogs . The jdt.ls log file is called .log (for some reason).

If you get a message about "classpath is incomplete", then make sure you have correctly configured the project files.

If you get messages about unresolved imports, then make sure you have correctly configured the project files, in particular check that the classpath is set correctly.

C# Semantic Completion

YCM relies on OmniSharp-Roslyn to provide completion and code navigation. OmniSharp-Roslyn needs a solution file for a C# project and there are two ways of letting YCM know about your solution files.

Automatically discovered solution files

YCM will scan all parent directories of the file currently being edited and look for a file with .sln extension.

Manually specified solution files

If YCM loads .ycm_extra_conf.py which contains CSharpSolutionFile function, YCM will try to use that to determine the solution file. This is useful when one wants to override the default behaviour and specify a solution file that is not in any of the parent directories of the currently edited file. Example:

def CSharpSolutionFile( filepath ):
  # `filepath` is the path of the file user is editing
  return '/path/to/solution/file' # Can be relative to the `.ycm_extra_conf.py`

If the path returned by CSharpSolutionFile is not an actual file, YCM will fall back to the other way of finding the file.

Use with .NET 6.0 and .NET SDKs

YCM ships with older version of OmniSharp-Roslyn based on Mono runtime. It is possible to use it with .NET 6.0 and newer, but it requires manual setup.

  1. Download NET 6.0 version of the OmniSharp server for your system from releases
  2. Set g:ycm_roslyn_binary_path to the unpacked executable OmniSharp
  3. Create a solution file if one doesn't already exist, it is currently required by YCM for internal bookkeeping
    1. Run dotnet new sln at the root of your project
    2. Run dotnet sln add <project1.csproj> <project2.csproj> ... for all of your projects
  4. Run :YcmRestartServer

Python Semantic Completion

YCM relies on the Jedi engine to provide completion and code navigation. By default, it will pick the version of Python running the ycmd server and use its sys.path. While this is fine for simple projects, this needs to be configurable when working with virtual environments or in a project with third-party packages. The next sections explain how to do that.

Working with virtual environments

A common practice when working on a Python project is to install its dependencies in a virtual environment and develop the project inside that environment. To support this, YCM needs to know the interpreter path of the virtual environment. You can specify it by creating a .ycm_extra_conf.py file at the root of your project with the following contents:

def Settings( **kwargs ):
  return {
    'interpreter_path': '/path/to/virtual/environment/python'
  }

Here, /path/to/virtual/environment/python is the path to the Python used by the virtual environment you are working in. Typically, the executable can be found in the Scripts folder of the virtual environment directory on Windows and in the bin folder on other platforms.

If you don't like having to create a .ycm_extra_conf.py file at the root of your project and would prefer to specify the interpreter path with a Vim option, read the Configuring through Vim options section.

Working with third-party packages

Another common practice is to put the dependencies directly into the project and add their paths to sys.path at runtime in order to import them. YCM needs to be told about this path manipulation to support those dependencies. This can be done by creating a .ycm_extra_conf.py file at the root of the project. This file must define a Settings( **kwargs ) function returning a dictionary with the list of paths to prepend to sys.path under the sys_path key. For instance, the following .ycm_extra_conf.py adds the paths /path/to/some/third_party/package and /path/to/another/third_party/package at the start of sys.path:

def Settings( **kwargs ):
  return {
    'sys_path': [
      '/path/to/some/third_party/package',
      '/path/to/another/third_party/package'
    ]
  }

If you would rather prepend paths to sys.path with a Vim option, read the Configuring through Vim options section.

If you need further control on how to add paths to sys.path, you should define the PythonSysPath( **kwargs ) function in the .ycm_extra_conf.py file. Its keyword arguments are sys_path which contains the default sys.path, and interpreter_path which is the path to the Python interpreter. Here's a trivial example that inserts the /path/to/third_party/package path at the second position of sys.path:

def PythonSysPath( **kwargs ):
  sys_path = kwargs[ 'sys_path' ]
  sys_path.insert( 1, '/path/to/third_party/package' )
  return sys_path

A more advanced example can be found in YCM's own .ycm_extra_conf.py.

Configuring through Vim options

You may find it inconvenient to have to create a .ycm_extra_conf.py file at the root of each one of your projects in order to set the path to the Python interpreter and/or add paths to sys.path and would prefer to be able to configure those through Vim options. Don't worry, this is possible by using the g:ycm_extra_conf_vim_data option and creating a global extra configuration file. Let's take an example. Suppose that you want to set the interpreter path with the g:ycm_python_interpreter_path option and prepend paths to sys.path with the g:ycm_python_sys_path option. Suppose also that you want to name the global extra configuration file global_extra_conf.py and that you want to put it in your HOME folder. You should then add the following lines to your vimrc:

let g:ycm_python_interpreter_path = ''
let g:ycm_python_sys_path = []
let g:ycm_extra_conf_vim_data = [
  \  'g:ycm_python_interpreter_path',
  \  'g:ycm_python_sys_path'
  \]
let g:ycm_global_ycm_extra_conf = '~/global_extra_conf.py'

Then, create the ~/global_extra_conf.py file with the following contents:

def Settings( **kwargs ):
  client_data = kwargs[ 'client_data' ]
  return {
    'interpreter_path': client_data[ 'g:ycm_python_interpreter_path' ],
    'sys_path': client_data[ 'g:ycm_python_sys_path' ]
  }

That's it. You are done. Note that you don't need to restart the server when setting one of the options. YCM will automatically pick the new values.

Rust Semantic Completion

YCM uses rust-analyzer for Rust semantic completion.

NOTE: Previously, YCM used rls for rust completion. This is no longer supported, as the Rust community has decided on rust-analyzer as the future of Rust tooling.

Completions and GoTo commands within the current crate and its dependencies should work out of the box with no additional configuration (provided that you built YCM with the --rust-completer flag; see the Installation section for details). The install script takes care of installing the Rust source code, so no configuration is necessary.

rust-analyzer supports a myriad of options. These are configured using LSP configuration, and are documented here.

Go Semantic Completion

Completions and GoTo commands should work out of the box (provided that you built YCM with the --go-completer flag; see the Installation section for details). The server only works for projects with the "canonical" layout.

gopls also has a load of documented options.

You can set these in your .ycm_extra_conf.py. For example, to set the build tags:

def Settings( **kwargs ):
  if kwargs[ 'language' ] == 'go':
    return {
       'ls': {
         'build.buildFlags': [ '-tags=debug' ] }
       }
    }

JavaScript and TypeScript Semantic Completion

NOTE: YCM originally used the Tern engine for JavaScript but due to Tern not being maintained anymore by its main author and the TSServer engine offering more features, YCM is moving to TSServer. This won't affect you if you were already using Tern but you are encouraged to do the switch by deleting the third_party/ycmd/third_party/tern_runtime/node_modules directory in YCM folder. If you are a new user but still want to use Tern, you should pass the --js-completer option to the install.py script during installation. Further instructions on how to set up YCM with Tern are available on the wiki.

All JavaScript and TypeScript features are provided by the TSServer engine, which is included in the TypeScript SDK. To enable these features, install Node.js 18+ and npm and call the install.py script with the --ts-completer flag.

TSServer relies on the jsconfig.json file for JavaScript and the tsconfig.json file for TypeScript to analyze your project. Ensure the file exists at the root of your project.

To get diagnostics in JavaScript, set the checkJs option to true in your jsconfig.json file:

{
    "compilerOptions": {
        "checkJs": true
    }
}

Semantic Completion for Other Languages

C-family, C#, Go, Java, Python, Rust, and JavaScript/TypeScript languages are supported natively by YouCompleteMe using the Clang, OmniSharp-Roslyn, Gopls, jdt.ls, Jedi, rust-analyzer, and TSServer engines, respectively. Check the installation section for instructions to enable these features if desired.

Plugging an arbitrary LSP server

Similar to other LSP clients, YCM can use an arbitrary LSP server with the help of g:ycm_language_server option. An example of a value of this option would be:

let g:ycm_language_server = 
  \ [ 
  \   {
  \     'name': 'yaml',
  \     'cmdline': [ '/path/to/yaml/server/yaml-language-server', '--stdio' ],
  \     'filetypes': [ 'yaml' ]
  \   },
  \   {
  \     'name': 'rust',
  \     'cmdline': [ 'ra_lsp_server' ],
  \     'filetypes': [ 'rust' ],
  \     'project_root_files': [ 'Cargo.toml' ]
  \   },
  \   {
  \     'name': 'godot',
  \     'filetypes': [ 'gdscript' ],
  \     'port': 6008,
  \     'project_root_files': [ 'project.godot' ]
  \    }
  \ ]

Each dictionary contains the following keys:

  • name (string, mandatory): When configuring a LSP server the value of the name key will be used as the kwargs[ 'language' ]. Can be anything you like.
  • filetypes (list of string, mandatory): List of Vim filetypes this server should be used for.
  • project_root_files (list of string, optional): List of filenames to search for when trying to determine the project's root.
  • cmdline (list of strings, optional): If supplied, the server is started with this command line (each list element is a command line word). Typically, the server should be started with STDIO communication. If not supplied, port must be supplied.
  • port (number, optional): If supplied, ycmd will connect to the server at localhost:<port> using TCP (remote servers are not supported).
  • capabilities (dict, optional): If supplied, this is a dictionary that is merged with the LSP client capabilities reported to the language server. This can be used to enable or disable certain features, such as the support for configuration sections (workspace/configuration).

See the LSP Examples project for more examples of configuring the likes of PHP, Ruby, Kotlin, and D.

LSP Configuration

Many LSP servers allow some level of user configuration. YCM enables this with the help of .ycm_extra_conf.py files. Here's an example of jdt.ls user examples of configuring the likes of PHP, Ruby, Kotlin, D, and many, many more.

def Settings( **kwargs ):
  if kwargs[ 'language' ] == 'java':
    return {
      'ls': {
        'java.format.onType.enabled': True
      }
    }

The ls key tells YCM that the dictionary should be passed to the LSP server. For each of the LSP server's configuration, you should look up the respective server's documentation.

Some servers request settings from arbitrary 'sections' of configuration. There is no concept of configuration sections in Vim, so you can specify an additional config_sections dictionary which maps section to a dictionary of config required by the server. For example:

def Settings( **kwargs ):
  if kwargs[ 'language' ] == 'java':
    return {
      'ls': {
        'java.format.onType.enabled': True
      },
      'config_sections': {
        'some section': {
          'some option': 'some value'
        }
    }

The sections and options/values are completely server-specific and rarely well documented.

Using omnifunc for semantic completion

YCM will use your omnifunc (see :h omnifunc in Vim) as a source for semantic completions if it does not have a native semantic completion engine for your file's filetype. Vim comes with rudimentary omnifuncs for various languages like Ruby, PHP, etc. It depends on the language.

You can get a stellar omnifunc for Ruby with Eclim. Just make sure you have the latest Eclim installed and configured (this means Eclim >= 2.2.* and Eclipse >= 4.2.*).

After installing Eclim remember to create a new Eclipse project within your application by typing :ProjectCreate <path-to-your-project> -n ruby inside Vim and don't forget to have let g:EclimCompletionMethod = 'omnifunc' in your vimrc. This will make YCM and Eclim play nice; YCM will use Eclim's omnifuncs as the data source for semantic completions and provide the auto-triggering and subsequence-based matching (and other YCM features) on top of it.

Writing New Semantic Completers

You have two options here: writing an omnifunc for Vim's omnicomplete system that YCM will then use through its omni-completer, or a custom completer for YCM using the Completer API.

Here are the differences between the two approaches:

  • You have to use VimScript to write the omnifunc, but get to use Python to write for the Completer API; this by itself should make you want to use the API.
  • The Completer API is a much more powerful way to integrate with YCM and it provides a wider set of features. For instance, you can make your Completer query your semantic back-end in an asynchronous fashion, thus not blocking Vim's GUI thread while your completion system is processing stuff. This is impossible with VimScript. All of YCM's completers use the Completer API.
  • Performance with the Completer API is better since Python executes faster than VimScript.

If you want to use the omnifunc system, see the relevant Vim docs with :h complete-functions. For the Completer API, see the API docs.

If you want to upstream your completer into YCM's source, you should use the Completer API.

Diagnostic Display

YCM will display diagnostic notifications for the C-family, C#, Go, Java, JavaScript, Rust, and TypeScript languages. Since YCM continuously recompiles your file as you type, you'll get notified of errors and warnings in your file as fast as possible.

Here are the various pieces of the diagnostic UI:

  • Icons show up in the Vim gutter on lines that have a diagnostic.
  • Regions of text related to diagnostics are highlighted (by default, a red wavy underline in gvim and a red background in vim).
  • Moving the cursor to a line with a diagnostic echoes the diagnostic text.
  • Vim's location list is automatically populated with diagnostic data (off by default, see options).

The new diagnostics (if any) will be displayed the next time you press any key on the keyboard. So if you stop typing and just wait for the new diagnostics to come in, that will not work. You need to press some key for the GUI to update.

Having to press a key to get the updates is unfortunate, but cannot be changed due to the way Vim internals operate; there is no way that a background task can update Vim's GUI after it has finished running. You have to press a key. This will make YCM check for any pending diagnostics updates.

You can force a full, blocking compilation cycle with the :YcmForceCompileAndDiagnostics command (you may want to map that command to a key; try putting nnoremap <F5> :YcmForceCompileAndDiagnostics<CR> in your vimrc). Calling this command will force YCM to immediately recompile your file and display any new diagnostics it encounters. Do note that recompilation with this command may take a while and during this time the Vim GUI will be blocked.

YCM will display a short diagnostic message when you move your cursor to the line with the error. You can get a detailed diagnostic message with the <leader>d key mapping (can be changed in the options) YCM provides when your cursor is on the line with the diagnostic.

You can also see the full diagnostic message for all the diagnostics in the current file in Vim's locationlist, which can be opened with the :lopen and :lclose commands (make sure you have set let g:ycm_always_populate_location_list = 1 in your vimrc). A good way to toggle the display of the locationlist with a single key mapping is provided by another (very small) Vim plugin called ListToggle (which also makes it possible to change the height of the locationlist window), also written by yours truly.

Diagnostic Highlighting Groups

You can change the styling for the highlighting groups YCM uses. For the signs in the Vim gutter, the relevant groups are:

  • YcmErrorSign, which falls back to group SyntasticErrorSign and then error if they exist
  • YcmWarningSign, which falls back to group SyntasticWarningSign and then todo if they exist

You can also style the line that has the warning/error with these groups:

  • YcmErrorLine, which falls back to group SyntasticErrorLine if it exists
  • YcmWarningLine, which falls back to group SyntasticWarningLine if it exists

Finally, you can also style the popup for the detailed diagnostics (it is shown if g:ycm_show_detailed_diag_in_popup is set) using the group YcmErrorPopup, which falls back to ErrorMsg.

Note that the line highlighting groups only work when the g:ycm_enable_diagnostic_signs option is set. If you want highlighted lines but no signs in the Vim gutter, set the signcolumn option to no in your vimrc:

set signcolumn=no

The syntax groups used to highlight regions of text with errors/warnings:

  • YcmErrorSection, which falls back to group SyntasticError if it exists and then SpellBad
  • YcmWarningSection, which falls back to group SyntasticWarning if it exists and then SpellCap

Here's how you'd change the style for a group:

highlight YcmErrorLine guibg=#3f0000

Symbol Search

This feature requires Vim and is not supported in Neovim

YCM provides a way to search for and jump to a symbol in the current project or document when using supported languages.

You can search for symbols in the current workspace when the GoToSymbol request is supported and the current document when GoToDocumentOutline is supported.

Here's a quick demo:

asciicast

As you can see, you can type and YCM filters down the list as you type. The current set of matches are displayed in a popup window in the centre of the screen and you can select an entry with the keyboard, to jump to that position. Any matches are then added to the quickfix list.

To enable:

  • nmap <something> <Plug>(YCMFindSymbolInWorkspace)
  • nmap <something> <Plug>(YCMFindSymbolInDocument)

e.g.

  • nmap <leader>yfw <Plug>(YCMFindSymbolInWorkspace)
  • nmap <leader>yfd <Plug>(YCMFindSymbolInDocument)

When searching, YCM opens a prompt buffer at the top of the screen for the input and puts you in insert mode. This means that you can hit <Esc> to go into normal mode and use any other input commands that are supported in prompt buffers. As you type characters, the search is updated.

Initially, results are queried from all open filetypes. You can hit <C-f> to switch to just the current filetype while the popup is open.

While the popup is open, the following keys are intercepted:

  • <C-j>, <Down>, <C-n>, <Tab> - select the next item
  • <C-k>, <Up>, <C-p>, <S-Tab> - select the previous item
  • <PageUp>, <kPageUp> - jump up one screenful of items
  • <PageDown>, <kPageDown> - jump down one screenful of items
  • <Home>, <kHome> - jump to first item
  • <End>, <kEnd> - jump to last item
  • <CR> - jump to the selected item
  • <C-c> cancel/dismiss the popup
  • <C-f> - toggle results from all file types or just the current filetype

The search is also cancelled if you leave the prompt buffer window at any time, so you can use window commands <C-w>... for example.

Closing the popup

NOTE: Pressing <Esc> does not close the popup - you must use Ctrl-c for that, or use a window command (e.g. <Ctrl-w>j) or the mouse to leave the prompt buffer window.

Commands

The :YcmRestartServer command

If the ycmd completion server suddenly stops for some reason, you can restart it with this command.

The :YcmForceCompileAndDiagnostics command

Calling this command will force YCM to immediately recompile your file and display any new diagnostics it encounters. Do note that recompilation with this command may take a while and during this time the Vim GUI will be blocked.

You may want to map this command to a key; try putting nnoremap <F5> :YcmForceCompileAndDiagnostics<CR> in your vimrc.

The :YcmDiags command

Calling this command will fill Vim's locationlist with errors or warnings if any were detected in your file and then open it. If a given error or warning can be fixed by a call to :YcmCompleter FixIt, then (FixIt available) is appended to the error or warning text. See the FixIt completer subcommand for more information.

NOTE: The absence of (FixIt available) does not strictly imply a fix-it is not available as not all completers are able to provide this indication. For example, the c-sharp completer provides many fix-its but does not add this additional indication.

The g:ycm_open_loclist_on_ycm_diags option can be used to prevent the location list from opening, but still have it filled with new diagnostic data. See the Options section for details.

The :YcmShowDetailedDiagnostic command

This command shows the full diagnostic text when the user's cursor is on the line with the diagnostic.

An options argument can be passed. If the argument is popup the diagnostic text will be displayed in a popup at the cursor position.

If you prefer the detailed diagnostic to always be shown in a popup, then let g:ycm_show_detailed_diag_in_popup=1.

The :YcmDebugInfo command

This will print out various debug information for the current file. Useful to see what compile commands will be used for the file if you're using the semantic completion engine.

The :YcmToggleLogs command

This command presents the list of logfiles created by YCM, the ycmd server, and the semantic engine server for the current filetype, if any. One of these logfiles can be opened in the editor (or closed if already open) by entering the corresponding number or by clicking on it with the mouse. Additionally, this command can take the logfile names as arguments. Use the <TAB> key (or any other key defined by the wildchar option) to complete the arguments or to cycle through them (depending on the value of the wildmode option). Each logfile given as an argument is directly opened (or closed if already open) in the editor. Only for debugging purposes.

The :YcmCompleter command

This command gives access to a number of additional IDE-like features in YCM, for things like semantic GoTo, type information, FixIt, and refactoring.

This command accepts a range that can either be specified through a selection in one of Vim's visual modes (see :h visual-use) or on the command line. For instance, :2,5YcmCompleter will apply the command from line 2 to line 5. This is useful for the Format subcommand.

Call YcmCompleter without further arguments for a list of the commands you can call for the current completer.

See the file type feature summary for an overview of the features available for each file type. See the YcmCompleter subcommands section for more information on the available subcommands and their usage.

Some commands, like Format accept a range, like :%YcmCompleter Format.

Some commands like GetDoc and the various GoTo commands respect modifiers, like :rightbelow YcmCompleter GetDoc, :vertical YcmCompleter GoTo.

YcmCompleter Subcommands

NOTE: See the docs for the YcmCompleter command before tackling this section.

The invoked subcommand is automatically routed to the currently active semantic completer, so :YcmCompleter GoToDefinition will invoke the GoToDefinition subcommand on the Python semantic completer if the currently active file is a Python one and on the Clang completer if the currently active file is a C-family language one.

You may also want to map the subcommands to something less verbose; for instance, nnoremap <leader>jd :YcmCompleter GoTo<CR> maps the <leader>jd sequence to the longer subcommand invocation.

GoTo Commands

These commands are useful for jumping around and exploring code. When moving the cursor, the subcommands add entries to Vim's jumplist so you can use CTRL-O to jump back to where you were before invoking the command (and CTRL-I to jump forward; see :h jumplist for details). If there is more than one destination, the quickfix list (see :h quickfix) is populated with the available locations and opened to the full width at the bottom of the screen. You can change this behavior by using the YcmQuickFixOpened autocommand.

The GoToInclude subcommand

Looks up the current line for a header and jumps to it.

Supported in filetypes: c, cpp, objc, objcpp, cuda

The GoToAlternateFile subcommand

Jump to the associated file, as defined by the language server. Typically this will jump you to the associated header file for a C or C++ translation unit.

Supported in filetypes: c, cpp, objc, objcpp, cuda (clangd only)

The GoToDeclaration subcommand

Looks up the symbol under the cursor and jumps to its declaration.

Supported in filetypes: c, cpp, objc, objcpp, cuda, cs, go, java, javascript, python, rust, typescript

The GoToDefinition subcommand

Looks up the symbol under the cursor and jumps to its definition.

NOTE: For C-family languages this only works in certain situations, namely when the definition of the symbol is in the current translation unit. A translation unit consists of the file you are editing and all the files you are including with #include directives (directly or indirectly) in that file.

Supported in filetypes: c, cpp, objc, objcpp, cuda, cs, go, java, javascript, python, rust, typescript

The GoTo subcommand

This command tries to perform the "most sensible" GoTo operation it can. Currently, this means that it tries to look up the symbol under the cursor and jumps to its definition if possible; if the definition is not accessible from the current translation unit, jumps to the symbol's declaration. For C-family languages, it first tries to look up the current line for a header and jump to it. For C#, implementations are also considered and preferred.

Supported in filetypes: c, cpp, objc, objcpp, cuda, cs, go, java, javascript, python, rust, typescript

The GoToImprecise subcommand

WARNING: This command trades correctness for speed!

Same as the GoTo command except that it doesn't recompile the file with libclang before looking up nodes in the AST. This can be very useful when you're editing files that take time to compile but you know that you haven't made any changes since the last parse that would lead to incorrect jumps. When you're just browsing around your codebase, this command can spare you quite a bit of latency.

Supported in filetypes: c, cpp, objc, objcpp, cuda