cquery is no longer under development. clangd and ccls are both good replacements.

cquery is a highly-scalable, low-latency language server for C/C++/Objective-C. It is tested and designed for large code bases like Chromium. cquery provides accurate and fast semantic analysis without interrupting workflow.

cquery implements almost the entire language server protocol and provides some extra features to boot:

• code completion (with both signature help and snippets)
• finding definition/references
• call (caller/callee) hierarchy, inheritance (base/derived) hierarchy
• symbol rename
• document symbols and approximate search of workspace symbol
• hover information
• diagnostics
• code actions (clang FixIts)
• preprocessor skipped regions
• #include auto-complete, undefined type include insertion, include quick-jump (goto definition, document links)
• auto-implement functions without a definition
• semantic highlighting, including support for rainbow semantic highlighting

cquery is able to respond to queries quickly because it caches a huge amount of information. When a request comes in, cquery just looks it up in the cache without running many computations. As a result, there's a large memory overhead. For example, a full index of Chrome will take about 10gb of memory. If you exclude v8, webkit, and third_party, it goes down to about 6.5gb.

MIT