Irony is a C/C++ minor mode based on libclang, which aims context-sensitive completion and syntax checking using the clang compiler. To do so the following packages from (M)ELPA are needed:

• irony,
• company-irony (context-sensitive completion)
• company-irony-c-headers (header file name completion)
• flycheck (generic syntax checker)
• flycheck-irony (C/C++ syntax checking with clang)

But secondly (to get all that stuff working) the clang compiler must know the exact command line options. For that purpose a so called Compiler Database (CDB) must exist in a (sub-) directory, from irony-cdb-search-directory-list. For details on that issue see:

• https://github.com/Sarcasm/irony-mode#Compilation-Database,
• https://sarcasm.github.io/notes/dev/compilation-database.html,
• variable irony-cdb-compilation-databases

Sometimes i simply use a static .clang_complete file in project root directory, according to this one:

-std=c99
-Wall
-Wextra
-DDEBUG
-D_DEBUG_
-D_GNU_SOURCE
-D_REENRANT
-I.
-I./include
-I/usr/include
-I/usr/include/clang/3.8/include
-I/usr/include/gsl
-I/usr/include/glib-2.0
-I/usr/include/cairo
-I/usr/include/pango-1.0
-I/usr/include/gtk-3.0

In case a project is using CMake then a file compile_commands.json can be generated on each build. For that purpose one has simply to add the following line to CMakeLists.txt:

set(CMAKE_EXPORT_COMPILE_COMMANDS ON)

More information on Irony setup can be found on:

• Zuogong Yue: Emacs as C++ IDE,
• Nils Deppe: Using Emacs as a C++ IDE,
• Gregory J. Stein: C/C++ Completion In Emacs,
• Soonho Kong: How to Setup emacs irony-mode in Ubuntu-12.04,
• Martin Törnqvist: How to Setup emacs `irony-mode` in Ubuntu-12.04,
• Stack Exchange: Add include paths to flycheck and to company-irony?