Eclipse CDT: index not working

Eclipse CDT indexing of source codes is very important for large projects. Without it you cannot highlight a class or function and right click "open declaration". Manually go through the source tree to find a file is a nightmare.

The index of my C++ project was suddenly broken. I cannot highlight a class member and right click "open declaration". Also when I edit a header file, the outline panel does not show the class members.

I find this link
http://wiki.eclipse.org/CDT/User/FAQ#Why_does_Open_Declaration_.28F3.29_not_work.3F_.28also_applies_to_other_functions_using_the_indexer.29
but it does not work for me.

Then it occurs to me Eclipse no longer treated my project as a C++ project. Finally I got a solution:

Choose menu File/New/Convert to a C/C++ project. It took a while to index the source code. After that, everything works like a charm.