File Size Limit
By default, the maximum C/C++ source/header file length in CLion is limited to 500 000 characters. If that limit is exceeded, all the code insight features, such as code completion and syntax highlighting are disabled. In this case the IDE notifies you about the current configured limit and the actual file length.
Change the file size limit in the Maximum FIle Length dialog
Click the link in the upper right corner of the IDE notification:
In the dialog that opens, specify the file length limit and press OK to proceed.
Note that the IDE rebuilds the project's symbols upon that action.
Change the file size limit using the Registry
From the main menu, chooseor press Ctrl+Shift+A.
In the dialog that opens, type Registry and press Enter.
In the dialog that appears, start typing cidr.max.intellisense.file.length. Specify the desired file size limit in the Value field. Click Close to proceed. In this case you need to invalidate project symbols manually by calling .