Well I am sure everyone must have come across many chrome or mozilla extensions like adblock, pocket etc… bus
but there is something called compiler extensions
compiler extensions are some features that are provided by specific compilers (like GCC) which are not a part standards of that particular language.
e.g:– typeof() is a compiler extension for C. it is used to find out the type of the variable. but this is not a standard ISO feature of C language, but just provided that compiler.
well here is a list of C language extensions provided by GCC here . (also you can look there for such C++ extensions.
so next time when you see something autocompleted by an IDE like codeblocks though you have not included the respective library for that function, chances are that it is given as a compiler extension !!!