Scheduler::instance().clock()
Can anyone explain what this line of code means

Thanks in advance

First, what is Scheduler? Then look inside it for instance(). Since you're calling something on the returned result of instance(), you'll want to look at whatever that is, too, as it should have clock() inside of it.

This should be enough for you to figure out the line yourself, but if you keep having trouble, post again with what you were able to find out.

Thanks for quick reply
so it means that there is instance method inside Scheduler class , and there is clock method inside instance method, Is it like that??

Not exactly. Methods themselves can't have methods; if you look at instance()'s implementation though, you should be able to figure out where clock() is coming from.