How can we make our code compile fast?

Let’s say we have a function which takes a string as an argument. Our string class is called COLstring. Best practice is to predeclare the COLstring class in the header file so that we can avoid including the COLstring.h header file like this:

In this case it doesn’t make much difference in compilation speed but it adds up through the network effect:

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I say the same thing in other language here.