so many people here haha

@MathiasSiigNørregaard Classes which are used by reference (= any type of smart pointer) don't need to be included in the header file anyway. But for 'by value" usage in private member variables, you would have to include class in the header file, if you don't use the pimpl-idiom or something similar.

Why does this make compile time faster? Whether we #include or use smart pointers, the file still needs to be compiled (?)

The point is not the compile time of the (pimpled) class itself, but the compile time of code using that (pimpled) class. And even more important than compile times, you get rid of annoying third party include dependencies in the code using that class.

huh. why is compile-time on whatever is using the pimpled class faster? I don't quite follow.

Because the source files of that other code no longer implicitly includes the header files of the classes used by the (pimpled) class. Especially for preprocessor and template heavy code like the boost-libraries, this can have a significant impact on compile time.

Boost uses PIMPL?

Boost only uses pimpl in very few places, but that's not the point. You wanted to know how pimpl can reduce compile times, and I gave you one of the most significant practical examples.

Thank you

Thanks vzn for sharing the questions. :)