This. And the reason that C++ compilation is so painfully slow is that unlike languages who saw C++'s problems and corrected them (Java, C#), a C++ compiler can

This. And the reason that C++ compilation is so painfully slow is that unlike languages who saw C++'s problems and corrected them (Java, C#), a C++ compiler can not glean definitions from compiled code. In Java, the compiler reads a Java file once, produces a class file and the class file contains all the information needed to compile other files that refer to its contents. In C++, the compiler has to read all of source declarations that are used by a compilation unit, over and over again. An arbitrary cpp file can transitively include tens to hundreds of header files. Modern C++ compilers can play caching tricks, but the model is intrinsically O(n^2) compilation rather than O(n) compilation where n is the number of source files.