How do languages like C# and Java avoid C/C++-like independent compilation?

Question

For my programming languages class, I'm writing a research paper on some papers by some important people in the history of language design. One by CAR Hoare struck me as odd because it speaks against independent compilation techniques used in C and later C++ before C even became popular.

Since this is primarily an optimization to speed up compilation times, what is it about Java and C# that make them able to avoid reliance on independent compilation? Is it a compiler technique or are there elements of the language that facilitate this? And are there any other compiled languages that used these techniques before them?

Answer 1

Short answer: Java and C# don't avoid separate compilation; they make full use of it.

Where they differ is that they don't require the programmer to write a pair of separate header/implementation files when writing a reusable library. The user writes the definition of a class once, and the compiler extracts the information equivalent to the "header" from that single definition and includes it in the output file as "type metadata". So the output file (a .jar full of .class files in Java, or an .dll assembly in .NET-based languages) is a combination of binaries AND headers in a single package.

Then when another class is compiled and it depends on the first class, it can look at the metadata instead of having to find a separate include file.

It happens that they target a virtual machine rather than a specific chip architecture, but that's a separate issue; they could put x86 machine code in as the binary and still have the header-like metadata in the same file as well (this is in fact an option in .NET, albeit rarely used).

In C++ compilers it is common to try to speed up compilation by using "pre-compiled headers". The metadata in .NET .dll and .class files is much like a pre-compiled header - already parsed and indexed, ready for rapid look-ups.

The upshot is that in these modern languages, there is one way of doing modularization, and it has the characteristics of a perfectly organised and hand-optimised C++ modular build system - pretty nifty, speaking ASFAC++B.