There is no direct relationship between headers and object files in C or C++. Compilation process is two stage:
1. Source files are compiled into object files 2. Object files are linked together into executable of sorts (Actually it's a three stage process, but i'm going to describe it using two stages). Stages are isolated from each other and to some extent autonomous. Object files are kind of intermediate representation of source code that is later used to produce machine code. I'm sure someone can correct me on this, but for the sake of simplicity, i think it's OK to think of them as of IR. When *compilation unit* is compiled into an object file, it is also separated from other units. To compile it separately from other units all relevant source code has to be pulled into the current unit and compiled. So `#include <something.h>` doesn't include just something.h, it includes something.h, then all includes that something.h includes and so on. This is a process similar to amalgamation of source code, everything is copied into one place and then compiled as a single unit. After all units are compiled, they might be joined together by a linker either into a static library, dynamic library or executable. This is actually more sophisticated than that, but it does allow you to do some cool stuff like you can compile your source code into objects, then ship object files and then link them elsewhere. In fact, static libraries are just a bunch of object files packed together, but headers are still required because you need symbol names to refer to on source code level, therefore libraries are shipped with headers: you compile with headers and then link with objects. Since it's the separate stages, you could, for instance, write your own headers for 3rd party objects, think open source headers for closed source DirectX SDK. This description is very superficial and doesn't cover a lot of what is really going on. The process is very flexible and allows to do all kinds of stuff in various combinations. Alas this process is also not very fast and requires some costly steps like you need to pull all required source code into a single unit to compile it. Modern C++ is also using a lot of templates, even if you're not writing templates, you're going to use templates from the standard library and to use templates you need to transform (instantiate) each template into concrete code and then (simply put) compile instantiated template as regular non-templated source code. Because every compilation unit is being "amalgamated", this process has to be repeated for every unit, which also takes some time. There is such thing as C++ modules, but they are quite new (standardized like a month ago) and not yet widespread. I think they should be more similar to Go *packages* when source code files are logically joined into a single entity and for that entity another intermediate representation is created which is called BMI (binary module interface) even though it doesn't have to be binary, so sometimes it's called CMI (compiler module interface). This CMI is basically a compiler cache, a package, or in terms of C++, a module interface, can be compiled once and then reused to compile object files without recompiling the same source code for every unit. Regarding how packages compilation actually works in Go - this is an interesting topic. I'm afraid i won't be able to explain it more or less correctly and i would be glad to read about it too. сб, 14 нояб. 2020 г. в 04:17, kev kev <kevthemusic...@gmail.com>: > > > Thanks for the answer. If C/C++ has object files, is it not possible to see > “something.h” and then fetch the corresponding object file? > > With go, if I import “package something” and that package imports another > package called “package bar” then at some point I will need to compile “bar” > and “something”. This to me is like your header example. > > I think you are maybe saying that this traversal is only done once for > golang and the information is stored in an object file? While in C, the > header traversal is done each time I see include? > On Saturday, 14 November 2020 at 00:14:41 UTC Kevin Chowski wrote: >> >> C/C++ also has object file caching (depending on how your build is set up, I >> guess). In C/C++ the issue is that you need to possibly open a large number >> of header files when you import any header file. >> >> For example, if I write a file "main.c" which imports "something.h", which >> in turn imports "another.h" and "big.h", and compile just main.c, the >> compiler has to open all three header files and include them in the parsing >> of main.c in order for the compilation to correctly move forward. In Go, the >> compiler arranges things such that it only has to open one file per package >> that is imported. The post you linked goes into greater detail, so I will >> avoid duplicating the details for now, but feel free to ask a more specific >> question and I can try to answer. >> >> There's a bit of nuance there, which the post also goes into: Go's strategy >> ends up requiring that some package much be compiled before any package >> which imports it is compiled. In C/C++ the ordering is a little more >> flexible due to the more decoupled nature of header files, meaning that >> theoretically more builds could occur in parallel. But I suspect that in >> your average Go program the dependency tree would still allow you to execute >> a large number of builds in parallel. >> >> Also note that the article claims this is "the single biggest reason" Go >> compilation is fast, not the only one. There are lots of smaller, yet >> important, reasons as well. For example, parsing the language is pretty >> straightforward because it is not very complex, and linking the final binary >> together is continually being optimized. Plus there are no turing-complete >> meta-language features like the templates C++ compilers have to deal with ;) >> >> As for your following, the whole set of files in some package are the >> compilation unit, at least as far as I understand the terms. This is because >> if a.go and b.go are both in the same package (e.g. in the same directory), >> code in a.go can call code in b.go without explicitly declaring anything. So >> before the code in a.go can be fully compiled into an object file, b.go must >> be considered as well. >> On Friday, November 13, 2020 at 3:54:34 PM UTC-7 kev kev wrote: >>> >>> I recently read the post by Rob Pike about language choices for Golang: >>> https://talks.golang.org/2012/splash.article#TOC_5. >>> >>> The seventh point refers to how Golang handles dependencies. It mentions an >>> "object file" for packages that a _dependent_ reads. >>> >>> Below I go through my interpretation of this section: >>> >>> Example: >>> >>> package A imports package B. >>> >>> When I compile package A, package B would have already been compiled. What >>> package A receives is not the AST of package B, but an "Object file". This >>> object file only reveals data about the publicly accessible symbols in that >>> package. From the example, if B had a private struct defined inside of it, >>> this private struct would not be in the object file. >>> >>> This part seems to make sense for me, hopefully I did not make any mistakes. >>> >>> It seems that the speedup compared to C/C++ is because the object file is >>> created once per package, while in C/C++ you need to re-compile the thing >>> you are including each time? >>> >>> Followup question: >>> >>> Is a single file a compilation unit or is it a package? >>> >>> Thanks >>> > -- > You received this message because you are subscribed to the Google Groups > "golang-nuts" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to golang-nuts+unsubscr...@googlegroups.com. > To view this discussion on the web visit > https://groups.google.com/d/msgid/golang-nuts/2e237e13-37c9-4741-8ea1-67f813923fafn%40googlegroups.com. -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. 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