Related
There are already two compilers that support C++ modules:
Clang: http://clang.llvm.org/docs/Modules.html
MS VS 2015: http://blogs.msdn.com/b/vcblog/archive/2015/12/03/c-modules-in-vs-2015-update-1.aspx
When starting a new project now, what should I pay attention to in order to be able to adopt the modules feature when it is eventually released in my compiler?
Is it possible to use modules and still maintain compatibility with older compilers that do not support it?
There are already two compilers that support C++ modules
clang: http://clang.llvm.org/docs/Modules.html
MS VS 2015: http://blogs.msdn.com/b/vcblog/archive/2015/12/03/c-modules-in-vs-2015-update-1.aspx
The Microsoft approach appears to be the one gaining the most traction, mainly because Microsoft are throwing a lot more resources at their implementation than any of the clang folk currently. See https://llvm.org/bugs/buglist.cgi?list_id=100798&query_format=advanced&component=Modules&product=clang for what I mean, there are some big showstopper bugs in Modules for C++, whereas Modules for C or especially Objective C look much more usable in real world code. Visual Studio's biggest and most important customer, Microsoft, is pushing hard for Modules because it solves a whole ton of internal build scalability problems, and Microsoft's internal code is some of the hardest C++ to compile anywhere in existence so you can't throw any compiler other than MSVC at it (e.g. good luck getting clang or GCC to compile 40k line functions). Therefore the clang build tricks used by Google etc aren't available to Microsoft, and they have a huge pressing need to get it fixed sooner rather than later.
This isn't to say there aren't some serious design flaws with the Microsoft proposal when applied in practice to large real world code bases. However Gaby is of the view you should refactor your code for Modules, and whilst I disagree, I can see where he is coming from.
When starting a new project now, what should I pay attention to in order to be able to adopt the modules feature when it is eventually released in my compiler?
In so far as Microsoft's compiler is currently expected to implement Modules, you ought to make sure your library is usable in all of these forms:
Dynamic library
Static library
Header only library
Something very surprising to many people is that C++ Modules as currently expected to be implemented keeps those distinctions, so now you get a C++ Module variant for all three of the above, with the first most looking like what people expect a C++ Module to be, and the last looking most like a more useful precompiled header. The reason you ought to support those variants is because you can reuse most of the same preprocessor machinery to also support C++ Modules with very little extra work.
A later Visual Studio will allow linking of the module definition file (the .ifc file) as a resource into DLLs. This will finally eliminate the need for the .lib and .dll distinction on MSVC, you just supply a single DLL to the compiler and it all "just works" on module import, no headers or anything else needed. This of course smells a bit like COM, but without most of the benefits of COM.
Is it possible to use modules in a single codebase and still maintain compatibility with older compilers that do not support it?
I'm going to assume you meant the bold text inserted above.
The answer is generally yes with even more preprocessor macro fun. #include <someheader> can turn into an import someheader within the header because the preprocessor still works as usual. You can therefore mark up individual library headers with C++ Modules support along something like these lines:
// someheader.hpp
#if MODULES_ENABLED
# ifndef EXPORTING_MODULE
import someheader; // Bring in the precompiled module from the database
// Do NOT set NEED_DEFINE so this include exits out doing nothing more
# else
// We are at the generating the module stage, so mark up the namespace for export
# define SOMEHEADER_DECL export
# define NEED_DEFINE
# endif
#else
// Modules are not turned on, so declare everything inline as per the old way
# define SOMEHEADER_DECL
# define NEED_DEFINE
#endif
#ifdef NEED_DEFINE
SOMEHEADER_DECL namespace someheader
{
// usual classes and decls here
}
#endif
Now in your main.cpp or whatever, you simply do:
#include "someheader.hpp"
... and if the compiler had /experimental:modules /DMODULES_ENABLED then your application automagically uses the C++ Modules edition of your library. If it doesn't, you get inline inclusion as we've always done.
I reckon these are the minimum possible set of changes to your source code to make your code Modules-ready now. You will note I have said nothing about build systems, this is because I am still debugging the cmake tooling I've written to get all this stuff to "just work" seamlessly and I expect to be debugging it for some months yet. Expect to see it maybe at a C++ conference next year or the year after :)
Is it possible to use modules and still maintain compatibility with older compilers that do not support it?
No, it is not possible. It might be possible using some #ifdef magic like this:
#ifdef CXX17_MODULES
...
#else
#pragma once, #include "..." etc.
#endif
but this means you still need to provide .h support and thus lose all the benefits, plus your codebase looks quite ugly now.
If you do want to follow this approach, the easiest way to detect "CXX17_MODULES" which I just made up is to compile a small test program that uses modules with a build system of your choice, and define a global for everyone to see telling whether the compilation succeeded or not.
When starting a new project now, what should I pay attention to in order to be able to adopt the modules feature when it is eventually released in my compiler?
It depends. If your project is enterprise and gets you food on the plate, I'd wait a few years once it gets released in stables so that it becomes widely adapted. On the other hand, if your project can afford to be bleeding-edge, by all means, use modules.
Basically, it's the same story ast with Python3 and Python2, or less relevantly, PHP7 and PHP5. You need to find a balance between being a good up-to-date programmer and not annoying people on Debian ;-)
I am wondering if it is possible to enforce direct #include requirements with GCC. Let say I have these files:
abc.h:
typedef struct {
int useful;
} str;
file1.h:
#include <abc.h>
#ifndef GUARD
#define GUARD
#include <deh.h>
typedef struct {
int useful;
} str2;
#endif
file2.h:
#ifndef GUARD2
#define GUARD2
#include <file1.h>
void a_function (str* my_str);
void a_function2(str2* my_str);
#endif
The problem is that "file2.h" is using "str" defined in "abc.h". Let say "file1.h" is provided by the system on some Linux systems. I have no control of "file1.h" content. If may or may not include , it may or may not be inside include guards and it may or may not change over time.
The issue is when it come to support multiple distributions and system. If file2.h is accidentally using "str" without including , it may compile anyway on most systems, but may fail on others, or in the future when "file1.h" change.
Is there a way to force GCC (or LLVM) to use only types directly defined in file2.h? I understand that "#include" are just that, includes, so the compiler internal may not be aware of those issues after the preprocessor phase, however, I am wondering if this is currently possible and, if so, how?
I had this problem a few time with "normal" Linux distributions, but it was even worst with early Android NDK versions.
No, #include instructs the compiler to treat the other file's content as if it were placed at the #include directive -- you're asking for the other file's content to be treated somehow differently.
Your best hope in this scenario is to use a static analysis tool that performs dependency analysis, and check that there are no direct dependencies on types (or functions or objects) obtained through indirect (nested) inclusion.
The free doxygen documentation tool extracts information about inclusion and dependencies, which it makes available in XML format. Of course, it isn't as accurate as a true compiler, in terms of overload resolution and template processing. I'm sure there are paid tools that will be more accurate (user Ira Baxter pops up from time to time mentioning a commercial product his company sells, DMS Toolkit or something like that, which sounded like it would get at this information). But I'm guessing that doxygen will give you the right results for most "normal" code.
There isn't anything in the C++ language which would verify that all headers are included correctly. However, there is include-what-you-use which is based on clang. I haven't tried using it but it seems to be in the direction of what you are looking for. For C, implementing an analyzer detecting dependencies and report missing direct includes seems to be fairly straight forward. When trying the same with C++ things get somewhat harder due to the need of detecting dependencies for template instantiations.
Based on last weeks discussion at the C++ committee meeting, refactoring sources and headers to properly include what is actually used may be helpful for future module support in C++.
It's difficult to tell what is being asked here. This question is ambiguous, vague, incomplete, overly broad, or rhetorical and cannot be reasonably answered in its current form. For help clarifying this question so that it can be reopened, visit the help center.
Closed 12 years ago.
When is doing conditional compilation a good idea and when is it a horribly bad idea?
By conditional compile I mean using #ifdefs to only compile certain bits of code in certain conditions. The #defineds themselves may be in either a common header file or introduced via the -D compiler directive.
The good ideas:
header guards (you can't do much better for portability)
conditional implementation (juggling with platform differences)
debug specific checks (asserts, etc...)
per suggestion: extern "C" { and } so that the same headers may be used by the C++ implementation and by the C clients of the API
The bad idea:
changing the API between compile flags, since it forces the client to changes its uses with the same compile flags... urk!
Don't put ifdef in your code.
It makes it really hard to read and understand. Please make the code as easy to read as possable for the maintainer (he knows where you live and owns an Axe).
Hide the conditional code in separate functions and use the ifdef to define what functions are being used.
DONT use the else part to make a definition. If you are ding that you are saying one platform is unique and all others are the same. This is unlikely, what is more likely is that you know what happens on a couple of platforms but you should use the #else section to stick a #error so when it is ported to a new platform a developer has to explicitly fix the condition for his platform.
x.h
#if defined(WINDOWS)
#define MyPlatfromSleepSeconds(x) sleep(x * 1000)
#elif defined (UNIX)
#define MyPlatfromSleepSeconds(x) Sleep(x)
#else
#error "Please define appropriate sleep for your platform"
#endif
Don;t be tempted to expand a macro into multiple lines of code. That leads to madness.
p.h
#if defined(SOLARIS_3_1_1)
#define DO_SOME_TASK(x,y) doPartA(x); \
doPartB(y); \
couple(x,y)
#elif defined(WINDOWS)
#define DO_SOME_TASK(x,y) doAndCouple(x,y)
#else
#error "Please define appropriate DO_SOME_TASK for your platform"
#endif
If you develop the code on windows then test on solaris 3_1_1 later you may find unexpected bugs when people do things like:
int loop;
for(loop = 0;loop < 10;++loop)
DO_SOME_TASK(loop,loop); // Windows works fine()
// Solaras. Only doPartA() is in the loop.
// The other statements are done when the loop finishes
Basically, you should try to keep the amount of code that is conditionally compiled to a minimum, because you should be trying to test all that and having lots of conditions makes that more difficult. It also reduces the readability of the code; conditionally compiling whole files is clearer, e.g., by putting platform-specific code in a separate file for each platform and having that all have the same API from the perspective of the rest of the problem. Also try to avoid using it in function headers; again, that's because that's a place where it is particularly confusing.
But that's not to say that you should never use conditional compilation. Just try to keep it short and minimal. (Where I can, I use conditional compilation to control the definitions of other macros which are then just used in the rest of the code; that seems to be clearer to me at least.)
It's a bad idea whenever you don't know what you're doing. It can be a good idea when you're effectively solving an issue this way :).
The way you describe conditional compiling, include guards are part of it. It's not only a good idea to use it. It's a way to avoid compilation errors.
For me, conditional compiling is also a way to target multiple compilers and operating systems. I'm involved in a lib that's supposed to be compileable on Windows XP and newer, 32 or 64 bit, using MinGW and Visual C++, on Linux 32 and 64 bit using gcc/g++ and on MacOS using I-don't-know-what (I'm not maintaining that, but I assume it's a gcc port). Without the preprocessor conditions, it would be pretty much impossible to create a single source file that's compileable anywhere.
Another pragmatic use of conditional compiles is to "comment out" sections of code which contain standard "C" comments (i.e. /* */). Some compilers do not allow nesting of these comments, for example:
/* comment out block of code
.... code ....
/* This is a standard
* comment.
*/ ... oopos! Some compilers try to compile code after this closing comment.
.... code ....
end of block of code*/
(As you can see in the syntax highlighting, StackOverflow does not nest comments.)
instead you can use#ifdef to get the right effect, for example:
#ifdef _NOT_DEFINED_
.... code ....
/* This is a standard
* comment.
*/
.... code ....
#endif
In the past if you wanted to produce truly portable code, you'd have to resort to some form of conditional compilation. With there being a proliferation of portable libraries (such as APR, boost etc.) this reason has little weight IMHO. If you are using conditional compilation simply compile out blocks of code that are not need for particular builds, you should really revisit your design - I should imagine that this would become a nightmare to maintain.
Having said all that, if you do need to use conditional compilation, I would hide as much as I can away from the main body of the code and limit to to very specific cases that are very well understood.
Good/justifiable uses are based on cost/benefit analysis. Obviously, people here are very conscious of the risks:
in linking objects that saw different versions of classes, functions etc.
in making code hard to understand, test and reason about
But, there are uses which often fall into the net-benefit category:
header guards
code customisations for distinct software "ecosystems", such as Linux versus Windows, Visual C++ versus GCC, CPU-specific optimisations, sometimes word size and endianness factors (though with C++ you can often determine these at compile via template hackery, but that may prove messier still) - abstracts away lower-level differences to provide a consistent API across those environments
interacting with existing code that uses preprocessor defines to select versions of APIs, standards, behaviours, thread safety, protocols etc. (sad but true)
compilation that may use optional features when available (think of GNU configure scripts and all the tests they perform on OS interfaces etc)
request that extra code be generated in a translation unit, such as adding main() for a standalone app versus without for a library
controlling code inclusion for distinct logical build modes such as debug and release
It is always a bad idea. What it does is effectively create multiple versions of your source code, all of which need to be tested, which is a pain, to say the least. Unfortunately, like many bad things it is sometimes unavoidable. I use it in very small amounts when writing code that needs to be ported between Windows and Linux, but if I found myself doing it a lot, I would consider alternatives, such as having two separate development sub-trees.
Is there any way to not have to write function declarations twice (headers) and still retain the same scalability in compiling, clarity in debugging, and flexibility in design when programming in C++?
Use Lzz. It takes a single file and automatically creates a .h and .cpp for you with all the declarations/definitions in the right place.
Lzz is really very powerful, and handles 99% of full C++ syntax, including templates, specializations etc etc etc.
Update 150120:
Newer C++ '11/14 syntax can only be used within Lzz function bodies.
I felt the same way when I started writing C, so I also looked into this. The answer is that yes, it's possible and no, you don't want to.
First with the yes.
In GCC, you can do this:
// foo.cph
void foo();
#if __INCLUDE_LEVEL__ == 0
void foo() {
printf("Hello World!\n");
}
#endif
This has the intended effect: you combine both header and source into one file that can both be included and linked.
Then with the no:
This only works if the compiler has access to the entire source. You can't use this trick when writing a library that you want to distribute but keep closed-source. Either you distribute the full .cph file, or you have to write a separate .h file to go with your .lib. Although maybe you could auto-generate it with the macro preprocessor. It would get hairy though.
And reason #2 why you don't want this, and that's probably the best one: compilation speed. Normally, C sources files only have to be recompiled when the file itself changes, or any of the files it includes changes.
The C file can change frequently, but the change only involves recompiling the one file that changed.
Header files define interfaces, so they shouldn't change as often. When they do however, they trigger a recompile of every source file that includes them.
When all your files are combined header and source files, every change will trigger a recompile of all source files. C++ isn't known for its fast compile times even now, imagine what would happen when the entire project had to be recompiled every time. Then extrapolate that to a project of hundreds of source files with complicated dependencies...
Sorry, but there's no such thing as a "best practice" for eliminating headers in C++: it's a bad idea, period. If you hate them that much, you have three choices:
Become intimately familiar with C++ internals and any compilers you're using; you're going to run into different problems than the average C++ developer, and you'll probably need to solve them without a lot of help.
Pick a language you can use "right" without getting depressed
Get a tool to generate them for you; you'll still have headers, but you save some typing effort
In his article Simple Support for Design by Contract in C++, Pedro Guerreiro stated:
Usually, a C++ class comes in two
files: the header file and the
definition file. Where should we write
the assertions: in the header file,
because assertions are specification?
Or in the definition file, since they
are executable? Or in both, running
the risk of inconsistency (and
duplicating work)? We recommend,
instead, that we forsake the
traditional style, and do away with
the definition file, using only the
header file, as if all functions were
defined inline, very much like Java
and Eiffel do.
This is such a drastic
change from the C++ normality that it
risks killing the endeavor at the
outset. On the other hand, maintaining
two files for each class is so
awkward, that sooner or later a C++
development environment will come up
that hides that from us, allowing us
to concentrate on our classes, without
having to worry about where they are
stored.
That was 2001. I agreed. It is 2009 now and still no "development environment that hides that from us, allowing us to concentrate on our classes" has come up. Instead, long compile times are the norm.
Note: The link above seems to be dead now. This is the full reference to the publication, as it appears in the Publications section of the author's website:
Pedro Guerreiro, Simple Support for Design by Contract in C++, TOOLS USA 2001, Proceedings, pages 24-34, IEEE, 2001.
There is no practical way to get around headers. The only thing you could do is to put all code into one big c++ file. That will end up in an umaintainable mess, so please don't do it.
At the moment C++ header-files are a nessesary evil. I don't like them, but there is no way around them. I'd love to see some improvements and fresh ideas on the problem though.
Btw - once you've got used to it it's not that bad anymore.. C++ (and any other language as well) has more anoying things.
What I have seen some people like you do is write everything in the headers. That gives your desired property of only having to write the method profiles once.
Personally I think there are very good reasons why it is better to separate declaration and definition, but if this distresses you there is a way to do what you want.
There's header file generation software. I've never used it, but it might be worth looking into. For instance, check out mkhdr! It supposedly scans C and C++ files and generates the appropriate header files.
(However, as Richard points out, this seems to limit you from using certain C++ functionality. See Richard's answer instead here right in this thread.)
You have to write function declaration twice, actually (once in header file, once in implementation file). The definition (AKA implementation) of the function will be written once, in the implementation file.
You can write all the code in header files (it is actually a very used practice in generic programming in C++), but this implies that every C/CPP file including that header will imply recompilation of the implementation from those header files.
If you are thinking to a system similar to C# or Java, it is not possible in C++.
Nobody has mentioned Visual-Assist X under Visual Studio 2012 yet.
It has a bunch of menus and hotkeys that you can use to ease the pain of maintaining headers:
"Create Declaration" copies the function declaration from the current function into the .hpp file.
"Refactor..Change signature" allows you to simultaneously update the .cpp and .h file with one command.
Alt-O allows you to instantly flip between .cpp and .h file.
C++ 20 modules solve this problem. There is no need for copy-pasting anymore! Just write your code in a single file and export things using "export".
export module mymodule;
export int myfunc() {
return 1
}
Read more about modules here: https://en.cppreference.com/w/cpp/language/modules
At the time of writing this answer (2022 Feb), these compilers support it:
See here for the supported compilers:
https://en.cppreference.com/w/cpp/compiler_support
See this answer if you want to use modules with CMake:
https://stackoverflow.com/a/71119196/7910299
Actually... You can write the entire implementation in a file. Templated classes are all defined in the header file with no cpp file.
You can also save then with whatever extensions you want. Then in #include statements, you would include your file.
/* mycode.cpp */
#pragma once
#include <iostreams.h>
class myclass {
public:
myclass();
dothing();
};
myclass::myclass() { }
myclass::dothing()
{
// code
}
Then in another file
/* myothercode.cpp */
#pragma once
#include "mycode.cpp"
int main() {
myclass A;
A.dothing();
return 0;
}
You may need to setup some build rules, but it should work.
You can avoid headers. Completely. But I don't recommend it.
You'll be faced with some very specific limitations. One of them is you won't be able to have circular references (you won't be able to have class Parent contain a pointer to an instance of class ChildNode, and class ChildNode also contain a pointer to an instance of class Parent. It'd have to be one or the other.)
There are other limitations which just end up making your code really weird. Stick to headers. You'll learn to actually like them (since they provide a nice quick synopsis of what a class can do).
To offer a variant on the popular answer of rix0rrr:
// foo.cph
#define INCLUDEMODE
#include "foo.cph"
#include "other.cph"
#undef INCLUDEMODE
void foo()
#if !defined(INCLUDEMODE)
{
printf("Hello World!\n");
}
#else
;
#endif
void bar()
#if !defined(INCLUDEMODE)
{
foo();
}
#else
;
#endif
I do not recommend this, bit I think this construction demonstrates the removal of content repetition at the cost of rote repetition. I guess it makes copy-pasta easier? That's not really a virtue.
As with all the other tricks of this nature, a modification to the body of a function will still require recompilation of all files including the file containing that function. Very careful automated tools can partially avoid this, but they would still have to parse the source file to check, and be carefully constructed to not rewrite their output if it's no different.
For other readers: I spent a few minutes trying to figure out include guards in this format, but didn't come up with anything good. Comments?
I understand your problems. I would say that the C++ main problem is the compilation/build method that it inherited from the C. The C/C++ header structure has been designed in times when coding involved less definitions and more implementations. Don't throw bottles on me, but that's how it looks like.
Since then the OOP has conquered the world and the world is more about definitions then implementations. As the result, including headers makes pretty painful to work with a language where the fundamental collections such as the ones in the STL made with templates which are notoriously difficult job for the compiler to deal with. All those magic with the precompiled headers doesn't help so much when it comes to TDD, refactoring tools, the general development environment.
Of course C programmers are not suffering from this too much since they don't have compiler-heavy header files and so they are happy with the pretty straightforward, low-level compilation tool chain. With C++ this is a history of suffering: endless forward declarations, precompiled headers, external parsers, custom preprocessors etc.
Many people, however, does not realize that the C++ is the ONLY language that has strong and modern solutions for high- and low-level problems. It's easy to say that you should go for an other language with proper reflection and build system, but it is non-sense that we have to sacrifice the low-level programming solutions with that and we need to complicate things with low-level language mixed with some virtual-machine/JIT based solution.
I have this idea for some time now, that it would be the most cool thing on earth to have a "unit" based c++ tool-chain, similar to that in D. The problem comes up with the cross-platform part: the object files are able to store any information, no problem with that, but since on windows the object file's structure is different that of the ELF, it would be pain in the ass to implement a cross-platform solution to store and process the half-way-compilation units.
After reading all the other answers, I find it missing that there is ongoing work to add support for modules in the C++ standard. It will not make it to C++0x, but the intention is that it will be tackled in a later Technical Review (rather than waiting for a new standard, that will take ages).
The proposal that was being discussed is N2073.
The bad part of it is that you will not get that, not even with the newest c++0x compilers. You will have to wait. In the mean time, you will have to compromise between the uniqueness of definitions in header-only libraries and the cost of compilation.
As far as I know, no. Headers are an inherent part of C++ as a language. Don't forget that forward declaration allows the compiler to merely include a function pointer to a compiled object/function without having to include the whole function (which you can get around by declaring a function inline (if the compiler feels like it).
If you really, really, really hate making headers, write a perl-script to autogenerate them, instead. I'm not sure I'd recommend it though.
It's completely possible to develop without header files. One can include a source file directly:
#include "MyModule.c"
The major issue with this is one of circular dependencies (ie: in C you must declare a function before calling it). This is not an issue if you design your code completely top-down, but it can take some time to wrap ones head around this sort of design pattern if you're not used to it.
If you absolutely must have circular dependencies, one may want to consider creating a file specifically for declarations and including it before everything else. This is a little inconvenient, but still less pollution than having a header for every C file.
I am currently developing using this method for one of my major projects. Here is a breakdown of advantages I've experienced:
Much less file pollution in your source tree.
Faster build times. (Only one object file is produced by the compiler, main.o)
Simpler make files. (Only one object file is produced by the compiler, main.o)
No need to "make clean". Every build is "clean".
Less boiler plate code. Less code = less potential bugs.
I've discovered that Gish (a game by Cryptic Sea, Edmund McMillen) used a variation on this technique inside its own source code.
You can carefully lay out your functions so that all of the dependent functions are compiled after their dependencies, but as Nils implied, that is not practical.
Catalin (forgive the missing diacritical marks) also suggested a more practical alternative of defining your methods in the header files. This can actually work in most cases.. especially if you have guards in your header files to make sure they are only included once.
I personally think that header files + declaring functions is much more desirable for 'getting your head around' new code, but that is a personal preference I suppose...
You can do without headers. But, why spend effort trying to avoid carefully worked out best practices that have been developed over many years by experts.
When I wrote basic, I quite liked line numbers. But, I wouldn't think of trying to jam them into C++, because that's not the C++ way. The same goes for headers... and I'm sure other answers explain all the reasoning.
For practical purposes no, it's not possible. Technically, yes, you can. But, frankly, it's an abuse of the language, and you should adapt to the language. Or move to something like C#.
It is best practice to use the header files, and after a while it will grow into you.
I agree that having only one file is easier, but It also can leed to bad codeing.
some of these things, althoug feel awkward, allow you to get more then meets the eye.
as an example think about pointers, passing parameters by value/by reference... etc.
for me the header files allow-me to keep my projects properly structured
Learn to recognize that header files are a good thing. They separate how codes appears to another user from the implementation of how it actually performs its operations.
When I use someone's code I do now want to have to wade through all of the implementation to see what the methods are on a class. I care about what the code does, not how it does it.
This has been "revived" thanks to a duplicate...
In any case, the concept of a header is a worthy one, i.e. separate out the interface from the implementation detail. The header outlines how you use a class / method, and not how it does it.
The downside is the detail within headers and all the workarounds necessary. These are the main issues as I see them:
dependency generation. When a header is modified, any source file that includes this header requires recompilation. The issue is of course working out which source files actually use it. When a "clean" build is performed it is often necessary to cache the information in some kind of dependency tree for later.
include guards. Ok, we all know how to write these but in a perfect system it would not be necessary.
private details. Within a class, you must put the private details into the header. Yes, the compiler needs to know the "size" of the class, but in a perfect system it would be able to bind this in a later phase. This leads to all kinds of workaround like pImpl and using abstract base classes even when you only have one implementation just because you want to hide a dependency.
The perfect system would work with
separate class definition and declaration
A clear bind between these two so the compiler would know where a class declaration and its definition are, and would know what the size of a class.
You declare using class rather than pre-processor #include. The compiler knows where to find a class. Once you have done "using class" you can use that class name without qualifying it.
I'd be interested to know how D does it.
With regards to whether you can use C++ without headers, I would say no you need them for abstract base classes and standard library. Aside from that you could get by without them, although you probably would not want to.
Can I write C++ code without headers
Read more about C++, e.g. the Programming using C++ book then the C+11 standard n3337.
Yes, because the preprocessor is (conceptually) generating code without headers.
If your C++ compiler is GCC and you are compiling your translation unit foo.cc consider running g++ -O -Wall -Wextra -C -E foo.cc > foo.ii; the emitted file foo.ii does not contain any preprocessor directive, and could be compiled with g++ -O foo.ii -o foo-bin into a foo-bin executable (at least on Linux). See also Advanced Linux Programming
On Linux, the following C++ file
// file ex.cc
extern "C" long write(int fd, const void *buf, size_t count);
extern "C" long strlen(const char*);
extern "C" void perror(const char*);
int main (int argc, char**argv)
{
if (argc>1)
write(1, argv[1], strlen(argv[1]);
else
write(1, __FILE__ " has no argument",
sizeof(__FILE__ " has no argument"));
if (write(1, "\n", 1) <= 0) {
perror(__FILE__);
return 1;
}
return 0;
}
could be compiled using GCC as g++ ex.cc -O ex-bin into an executable ex-bin which, when executed, would show something.
In some cases, it is worthwhile to generate some C++ code with another program
(perhaps SWIG, ANTLR, Bison, RefPerSys, GPP, or your own C++ code generator) and configure your build automation tool (e.g. ninja-build or GNU make) to handle such a situation. Notice that the source code of GCC 10 has a dozen of C++ code generators.
With GCC, you might sometimes consider writing your own GCC plugin to analyze your (or others) C++ code (e.g. at the GIMPLE level). See also (in fall 2020) CHARIOT and DECODER European projects. You could also consider using the Clang static analyzer or Frama-C++.
Historically hearder files have been used for two reasons.
To provides symbols when compiling a program that wants to used a
library or a additional file.
To hide part of the implementing; keep things private.
For example say you have a function you don't want exposed to other
parts of your program, but want to use in your implementation. In that
case, you would write the function in the CPP file, but leave it out
of the header file. You can do this with variables and anything that
would want to keep private in the impregnation that you don't want
exposed to conumbers of that source code. In other programming
lanugases there is a "public" keyword that allows module parts to be
kept from being exposed to other parts of your program. In C and C++
no such facility exists at afile level, so header files are used
intead.
Header files are not perfect. Useing '#include' just copies the contents
of what ever file you provide. Single quotes for the current working
tree and < and > for system installed headers. In CPP for system
installed std components the '.h' is omitted; just another way C++
likes to do its own thing. If you want to give '#include' any kind of
file, it will be included. It really isn't a module system like Java,
Python, and most other programming lanuages have. Since headers are
not modules some extra steps need to be taken to get similar function
out of them. The Prepossesser (the thing that works with all the
#keywords) will blindly include what every you state is needed to be
consumed in that file, but C or C++ want to have your symbals or
implications defined only one in compilation. If you use a library, no
it main.cpp, but in the two files that main includes, then you only
want that library included once and not twice. Standard Library
components are handled special, so you don't need to worry about using
the same C++ include everywhere. To make it so that the first time the
Prepossesser sees your library it doesn't include it again, you need
to use a heard guard.
A heard guard is the simplest thing. It looks like this:
#ifndef LIBRARY_H
#define LIBRARY_H
// Write your definitions here.
#endif
It is considered good to comment the ifndef like this:
#endif // LIBRARY_H
But if you don't do the comment the compiler wont care and it wont
hurt anthing.
All #ifndef is doing is checking whether LIBRARY_H is equal to 0;
undefined. When LIBRARY_H is 0, it provides what comes before the
#endif.
Then #define LIBRARY_H sets LIBRARY_H to 1, so the next time the
Preprocessor sees #ifndef LIBRARY_H, it wont provide the same contents
again.
(LIBRARY_H should be what ever the file name is and then _ and the
extension. This is not going break anything if you don't write the
same thing, but you should be consistent. At least put the file name
for the #ifndef. Otherwise it might get confusing what guards are for
what.)
Really nothing fancy going on here.
Now you don't want to use header files.
Great, say you don't care about:
Having things private by excluding them from header files
You don't intend to used this code in a library. If you ever do, it
may be easier to go with headers now so you don't have to reorganise
your code into headers later.
You don't want to repeat yourself once in a header file and then in
a C++ file.
The purpose of hearder files can seem ambiguous and if you don't care
about people telling out it's wrong for imaginary reasons, then save
your hands and don't bother repeating yourself.
How to include only hearder files
Do
#ifndef THING_CPP
#define THING_CPP
#include <iostream>
void drink_me() {
std::cout << "Drink me!" << std::endl;
}
#endif // THING_CPP
for thing.cpp.
And for main.cpp do
#include "thing.cpp"
int main() {
drink_me();
return 0;
}
then compile.
Basically just name your included CPP file with the CPP extension and
then treat it like a header file but write out the implementations in
that one file.
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Many languages, such as Java, C#, do not separate declaration from implementation. C# has a concept of partial class, but implementation and declaration still remain in the same file.
Why doesn't C++ have the same model? Is it more practical to have header files?
I am referring to current and upcoming versions of C++ standard.
Backwards Compatibility - Header files are not eliminated because it would break Backwards Compatibility.
Header files allow for independent compilation. You don't need to access or even have the implementation files to compile a file. This can make for easier distributed builds.
This also allows SDKs to be done a little easier. You can provide just the headers and some libraries. There are, of course, ways around this which other languages use.
Even Bjarne Stroustrup has called header files a kludge.
But without a standard binary format which includes the necessary metadata (like Java class files, or .Net PE files) I don't see any way to implement the feature. A stripped ELF or a.out binary doesn't have much of the information you would need to extract. And I don't think that the information is ever stored in Windows XCOFF files.
I routinely flip between C# and C++, and the lack of header files in C# is one of my biggest pet peeves. I can look at a header file and learn all I need to know about a class - what it's member functions are called, their calling syntax, etc - without having to wade through pages of the code that implements the class.
And yes, I know about partial classes and #regions, but it's not the same. Partial classes actually make the problem worse, because a class definition is spread across several files. As far as #regions go, they never seem to be expanded in the manner I'd like for what I'm doing at the moment, so I have to spend time expanding those little plus's until I get the view right.
Perhaps if Visual Studio's intellisense worked better for C++, I wouldn't have a compelling reason to have to refer to .h files so often, but even in VS2008, C++'s intellisense can't touch C#'s
C was made to make writing a compiler easily. It does a LOT of stuff based on that one principle. Pointers only exist to make writing a compiler easier, as do header files. Many of the things carried over to C++ are based on compatibility with these features implemented to make compiler writing easier.
It's a good idea actually. When C was created, C and Unix were kind of a pair. C ported Unix, Unix ran C. In this way, C and Unix could quickly spread from platform to platform whereas an OS based on assembly had to be completely re-written to be ported.
The concept of specifying an interface in one file and the implementation in another isn't a bad idea at all, but that's not what C header files are. They are simply a way to limit the number of passes a compiler has to make through your source code and allow some limited abstraction of the contract between files so they can communicate.
These items, pointers, header files, etc... don't really offer any advantage over another system. By putting more effort into the compiler, you can compile a reference object as easily as a pointer to the exact same object code. This is what C++ does now.
C is a great, simple language. It had a very limited feature set, and you could write a compiler without much effort. Porting it is generally trivial! I'm not trying to say it's a bad language or anything, it's just that C's primary goals when it was created may leave remnants in the language that are more or less unnecessary now, but are going to be kept around for compatibility.
It seems like some people don't really believe that C was written to port Unix, so here: (from)
The first version of UNIX was written
in assembler language, but Thompson's
intention was that it would be written
in a high-level language.
Thompson first tried in 1971 to use
Fortran on the PDP-7, but gave up
after the first day. Then he wrote a
very simple language he called B,
which he got going on the PDP-7. It
worked, but there were problems.
First, because the implementation was
interpreted, it was always going to be
slow. Second, the basic notions of B,
which was based on the word-oriented
BCPL, just were not right for a
byte-oriented machine like the new
PDP-11.
Ritchie used the PDP-11 to add types
to B, which for a while was called NB
for "New B," and then he started to
write a compiler for it. "So that the
first phase of C was really these two
phases in short succession of, first,
some language changes from B, really,
adding the type structure without too
much change in the syntax; and doing
the compiler," Ritchie said.
"The second phase was slower," he said
of rewriting UNIX in C. Thompson
started in the summer of 1972 but had
two problems: figuring out how to run
the basic co-routines, that is, how to
switch control from one process to
another; and the difficulty in getting
the proper data structure, since the
original version of C did not have
structures.
"The combination of the things caused
Ken to give up over the summer,"
Ritchie said. "Over the year, I added
structures and probably made the
compiler code somewhat better --
better code -- and so over the next
summer, that was when we made the
concerted effort and actually did redo
the whole operating system in C."
Here is a perfect example of what I mean. From the comments:
Pointers only exist to make writing a compiler easier? No. Pointers exist because they're the simplest possible abstraction over the idea of indirection. – Adam Rosenfield (an hour ago)
You are right. In order to implement indirection, pointers are the simplest possible abstraction to implement. In no way are they the simplest possible to comprehend or use. Arrays are much easier.
The problem? To implement arrays as efficiently as pointers you have to pretty much add a HUGE pile of code to your compiler.
There is no reason they couldn't have designed C without pointers, but with code like this:
int i=0;
while(src[++i])
dest[i]=src[i];
it will take a lot of effort (on the compilers part) to factor out the explicit i+src and i+dest additions and make it create the same code that this would make:
while(*(dest++) = *(src++))
;
Factoring out that variable "i" after the fact is HARD. New compilers can do it, but back then it just wasn't possible, and the OS running on that crappy hardware needed little optimizations like that.
Now few systems need that kind of optimization (I work on one of the slowest platforms around--cable set-top boxes, and most of our stuff is in Java) and in the rare case where you might need it, the new C compilers should be smart enough to make that kind of conversion on its own.
In The Design and Evolution of C++, Stroustrup gives out one more reason...
The same header file can have two or more implementation files which can be simultaneously worked-upon by more than one programmer without the need of a source-control system.
This might seem odd these days, but I guess it was an important issue when C++ was invented.
If you want C++ without header files then I have good news for you.
It already exists and is called D (http://www.digitalmars.com/d/index.html)
Technically D seems to be a lot nicer than C++ but it is just not mainstream enough for use in many applications at the moment.
One of C++'s goals is to be a superset of C, and it's difficult for it to do so if it cannot support header files. And, by extension, if you wish to excise header files you may as well consider excising CPP (the pre-processor, not plus-plus) altogether; both C# and Java do not specify macro pre-processors with their standards (but it should be noted in some cases they can be and even are used even with these languages).
As C++ is designed right now, you need prototypes -- just as in C -- to statically check any compiled code that references external functions and classes. Without header files, you would have to type out these class definitions and function declarations prior to using them. For C++ not to use header files, you'd have to add a feature in the language that would support something like Java's import keyword. That'd be a major addition, and change; to answer your question of if it'd be practical: I don't think so--not at all.
Many people are aware of shortcomings of header files and there are ideas to introduce more powerful module system to C++.
You might want to take a look at Modules in C++ (Revision 5) by Daveed Vandevoorde.
Well, C++ per se shouldn't eliminate header files because of backwards compatibility. However, I do think they're a silly idea in general. If you want to distribute a closed-source lib, this information can be extracted automatically. If you want to understand how to use a class w/o looking at the implementation, that's what documentation generators are for, and they do a heck of a lot better a job.
There is value in defining the class interface in a separate component to the implementation file.
It can be done with interfaces, but if you go down that road, then you are implicitly saying that classes are deficient in terms of separating implementation from contract.
Modula 2 had the right idea, definition modules and implementation modules. http://www.modula2.org/reference/modules.php
Java/C#'s answer is an implicit implementation of the same (albeit object-oriented.)
Header files are a kludge, because header files express implementation detail (such as private variables.)
In moving over to Java and C#, I find that if a language requires IDE support for development (such that public class interfaces are navigable in class browsers), then this is maybe a statement that the code doesn't stand on its own merits as being particularly readable.
I find the mix of interface with implementation detail quite horrendous.
Crucially, the lack of ability to document the public class signature in a concise well-commented file independent of implementation indicates to me that the language design is written for convenience of authorship, rather convenience of maintenance. Well I'm rambling about Java and C# now.
One advantage of this separation is that it is easy to view only the interface, without requiring an advanced editor.
No language exists without header files. It's a myth.
Look at any proprietary library distribution for Java (I have no C# experience to speak of, but I'd expect it's the same). They don't give you the complete source file; they just give you a file with every method's implementation blanked ({} or {return null;} or the like) and everything they can get away with hiding hidden. You can't call that anything but a header.
There is no technical reason, however, why a C or C++ compiler could count everything in an appropriately-marked file as extern unless that file is being compiled directly. However, the costs for compilation would be immense because neither C nor C++ is fast to parse, and that's a very important consideration. Any more complex method of melding headers and source would quickly encounter technical issues like the need for the compiler to know an object's layout.
If you want the reason why this will never happen: it would break pretty much all existing C++ software. If you look at some of the C++ committee design documentation, they looked at various alternatives to see how much code it would break.
It would be far easier to change the switch statement into something halfway intelligent. That would break only a little code. It's still not going to happen.
EDITED FOR NEW IDEA:
The difference between C++ and Java that makes C++ header files necessary is that C++ objects are not necessarily pointers. In Java, all class instances are referred to by pointer, although it doesn't look that way. C++ has objects allocated on the heap and the stack. This means C++ needs a way of knowing how big an object will be, and where the data members are in memory.
Header files are an integral part of the language. Without header files, all static libraries, dynamic libraries, pretty much any pre-compiled library becomes useless. Header files also make it easier to document everything, and make it possible to look over a library/file's API without going over every single bit of code.
They also make it easier to organize your program. Yes, you have to be constantly switching from source to header, but they also allow you define internal and private APIs inside the implementations. For example:
MySource.h:
extern int my_library_entry_point(int api_to_use, ...);
MySource.c:
int private_function_that_CANNOT_be_public();
int my_library_entry_point(int api_to_use, ...){
// [...] Do stuff
}
int private_function_that_CANNOT_be_public() {
}
If you #include <MySource.h>, then you get my_library_entry_point.
If you #include <MySource.c>, then you also get private_function_that_CANNOT_be_public.
You see how that could be a very bad thing if you had a function to get a list of passwords, or a function which implemented your encryption algorithm, or a function that would expose the internals of an OS, or a function that overrode privileges, etc.
Oh Yes!
After coding in Java and C# it's really annoying to have 2 files for every classes. So I was thinking how can I merge them without breaking existing code.
In fact, it's really easy. Just put the definition (implementation) inside an #ifdef section and add a define on the compiler command line to compile that file. That's it.
Here is an example:
/* File ClassA.cpp */
#ifndef _ClassA_
#define _ClassA_
#include "ClassB.cpp"
#include "InterfaceC.cpp"
class ClassA : public InterfaceC
{
public:
ClassA(void);
virtual ~ClassA(void);
virtual void methodC();
private:
ClassB b;
};
#endif
#ifdef compiling_ClassA
ClassA::ClassA(void)
{
}
ClassA::~ClassA(void)
{
}
void ClassA::methodC()
{
}
#endif
On the command line, compile that file with
-D compiling_ClassA
The other files that need to include ClassA can just do
#include "ClassA.cpp"
Of course the addition of the define on the command line can easily be added with a macro expansion (Visual Studio compiler) or with an automatic variables (gnu make) and using the same nomenclature for the define name.
Still I don't get the point of some statements. Separation of API and implementation is a very good thing, but header files are not API. There are private fields there. If you add or remove private field you change implementation and not API.