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I am having one big project in which I have kept all Headers in one common header file and where all I needed i included that header file.. by doing like this project is working fine but if I do changes in any header file its taking too much to build so I want to know is there any resolution to reduce the build time?
This isn't really specific to MFC, it is a general C++ thing. Basically don't put everything in 1 common header. Make use of forward declarations wherever possible. Use include guard macros in headers unless doing some special preprocessor magic.
Use a precompiled header and only put stuff in there that very rarely changes. Don't let this header get too big though as that can decrease build times.
Reduce the amount of code in headers. In some cases the pimpl idiom can make headers more terse and less prone to change due to 'internal' implementation changes at the cost of run-time efficiency.
http://www.cplusplus.com/forum/articles/10627/
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A similar issue has already been discussed (C/C++ include header file order), but this thread makes no mention of forward declarations. If I summarize what I've read online so far:
Everyone agrees that the corresponding header for the .cpp file should go first. This ensures that the header files has everything it needs.
Beyond that, there seems to be no consensus. The Google guidelines (https://google.github.io/styleguide/cppguide.html) suggest including headers from system -> other libs -> project. Many people on SO suggest the exact opposite. It seems to be a matter of personal preferences.
Regarding forward declarations, is there any reason to add them before/after the include headers? I don't see why it would matter (Is a class declaration allowed after a class definition?), but maybe I am missing something.
I think this is one of those questions that has no one right answer.
The way I think of it (and its by no means the absolute answer) is from an encapsulation point of view and I tend to do the opposite of the google guidelines for this reason:
If you include your project headers first you are less likley to hit issues where your header relies on some system include that went before it because you will just get a compile error... so therefore I find you see issues faster by putting the system includes last.
Same thing for the forward declarations - if you need them and they are not provided by a header - then add them after the includes because the headers should not need them (otherwise it would be in the header) and so you only declare it as/when you need it...
That is more-or-less my reasoning - but as I say, if your headers are designed correctly then you could do it either way...
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I'm just curious about whether using windows.h a good thing (PS : I don't care about OSindependent code) ,it seems really good with thousands of functions . So is it good ? and where is its Dir ?
Good, bad or indifferent depends on the view.
If you are programming Windows-specific applicaitons, then you'll probably not be able to avoid using it. For generic applications that work on any platform, it's obviously bad, and if you DON'T know that you need some headerfile, don't include it. But do include things that you do need, even if it "works" without - you never know when something changes that header file that dragged in something else you didn't include in your code, and breaks the code.
"Where is it's dir", I presume means "what directory will I find it in", and I'm afraid that's not something I, or anyone else, can tell you. It depends on which compiler you are using, and how/where it was installed - and in some cases, you need to install it separately, in other cases it's included with the compiler. Since there are at least half a dozen different current compilers that work under Windows, and several that no longer are being maintained, but still "works" [to varying degrees], it would be rather pointless to even try to answer this.
Microsoft does publish an "SDK", which contains the headers, should they not be part of your installation of the compiler:
https://dev.windows.com/en-us/downloads/windows-10-sdk
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I asked a question about global variables and one of the answers raised another question which is what is the risk of very large cpp file?
Is the concern here is the maintainability of the program or something else?
My original question
Only maintainability. There is no compilation issues, as it is common for compilers to combine all #include files into a translation unit and then compile that. Thus each .cpp file winds up being many times larger than the input anyway, before moving on to later stages of compilation.
For a single programmer working on his own program, when size become an issue is a personal choice. For a team of programmers at some company, having some reasonable number of C++ files for an application allows each team member to work on a separate file in parallel. Although there are tool sets that can merge separate edits made to the same source file(s), dealing with potential conflicts (someone has to check and/or fix the conflicts), is an issue.
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This was asked on a past exam. Given that the header file is custom I am assuming not because they are just variations of the main(){ header correct?
I'm not sure if I understand the question, but I would say yes, they are optional. You could write all of your (custom) functions, classes, etc. in one file if you wanted.
I'm new here so I can't comment, but the question is a bit confusing to me. But here is what I think, I hope it helps:
Header files contain functions, variables, classes etc. in C and C++. Header files that come pre-built with your compiler must be included before you can use any function or anything thing inside that file.
Now referring to a custom header files, you may choose to create a file containing specific information to use in your programs, often to make your code look more organize or to create reusable libraries. Those are OPTIONAL simply because you can manage to create all your functions, variables and classes in the same file containing your main(){}. It might look messy, impossible to read but possible to achieve.
BTW I'm not sure about what you mean by header files being variations of the main(), but agreeing with Trevor Hickey, they shouldn't have a main() function since they are not compilable, they don't execute the functions they just hold the information.
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I am developing some c++ code, but having code separated in to .h and .cpp is driving me mad, as it is slowing down re-factoring.
Is there a tool that lets one work on a single file. An editor that just hides the truth, or a per-processor that takes a single file and produces the two files .cpp and .h
clarification:
I want a single file per compilation unit (like Java, C♯, Eiffel). I will still have to have #include in files to include the headers of other modules. (but then Java and c♯ have import and using).
clarification 2:
Things are easier if everything that should be together is together.
i.e. one and only one file per class.
There's Lzz. I haven't tried it, but it seems like what you're looking for.
Whatever tool you try to use to do this will only hide some of the complexity or make your code C++-unlike and that will in turn make it harder for others to read/maintain.
I recommend that you just learn and get used to the compilation model of the language rather than fighting it. Deciding what goes into the header and/or the implementation is not an automated process, but rather part of the design and tools cannot design for you. Any automated tool to do that will end up generating a less than perfect result, probably longer compile times and/or leaking implementation details to the users of your headers.