In my project i have a Header file common.h which include many headers in it.Some of the files include Common.h and some other header which are already present in Common .h So In the Pre-Processing stage many functions get prototyped twice(Once from the Included header and other from Gui.h).I was wondering is this would cause any issue in long run.
Please suggest..Thanks in advance..
Headers should have include guards so that they are only processed once:
#ifndef SOME_UNIQUE_STRING
#define SOME_UNIQUE_STRING
// Everything else here
#endif
By "Everything" I mean "everything", starting with your #includes if any.
SOME_UNIQUE_STRING could be the name of the module as long as it is unlikely to coincide with another define somewhere else.
If you look in your library headers, you will notice they use include guards like this.
As Chemistpp suggested,
#pragma once
is a good option to try, though it is non-standard.
Check out the advantages and disadvantages listed in the link.
I have a precompiled header that contains includes for various 3rd party libraries, e.g.:
#ifndef PRECOMPILED_H
#define PRECOMPILED_H
#include "booststuff.h"
#include "luastuff.h"
#endif
Where booststuff.h and luastuff.h are header files in my project that just include various boost / lua related things and set up some typedefs / usings / namespace aliases.
I set up the precompiled header in the usual way inside visual studio (2012), and use the force include option to include it in every cpp file.
In the cpp files, I've also been fairly careful to #include "booststuff.h" where I actually use it as well (I sometimes disable precompiled headers to test this). However, I've been wondering lately whether that's a good idea. So:
Does anything bad happen if I include a file again that's already included in the precompiled header (I don't see why it would, but I've seen things about headers having to be included "in the same order", and not really understood what they were on about)?
Does it affect Intellisense (unusably slow with a fairly small project)? I'd be happy to give up some portability for better Intellisense since I currently have no desire to switch platforms.
If each include file has #pragma once in it, the compiler will completely skip reading the file on the second and subsequent attempts to include it. It isn't stated explicitly but I assume the precompiled header tracks this information as well.
#include "Ser.h"
#include ".\ser.h"
Is this .\ser.h is an executable of Ser.cpp...When right clicked on it and when pressed "open the document .\ser.h" its going to the Ser.h file...
Why does they again include the executable file(./ser.h) as header file.... isnt the header file(Ser.h) enough to get all the delarations needed to be defined in.
.\ser.h is not an executable file, it's simply another header file.
In systems where file names are case-irrelevant (and this seems likely here since it's using the Windows path separator \), it may well be just including the same file twice.
If include guards are used correctly, that won't matter. If case is relevant, they're most likely two different files.
I say "may" since the order of searches for header files, and even the mapping of the header-names to headers or files, is implementation defined and xyz.h and ./xyz.h may be found in different places because of this.
There is no way of telling for sure without knowing what your include path is. This is likely just including the same header file twice on a case insensitive platform such as Windows. Which is probably a mistake but a harmless one because of include guards (those #ifdef at the beginning and end of the file).
It seems as if you are having trouble understanding the purpose of header files. There is no such thing as the header file. A header file is just the place where functions and classes are declared but usually not defined. So if ser.cpp uses some function that is defined in common_functions.cpp, you should include the apropriate header - it will usually be called common_functions.h.
Be aware that, as for almost anything in the C or C++ world, there are plenty of exceptions, but the above holds true most of the time.
The ./ser.h is not an executable file. Please keep in mind, file extensions ending in .h are generally header files.
If your platform is windows you will find executable files taking the file extension .exe.
One Ser.h file is enough to receive all the declarations, so you do not need two of the same header files.
I don't understand why header guards are not used in pre-compiled headers... Any ideas?
Because "stdafx.h" has to be the first include in .cpp files, not anywhere else.
If all you do is include other headers, there's no need. If those files can not be included multiple times, they will have their own header guards. stdafx.h itself doesn't care how many times it is included unless you're using it wrong.
Usually, stdafx.h will be included only once per cpp file, as the first statement, and normally, no other files will include it. So, chances of recursively including stdafx.h are minimal, thus the "unnecessariness" of the include guard.
I would still advise to use one, just in case, or potentially use #pragma once at the top of the file.
I do not know the code of the precompiled header, but I guess it contains a "#pragma once", which has the same effect but only works in VS (at least it's not standard). I remember wizard created MFC files using these.
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What order should headers be declared in a header / cpp file? Obviously those that are required by subsequent headers should be earlier and class specific headers should be in cpp scope not header scope, but is there a set order convention / best practice?
In a header file you have to include ALL the headers to make it compilable. And don't forget to use forward declarations instead of some headers.
In a source file:
corresponded header file
necessary project headers
3rd party libraries headers
standard libraries headers
system headers
In that order you will not miss any of your header files that forgot to include libraries by their own.
Good practice: every .h file should have a .cpp that includes that .h first before anything else. This proves that any .h file can be put first.
Even if the header requires no implementation, you make a .cpp that just includes that .h file and nothing else.
This then means that you can answer your question any way you like. It doesn't matter what order you include them in.
For further great tips, try this book: Large-Scale C++ Software Design - it's a shame it's so expensive, but it is practically a survival guide for C++ source code layout.
In header files, I tend to put standard headers first, then my own headers (both lists being ordered alphabetically). In implementation files, I put first the header corresponding (if any), then standards headers and other dependency headers.
Order is of little importance, except if you make a great use of macros and #define ; in that case, you must checked that a macro you defined doesn't replace a previously included one (except if that's what you want, of course).
Concerning this statement
those that are required by subsequent headers should be earlier
A header shouldn't rely on other headers being included before it! If it requires headers, it just includes them. Header guards will prevent multiple inclusion:
#ifndef FOO_HEADER_H
#define FOO_HEADER_H
...
#endif
EDIT
Since I wrote this answer, I changed my way of ordering the include directives in my code. Now, I try to always put headers in increasing order of standardization, so the headers of my project come first, followed by 3rd party libraries headers, followed by standard headers.
For instance, if one of my file uses a library I wrote, Qt, Boost and the standard library, I will order the includes as follow:
//foo.cpp
#include "foo.hpp"
#include <my_library.hpp>
// other headers related to my_library
#include <QtCore/qalgorithms.h>
// other Qt headers
#include <boost/format.hpp> // Boost is arguably more standard than Qt
// other boost headers
#include <algorithms>
// other standard algorithms
The reason why I do that is to detect missing dependencies in my own headers: let's assume for instance that my_library.hpp uses std::copy, but doesn't include <algorithm>. If I include it after <algorithm> in foo.cpp, this missing dependency will go unnoticed. On the contrary, with the order I just presented, the compiler will complain that std::copy has not been declared, allowing me to correct my_library.hpp.
In each "library" group, I try to keep the include directives ordered alphabetically, to find them more easily.
On a sidenote, a good practice is also to limit at a maximum the dependency between header files. Files should include as little headers as possible, especially headers file. Indeed, the more headers you include, the more code needs to be recompiled when something changes. A good way to limit these dependencies is to use forward declaration, which is very often sufficient in header files (see When can I use a forward declaration?).
Google C++ Style Guide, Names and Order of Includes :
In dir/foo.cc, whose main purpose is to implement or test the stuff in dir2/foo2.h, order your includes as follows:
dir2/foo2.h (preferred location — see details below).
C system files.
C++ system files.
Other libraries' .h files.
Your project's .h files.
I used to order them in alphabetical order (easier to find)
The "how" is not obvious, but the "what" is.
Your goal is to make sure that the order in which you include header files never matters (and i mean "NEVER !").
A good help is to test whether header files compile when building cpp files (one for each header file) that only include one of them.
For .cpp files, you should include the header of the class or whatever you are implementing first, so you catch the case where this header is missing some includes. After that, most coding guidelines tend to include system headers first, project headers second, for example the Google C++ Style Guide.
It's a dependency thing and it depends largely on what you put in our headers. A fact is that you can be really notorious about this and minimize to keep your includes strict but you'll eventually run into a scenario where you'll wanna use inclusion guards.
#ifndef MY_HEADER_H
#define MY_HEADER_H
//...
#endif
The problem isn't that apparent in the beginning, but as the complexity of your software grows so does your dependencies. You can do well, and be smart about it but larger C++ projects are generally riddled with includes. You can try, but you can only do so much. So be diligent and think about your includes, YES! But you'll most certainly have cyclic dependencies at some point and that is why you need inclusion guards.
If a header needs other headers then it just includes them in that header.
Try to structure your code so you pass pointers or references and forward declare where you can.
In the implementation then the header that defines it should be listed first (except in Visual Studio if you are using pch then stdafx would go first).
I generally list them as I need.
I've found the following convention the most useful:
module.cpp:
// this is the header used to trigger inclusion of precompiled headers
#include <precompiled.h>
// this ensures that anything that includes "module.h" works
#include "module.h"
// other headers, usually system headers, the project
The important thing is to put the module's header as the first non-precompiled header. This ensures "module.h" has no unexpected dependencies.
If you're working on a large project with slow disk access times, I've seen this style used to decrease build times:
module.cpp:
// this is the header used to trigger inclusion of precompiled headers
#include <precompiled.h>
// this ensures that anything that includes "module.h" works
#include "module.h"
// other headers, usually system headers, the project
#if !defined _OTHER_MODULE_GUARD_
#include "other_module.h"
#endif
#if !defined _ANOTHER_MODULE_GUARD_
#include "another_module.h"
#endif
It's a bit verbose but does save on disk seeking since the header won't be searched for / opened if it's already been included. Without the guard check, the compiler will seek for and open the header file, parse the whole file to end up #ifdefing the whole file out.