I was wondering if there is some pro and contra having include statements directly in the include files as opposed to have them in the source file.
Personally I like to have my includes "clean" so, when I include them in some c/cpp file I don't have to hunt down every possible header required because the include file doesn't take care of it itself. On the other hand, if I have the includes in the include files compile time might get bigger, because even with the include guards, the files have to be parsed first. Is this just a matter of taste, or are there any pros/cons over the other?
What I mean is:
sample.h
#ifdef ...
#include "my_needed_file.h"
#include ...
class myclass
{
}
#endif
sample.c
#include "sample.h"
my code goes here
Versus:
sample.h
#ifdef ...
class myclass
{
}
#endif
sample.c
#include "my_needed_file.h"
#include ...
#include "sample.h"
my code goes here
There's not really any standard best-practice, but for most accounts, you should include what you really need in the header, and forward-declare what you can.
If an implementation file needs something not required by the header explicitly, then that implementation file should include it itself.
The language makes no requirements, but the almost universally
accepted coding rule is that all headers must be self
sufficient; a source file which consists of a single statement
including the include should compile without errors. The usual
way of verifying this is for the implementation file to include
its header before anything else.
And the compiler only has to read each include once. If it
can determine with certainty that it has already read the file,
and on reading it, it detects the include guard pattern, it has
no need to reread the file; it just checks if the controling
preprocessor token is (still) defined. (There are
configurations where it is impossible for the compiler to detect
whether the included file is the same as an earlier included
file. In which case, it does have to read the file again, and
reparse it. Such cases are fairly rare, however.)
A header file is supposed to be treated like an API. Let us say you are writing a library for a client, you will provide them a header file for including in their code, and a compiled binary library for linking.
In such scenario, adding a '#include' directive in your header file will create a lot of problems for your client as well as you, because now you will have to provide unnecessary header files just to get stuff compiling. Forward declaring as much as possible enables cleaner API. It also enables your client to implement their own functions over your header if they want.
If you are sure that your header is never going to be used outside your current project, then either way is not a problem. Compilation time is also not a problem if you are using include guards, which you should have been using anyway.
Having more (unwanted) includes in headers means having more number of (unwanted) symbols visible at the interface level. This may create a hell lot of havocs, might lead to symbol collisions and bloated interface
On the other hand, if I have the includes in the include files compile time might get bigger, because even with the include guards
If your compiler doesn't remember which files have include guards and avoid re-opening and re-tokenising the file then get a better compiler. Most modern compilers have been doing this for many years, so there's no cost to including the same file multiple times (as long as it has include guards). See e.g. http://gcc.gnu.org/onlinedocs/cpp/Once_002dOnly-Headers.html
Headers should be self-sufficient and include/declare what they need. Expecting users of your header to include its dependencies is bad practice and a great way to make users hate you.
If my_needed_file.h is needed before sample.h (because sample.h requires declarations/definitions from it) then it should be included in sample.h, no question. If it's not needed in sample.h and only needed in sample.c then only include it there, and my preference is to include it after sample.h, that way if sample.h is missing any headers it needs then you'll know about it sooner:
// sample.c
#include "sample.h"
#include "my_needed_file.h"
#include ...
#include <std_header>
// ...
If you use this #include order then it forces you to make sample.h self-sufficient, which ensures you don't cause problems and annoyances for other users of the header.
I think second approach is a better one just because of following reason.
when you have a function template in your header file.
class myclass
{
template<class T>
void method(T& a)
{
...
}
}
And you don't want to use it in the source file for myclass.cxx. But you want to use it in xyz.cxx, if you go with your first approach then you will end up in including all files that are required for myclass.cxx, which is of no use for xyz.cxx.
That is all what I think of now the difference. So I would say one should go with second approach as it makes your code each to maintain in future.
Related
I have a code that contains huge number of cpp / header files. My problem now is, that because many include each other, I occasionally get into a situation that my code doesn't compile, unless I reorder the #include directives in random files, which is now necessary basically with creation of any other header file.
This is really very annoying; is there any tip how should I write my c++ code in order to prevent complications with #include? I would prefer to split my source code to as many files as possible so that cooperation with other programmers (using git or svn) is easier (more files == lower number of edit conflicts).
One of things that help me now is forward declaration, when I declare the classes from other headers into other files. That helps sometimes, but doesn't resolve all issues; sometimes I just need to change order of #includes in random header files or merge multiple files.
Not a panacea, but the following guideline helps me a lot.
Assuming your code is composed of files like MyClassXyz.cpp with corresponding MyClassXyz.h, one class per source file, every cpp-file should include its corresponding header file first. That is, MyClassXyz.cpp must start with the following line:
// (possibly after comments)
#include "MyClassXyz.h"
This ensures that MyClassXyz.h includes all header files (or forward declarations) necessary for its compilation.
I often see code that uses an opposite convention (#includeing most general header files first), for example, MyClassXyz.cpp starts with
#include <vector>
#include <iosfwd>
#include "blah.h"
#include "mytypes.h"
#include "MyClassXyz.h"
And MyClassXyz.h "goes straight to the point" using stuff defined in the additional headers:
#pragma once
// "#include <vector>" missing - a hidden error!
// "#include <iosfwd>" missing - a hidden error!
class MyClassXyz
{
std::vector<int> v;
friend std::ostream& operator<<(...);
...
}
While this compiles OK, it gives enormous headaches of the type you describe, when trying to use the class MyClassXyz in some other source file.
If I have a project with
main .cpp
Knife .h and .cpp
Cucumber .h and .cpp
and I want to use Knife's members in Cucumber, does it matter whether I use the following:
#include "Knife.h"
#include <iostream>
using namespace std;
in Cucumber.h or Cucumber.cpp (assume that Cucumber.cpp already has an include for Cucumber.h)?
My recommendation is to minimize the number of files included in the header files.
So, if I have the choice, I prefer to include in the source file.
When you modify a header file, all the files that include this header file must be recompiled.
So, if cucumber.h includes knife.h, and main.cpp includes cucumber.h, and you modify knife.h, all the files will be recompiled (cucumber.cpp, knife.cpp and main.cpp).
If cucumber.cpp includes knife.h and main.cpp includes cucumber.h, and you modify knife.h, only cucumber.cpp and knife.cppwill be recompiled, so your compilation time is reduced.
If you need to use knife in cucumber you can proceed like this:
// Cucumber.hpp
#ifndef CUCUMBER_HPP
#define CUCUMBER_HPP
class Knife;
class Cucumber
{
public :
///...
private :
Knife* myKnife
}
#endif
//Cucumber.cpp
#include "Cucumber.hpp"
#include "Knife.hpp
// .. your code here
This "trick" is called "forward declaration". That is a well-known trick of C++ developers, who want to minimize compilation time.
yes, you should put it in the .cpp file.
you will have faster builds, fewer dependencies, and fewer artifacts and noise -- in the case of iostream, GCC declares:
// For construction of filebuffers for cout, cin, cerr, clog et. al.
static ios_base::Init __ioinit;
within namespace std. that declaration is going to produce a ton of (redundant) static data which must be constructed at startup, if Knife.h is included by many files. so there are a lot of wins, the only loss is that you must explicitly include the files you actually need -- every place you need them (which is also a very good thing in some regards).
The advice I've seen advocated was to only include the minimum necessary for a given source file, and to keep as many includes out of headers as possible. It potentially reduces the dependencies when it comes time to compile.
Another thing to note is your namespace usage. You definitely want to be careful about having that sort of thing in a header. It could change namespace usage in files you didn't plan on.
As a general rule, try to add your includes into the implementation file rather than the header for the following reasons:
Reduces potential unnecessary inclusion and keeps it to only where needed.
Reduces compile time (quite significantly in some cases I've seen.) Avoids over exposing implementation details.
Reduces risk of circular dependencies appearing.
Avoids locking users of your headers into having to indirectly include files that they may not want / need to.
If you reference a class by pointer or reference only in your header then it's not necessary to include the appropriate header; a class declaration will suffice.
Probably there are quite a few other reasons - the above are probably the most important / obvious ones.
I have 3 classes (it could be 300) , each one with its own header and implementation.
I'd like to write an 'elegant' way to organize the way I load of any class needed by every class of the three. Maybe this example helps...
I have : class1 class2 class3
Every header has:
#ifndef CLASS#_H
#define CLASS#_H
#define FORWARD_STYLE
#include "general.h"
#endif
Every implementation has:
#define DIRECT_STYLE
#include "general.h"
OK
I'm going to write a 'general.h' file in which I'd have :
#ifndef DIRECT_STYLE
#ifndef CLASS1_H
#include "class1.h"
#endif
#ifndef CLASS2_H
#include "class2.h"
#endif
#ifndef CLASS3_H
#include "class3.h"
#endif
#endif
#ifndef FORWARD_STYLE
class Class1;
class Class2;
class Class3;
#endif
// a lot of other elements needed
#include <string.h>
#include <stdio.h"
....
#include <vector.h"
( all the class I need now and in the future )
This is a good structure ? Or I'm doing some idiot thing ?
My goal is having one unique 'general.h' file to write all the elemenst I need...
Are this to work fine ?
Thanks
The basic rules to follow are:
Let each of your source file include all the header files it needs for getting compiled in a standalone manner. Avoid letting the header files include in the source file indirectly through other files.
If you have constructs which will be needed across most source files then put them in a common header and include the header in Only in those source files which need it.
Use Forward declarations wherever you can.There are several restrictions of when you can get away using them,read this to know more about those scenarios.
Overall it is a good idea to avoid including unnecessary code in source files through a common header because it just results in code bloat, so try and keep it to a minimum. Including a header just actually copy pastes the entire header to your source file and Including unnecessary files has several disadvantages, namely:
Increase in compilation time
Pollution of global namespace.
Potential clash of preprocessor names.
Increase in Binary size(in some cases though not always)
This might like a fine idea now, but won't scale and should be avoided. Your general.h file will include a vast amount of files, and thus all files that include it will (a) take ages to compile or not compile at all due to memory restrictions and (b) will have to be re-compiled every time anything changes.
Directly include the headers you need in each file, and define a few forward declaration files, and you should be fine.
The #define in a header will probably be ok, but it can propagate through lots of sources and potentially cause problems. More seriously, any time general.h or any of its includes change your entire project rebuilds. For small projects this isn't an issue, for larger projects it will result in unacceptable build times.
Instead, I utilize a few guidelines:
In headers, forward declare what you can, either explicitly or with #include "blah_fwd.h" as seen in the standard library.
All headers should be able to compile on their own and not rely on the source file including something earlier. This can be easily detected by all source files always including their own header first.
In source files, include what you need (usually you can't get away with forward declarations in source files).
Also note to never use using in headers because it will pollute the global namespace.
If this seems like a lot of work, unfortunately that's because it is. This is a system inherited from C and requires some level of programmer maintenance. If you want to be able to decide at a high level what's used by your project and let the compiler/runtime figure it out, perhaps C++ isn't the right language for your project.
I'm reading some c++ code and Notice that there are "#include" both in the header files and .cpp files . I guess if I move all the "#include" in the file, let's say foo.cpp, to its' header file foo.hh and let foo.cpp only include foo.hh the code should work anyway taking no account of issues like drawbacks , efficiency and etc .
I know my "all of sudden" idea must be in some way a bad idea, but what is the exact drawbacks of it? I'm new to c++ so I don't want to read lots of C++ book before I can answer this question by myself. so just drop the question here for your help . thanks in advance.
As a rule, put your includes in the .cpp files when you can, and only in the .h files when that is not possible.
You can use forward declarations to remove the need to include headers from other headers in many cases: this can help reduce compilation time which can become a big issue as your project grows. This is a good habit to get into early on because trying to sort it out at a later date (when its already a problem) can be a complete nightmare.
The exception to this rule is templated classes (or functions): in order to use them you need to see the full definition, which usually means putting them in a header file.
The include files in a header should only be those necessary to support that header. For example, if your header declares a vector, you should include vector, but there's no reason to include string. You should be able to have an empty program that only includes that single header file and will compile.
Within the source code, you need includes for everything you call, of course. If none of your headers required iostream but you needed it for the actual source, it should be included separately.
Include file pollution is, in my opinion, one of the worst forms of code rot.
edit: Heh. Looks like the parser eats the > and < symbols.
You would make all other files including your header file transitively include all the #includes in your header too.
In C++ (as in C) #include is handled by the preprocessor by simply inserting all the text in the #included file in place of the #include statement. So with lots of #includes you can literally boast the size of your compilable file to hundreds of kilobytes - and the compiler needs to parse all this for every single file. Note that the same file included in different places must be reparsed again in every single place where it is #included! This can slow down the compilation to a crawl.
If you need to declare (but not define) things in your header, use forward declaration instead of #includes.
While a header file should include only what it needs, "what it needs" is more fluid than you might think, and is dependent on the purpose to which you put the header. What I mean by this is that some headers are actually interface documents for libraries or other code. In those cases, the headers must include (and probably #include) everything another developer will need in order to correctly use your library.
Including header files from within header files is fine, so is including in c++ files, however, to minimize build times it is generally preferable to avoid including a header file from within another header unless absolutely necessary especially if many c++ files include the same header.
.hh (or .h) files are supposed to be for declarations.
.cpp (or .cc) files are supposed to be for definitions and implementations.
Realize first that an #include statement is literal. #include "foo.h" literally copies the contents of foo.h and pastes it where the include directive is in the other file.
The idea is that some other files bar.cpp and baz.cpp might want to make use of some code that exists in foo.cc. The way to do that, normally, would be for bar.cpp and baz.cpp to #include "foo.h" to get the declarations of the functions or classes that they wanted to use, and then at link time, the linker would hook up these uses in bar.cpp and baz.cpp to the implementations in foo.cpp (that's the whole point of the linker).
If you put everything in foo.h and tried to do this, you would have a problem. Say that foo.h declares a function called doFoo(). If the definition (code for) this function is in foo.cc, that's fine. But if the code for doFoo() is moved into foo.h, and then you include foo.h inside foo.cpp, bar.cpp and baz.cpp, there are now three definitions for a function named doFoo(), and your linker will complain because you are not allowed to have more than one thing with the same name in the same scope.
If you #include the .cpp files, you will probably end up with loads of "multiple definition" errors from the linker. You can in theory #include everything into a single translation unit, but that also means that everything must be re-built every time you make a change to a single file. For real-world projects, that is unacceptable, which is why we have linkers and tools like make.
There's nothing wrong with using #include in a header file. It is a very common practice, you don't want to burden a user a library with also remembering what other obscure headers are needed.
A standard example is #include <vector>. Gets you the vector class. And a raft of internal CRT header files that are needed to compile the vector class properly, stuff you really don't need nor want to know about.
You can avoid multiple definition errors if you use "include guards".
(begin myheader.h)
#ifndef _myheader_h_
#define _myheader_h_
struct blah {};
extern int whatsit;
#endif //_myheader_h_
Now if you #include "myheader.h" in other header files, it'll only get included once (due to _myheader_h_ being defined). I believe MSVC has a "#pragma once" with the equivalent functionality.
I have a question regarding "best-practice" when including headers.
Obviously include guards protect us from having multiple includes in a header or source file, so my question is whether you find it beneficial to #include all of the needed headers in a header or source file, even if one of the headers included already contains one of the other includes. The reasoning for this would be so that the reader could see everything needed for the file, rather than hunting through other headers.
Ex: Assume include guards are used:
// Header titled foo.h
#include "blah.h"
//....
.
// Header titled bar.h that needs blah.h and foo.h
#include "foo.h"
#include "blah.h" // Unnecessary, but tells reader that bar needs blah
Also, if a header is not needed in the header file, but is needed in it's related source file, do you put it in the header or the source?
In your example, yes, bar.h should #include blah.h. That way if someone modifies foo so that it doesn't need blah, the change won't break bar.
If blah.h is needed in foo.c but not in foo.h, then it should not be #included in foo.h. Many other files may #include foo.h, and more files may #include them. If you #include blah.h in foo.h, then you make all those files needlessly dependent on blah.h. Needless dependencies cause lots of headaches:
If you modify blah.h, all those files must be recompiled.
If you want to isolate one of them (say, to carry it over to another project or build a unit test around it) you have to take blah.h along.
If there's a bug in one of them, you can't rule out blah.h as the cause until you check.
If you are foolish enough to have something like a macro in blah.h... well, never mind, in that case there's no hope for you.
The basic rule is, #include any headers that you actually use in your code. So, if we're talking:
// foo.h
#include "utilities.h"
using util::foobar;
void func() {
foobar();
}
// bar.h
#include "foo.h"
#include "utilities.h"
using util::florg;
int main() {
florg();
func();
}
Where bar.h uses tools from the header included twice, then you should #include it, even if you don't necessarily have to. On the other hand, if bar.h doesn't need any functions from utilities.h, then even though foo.h includes it, don't #include it.
The header for a source file should define the interface that the users of the code need to use it accurately. It should contain all that they need to use the interface, but nothing extra. If they need the facility provided by xyz.cpp, then all that is required by the user is #include "xyz.h".
How 'xyz.h' provides that functionality is largely up to the implementer of 'xyz.h'. If it requires facilities that can only be specified by including a specific header, then 'xyz.h' should include that other header. If it can avoid including a specific header (by forward definition or any other clean means), it should do so.
In the example, my coding would probably depend on whether the 'foo.h' header was under the control of the same project as the 'blah.h' header. If so, then I probably would not make the explicit second include; if not, I might include it. However, the statements above should be forcing me to say "yes, include 'foo.h' just in case".
In my defense, I believe the C++ standard allows the inclusion of any one of the C++ headers to include others - as required by the implementation; this could be regarded as similar. The problem is that if you include just 'bar.h' and yet use features from 'blah.h', then when 'bar.h' is modified because its code no longer needs 'blah.h', then the user's code that used to compile (by accident) now fails.
However, if the user was accessing 'blah.h' facilities directly, then the user should have included 'blah.h' directly. The revised interface to the code in 'bar.h' does not need 'blah.h' any more, so any code that was using just the interface to 'bar.h' should be fine still. But if the code was using 'blah.h' too, then it should have been including it directly.
I suspect the Law of Demeter also should be considered - or could be viewed as influencing this. Basically, 'bar.h' should include the headers that are needed to make it work, whether directly or indirectly - and the consumers of 'bar.h' should not need to worry much about it.
To answer the last question: clearly, headers needed by the implementation but not needed by the interface should only be included in the implementation source code and absolutely not in the header. What the implementation uses is irrelevant to the user and compilation efficiency and information hiding both demand that the header only expose the minimum necessary information to the users of the header.
Including everything upfront in headers in C++ can cause compile times to explode
Better to encapsulate and forward declare as much as possible. The forward declarations provide enough hints to what is required to use the class. Its quite acceptable to have standard includes in there though (especially templates as they cannot be forward declared).
My comments might not be a direct answer to your question but useful.
IOD/IOP encourages that put less headers in INTERFACE headers as possible, the main benefits to do so:
less dependencies;
smaller link-time symbols scope;
faster compiling;
smaller size of final executables if header contains static C-style function definitions etc.
per IOD/IOP, should interfaces only be put in .h/.hxx headers. include headers in your .c/.cpp instead.
My Rules for header files are:
Rule #1
In the header file only #include class's that are members or base classes of your class.
If your class has pointers or references used forward declarations.
--Plop.h
#include "Base.h"
#include "Stop.h"
#include <string>
class Plat;
class Clone;
class Plop: public Base
{
int x;
Stop stop;
Plat& plat;
Clone* clone;
std::string name;
};
Caviat: If your define members in the header file (example template) then you may need to include Plat and Clone (but only do so if absolutely required).
Rule #2
In the source put header files from most specific to least specific order.
But don't include anything you do not explicitly need too.
So in this case you will inlcude:
Plop.h (The most specific).
Clone.h/Plat.h (Used directly in the class)
C++ header files (Clone and Plat may depend on these)
C header files
The argument here is that if Clone.h needs map (and Plop needs map) and you put the C++ header files closer to the top of the list then you hide the fact that Clone.h needs map thus you may not add it inside Clone.h.
Rule #3
Always use header guards
#ifndef <NAMESPACE1>_<NAMESPACE2>_<CLASSNAME>_H
#define <NAMESPACE1>_<NAMESPACE2>_<CLASSNAME>_H
// Stuff
#endif
PS: I am not suggesting using multiple nested namespaces. I am just demonstrating how I do if I do it. I normal put everything (apart from main) in a namespace. Nesting would depend on situation.
Rule #4
Avoid the using declaration.
Except for the current scope of the class I am working on:
-- Stop.h
#ifndef THORSANVIL_XXXXX_STOP_H
#define THORSANVIL_XXXXX_STOP_H
namespace ThorsAnvil
{
namespace XXXXX
{
class Stop
{
};
} // end namespace XXXX
} // end namespace ThorsAnvil
#endif
-- Stop.cpp
#include "Stop.h"
using namespace ThorsAnvil:XXXXX;