What to #include? - c++

I understand if a.h includes b.h and I don't declare anything from b.h in my header including only a.h is good practice. If I were to declare anything from b.h in my header then I should include both a.h and b.h to make my header self-sufficient.
In my header I do declare both class A from a.h and class B from b.h. However, class A depends on class B, so I always use them together. I never use class B independently of class A. In this case does it still make sense to include both a.h and b.h?
a.h
#include "b.h"
#include <queue>
class A
{
private:
std::queue<B> mFoo;
}
In my actual code I think it makes my intention clearer when I include just my event system for example and not a number of includes that seem superfluous to me.

You should always include direct dependencies: if a depends directly on b and c, even if b already includes c, a should include both.
Be explicit about your dependencies. You should not rely on the fact that some other module already depends on the module you'd have to include. You never know how the dependency tree will change in the future.
In this regard, having implicit dependencies makes your code fragile.
To make sure you don't redefine something by including it twice, you can put header guard inside your header files:
#ifndef GRANDFATHER_H
#define GRANDFATHER_H
struct foo {
int member;
};
#endif /* GRANDFATHER_H */
You could also use
#pragma once
But not every compiler supports it.
That way even if the preprocessor will step on same include more than once, It won't re-include the code.

My rule: Given some random header foo.h, the following should compile clean, and preferably should link and run as well:
#include "foo.h"
int main () {}
Suppose foo.h doesn't contain any syntax errors (i.e., a source file that includes it in the context you intended compiles clean) but the above nonetheless doesn't compile. This almost always means there are some missing #include directives in foo.h.
What this doesn't tell you is whether all of the headers included in foo.h truly are needed. I have a handy little script that checks the above. If it passes, it creates a copy of foo.h and progressively comments out #include directives and sees if those modified versions of foo.h pass the above test. Any that do pass are indicative of suspect superfluous #include directives.
The problem is that the #include directives deemed to be superfluous might not be superfluous. Suppose foo.h defines class Foo, which has data members of type Bar and Baz (not pointers, members). The header foo.h correctly includes both bar.h and baz.h because that is where those two classes are defined. Suppose bar.h happens to include baz.h. As the author of foo.h, I don't care. I should still include both of those headers in foo.h because foo.h has direct dependencies on both of those other headers. My script will complain about suspect superfluous headers in this case. Those complaints are hints, not mandates.
On the other hand, fixing foo.h so that my simple test program above compiles clean is a mandate.

Related

Forward declaration for member pointer with public access

Somewhat similar situation to what was asked here.
I've got a class A that has a member pointer to class B.
//A.h
class B;
class A {
B *b;
public:
B *GetB();
};
B is defined in its own file.
Now, whenever I include A.h and want to access an A's b member, I also have to include B.h. In the case where both A and B have rather large headers (think old nasty legacy code) is it better to continue including both headers whenever I include one or to just have A.h include B.h and be done with it?
The headers are pretty large but most of our code requires both anyway, I'm just curious if there is some kind of design pattern that decides what is the best decision to make in this case.
This is opinion, of course. For me, it boils down to whether it makes sense to use A without B. If A has a ton of operations and only one of them involves B, then no, I wouldn't include B.h. Why should someone who only calls A.Foo() and A.Bar() need to pay the overhead of including an extra header?
On the other hand, if A is a B factory (for example) and you can't imagine anyone using it and not using B too, then maybe it makes sense to include B.h in A.h.
And if A had a member variable of type B (not B*) with the consequence that anyone who included A.h would have to include B.h too in order to compile, then I would definitely include it in A.h.
wrap your header files with preprocessor to make sure they will be included only 1 time..
on B.h define
#ifndef __B_HEADER__
#define __B_HEADER__
.... B header files goes here....
#endif
then on A.h define
#ifndef __A_HEADER__
#define __A_HEADER__
#include <B.h>
.... A header files goes here....
#endif
and then include only A.h when it needs.
i personally prefer to include header files of what i know and want to use - i don't want to be bothered by the dependendcy tree of the components i use.
think when you include <iostream> in C++ STD library - do you really want to know and include explicitly all the <iostrem> dependencies (if there are)?

C++ : Best place to include header files

Let's say we have four files: a.h, a.cpp, b1.h, and b2.h. And, we need to include b1.h and b2.h in either a.h or a.cpp. Where should I include b1 and b2? Let's say only a.cpp needs b1 and b2.
If it's not needed for the header file then include it in the cpp.
This reduces compilation time and helps to understand the dependencies between the modules.
A general rule is that you should avoid including a header inside headers that do not use definitions from it.
If b1.h and/or b2.h have definitions like struct or typedef, etc. that are actually used in a.h (in a function prototype as parameters or return type, for example), then you should include them in the top of the header.
Otherwise, if b1.h/b2.h only provide definitions that are used internally to a.cpp (private members, etc), then include it at the top of a.cpp.
You should try to only include in a file what is actually needed for the compiler to understand that file. (Unlike windows, with the monstrosity that is <Windows.h>.)
Only include what is needed in the header.
Do the class definitions and function declarations of a.h require b1.h or b2.h?
Then include what is needed.
Otherwise only include in the .cpp.
Just remember, each time you include a file your compilation takes that much longer.
Here are a couple hints of when things are needed:
Return values or parameters do not need to be included.
For example std::string blahFunc(std::string a); does not need <string> included in the header file(still needs a forward declaration though)
Pointer types do not need to be included, they just need to be forward declared. For example randomType * f(); does not need to include randomType's header in it's header. All you have to do is forward declare with class randomType;
References can also just be forward declared.
I think the include directives should always be in your .h** files. The only include you should put in the a.cpp file should be
#include "a.hpp"
To understand why, suppose Bob uses your code inside his program, and your code is not disclosed (i.e., you provide it only as a library). Then all he can see are your headers. If you include everything you need in your headers, he will still be able to check what are the dependencies of your code and make sure he has everything needed by your code. If you put the include directives in the .c** files, then unless your code is open source (i.e. he has access to the .c** files) he won't be able to see what are the packages he must make sure to have installed.

#import in header file or implementation file

Some have the habit of adding header file imports/includes to the header file. Some on the other hand, write a forward declaration in the header file and write the actual #include or #import lines in the implementation file.
Is there a standard practice for this? Which is better and why?
Given X.h and X.c, if you #include everything from X.h then clients of "X" that #include <X.h> will also include all those headers, even though some may only be needed in X.c.
X.h should include only what's needed to parse X.h. It should assume no other headers will have been included by the translation unit to ensure that reordering inclusions won't break a client. X.c should include any extras needed for implementation.
This minimises recompilation dependencies. You don't want a change only to the implementation to affect the header and hence trigger a client recompilation. You should simply include directly from X.c.
Including forward references instead of headers is necessary when classes have shallow dependencies. For example:
A.h
#include "B.h"
class A {
B* pointer;
};
B.h
#include "A.h"
class B {
A* pointer;
};
Will break at compilation.
A.h
class B;
class A {
B* pointer;
};
B.h
class A;
class B {
A* pointer;
};
Will work as each class only needs to know the other class exists at the declaration.
I write my imports in header files, so that every implementation file has only one inclusion directive. This also has the advantage of hiding dependencies from the user of the module's code.
However, that same hiding has a disadvantage: your module's user may be importing all kinds of other headers included in your header that he may not need at all. From that point of view, it's better to have the inclusion directives in the implementation file, even if it means manually resolving dependencies, because it leads to lighter code.
I don't think there's a single answer. Considering the reasons I gave, I prefer the first approach, I think it leads to cleaner code (albeit heavier and possibly with unnecessary imports).
I don't remember who I'm quoting (and thus the phrase is not exact), but I always remember reading: "programs are written for human beings to read, and ocasionally for computers to execute". I don't particularly care if there are a few kilobytes of code the user of my module won't need, as long as he can cleanly, easily import it and use it with a single directive.
Again, I think it's a matter of taste, unless there's something I failed to consider. Comments are more than welcome in that case!
Cheers.

C++ Headers - Best practice when including

C++ headers
If I have A.cpp and A.h as well as b.h, c.h, d.h
Should I do:
in A.h:
#include "b.h"
#include "c.h"
#include "d.h"
in A.cpp:
#include "A.h"
or
in A.cpp:
#include "A.h"
#include "b.h"
#include "c.h"
#include "d.h"
Are there performance issues? Obvious benefits? Is something bad about this?
You should only include what is necessary to compile; adding unnecessary includes will hurt your compilation times, especially in large projects.
Each header file should be able to compile cleanly on its own -- that is, if you have a source file that includes only that header, it should compile without errors. The header file should include no more than is necessary for that.
Try to use forward declarations as much as possible. If you're using a class, but the header file only deals with pointers/references to objects of that class, then there's no need to include the definition of the class -- just use a forward declaration:
class SomeClass;
// Can now use pointers/references to SomeClass
// without needing the full definition
The key practice here is having around each foo.h file a guard such as:
#ifndef _FOO_H
#define _FOO_H
...rest of the .h file...
#endif
This prevents multiple-inclusions, with loops and all such attendant horrors. Once you do ensure every include file is thus guarded, the specifics are less important.
I like one guiding principle Adam expresses: make sure that, if a source file just includes a.h, it won't inevitably get errors due to a.h assuming other files have been included before it -- e.g. if a.h requires b.h to be included before, it can and should just include b.h itself (the guards will make that a noop if b.h was already included previously)
Vice versa, a source file should include the headers from which it is requiring something (macros, declarations, etc), not assume that other headers just come in magically because it has included some.
If you're using classes by value, alas, you need all the gory details of the class in some .h you include. But for some uses via references or pointers, just a bare class sic; will suffice. E.g., all other things being equal, if class a can get away with a pointer to an instance of class b (i.e. a member class b *my_bp; rather than a member class b *my_b;) the coupling between the include files can be made weaker (reducing much recompilation) -- e.g. b.h could have little more than class b; while all the gory details are in b_impl.h which is included only by the headers that really need it...
What Adam Rosenfield told you is spot on. An example of when you can use a forward declaration is:
#ifndef A_H_
#define A_H_
#include "D.h"
class B; //forward declaration
class C; //forward declaration
class A
{
B *m_pb; //you can forward declare this one bacause it's a pointer and the compilier doesn't need to know the size of object B at this point in the code. include B.h in the cpp file.
C &m_rc; //you can also forware declare this one for the same reason, except it's a reference.
D m_d; //you cannot forward declare this one because the complier need to calc the size of object D.
};
#endif
Answer: Let A.h include b.h, c.h and d.h only if it's needed to make the build succeed. The rule of thumb is: only include as much code in a header file as necessary. Anything which is not immediately needed in A.h should be included by A.cpp.
Reasoning: The less code is included into header files, the less likely you will need to recompile the code which uses the header file after doing some change somewhere. If there are lots of #include references between different header files, changing any of them will require rebuilding all other files which include the changed header file - recursivel. So if you decide to touch some toplevel header file, this might rebuild huge parts of your code.
By using forward declarations wherever possible in your header files, you reduce the coupling of the source files and thus make the build faster. Such forward declarations can be used in many more situations than you might think. As a rule of thumb, you only need the header file t.h (which defines a type T) if you
Declare a member variable of type T (note, this does not include declaring a pointer-to-T).
Write some inline functions which access members of an object of type T.
You do not need to include the declaration of T if your header file just
Declares constructors/functions which take references or pointers to a T object.
Declares functions which return a T object by pointer, reference or value.
Declares member variables which are references or pointers to a T object.
Consider this class declaration; which of the include files for A, B and C do you really need to include?:
class MyClass
{
public:
MyClass( const A &a );
void set( const B &b );
void set( const B *b );
B getB();
C getC();
private:
B *m_b;
C m_c;
};
You just need the include file for the type C, because of the member variable m_c. You can often remove this requirement as well by not declaring your member variables directly but using an opaque pointer to hide all the member variables in a private structure, so that they don't show up in the header file anymore.
Prefer not including headers in other headers - it slows down compilation and leas to circular reference
There would be no performance issues, it's all done at compile time, and your headers should be set up so they can't be included more than once.
I don't know if there's a standard way to do it, but I prefer to include all the headers I need in source files, and include them in the headers if something in the header itself needs it (for example, a typedef from a different header)

#include header style

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;