Header should be included once - c++

In header files I've seen two main ways for defining macro to avoid including the file more than once.
1.
#ifndef SOME_CLASS
#define SOME_CLASS
//code ...
#endif
2.
#ifndef SOME_CLASS
//code...
#define SOME_CLASS
#endif
Which is more preferable and why?

I prefer the first method, because it doesn't matter what happens after the ifndef because it will be defined straight after.

The first option is commonly optimized by compilers to behave like the non-standard #pragma once.
It is also safer in case of recursive includes. If, in the //code... part, you include another header which includes .... which includes the header you're currently editing, then the second version won't work.

I'd go for the first one.
Reason: If you ever want to change the guard name (say, SOME_CLASS to SOMECLASS), you don't have to scroll all the way down to the end of file to change it too.

The best option is to use #pragma once. With #define you must be very careful when using multiple libraries as the guard name may not be unique.

I prefer the first option. Suppose you include more files, and these files in turn include the file containing #ifndef SOME_CLASS.
I think it's fairly easy to spot include errors, if the #define SOME_CLASS isn't adjacent to #ifndef SOME_CLASS.
// SomeClass.h
#ifndef SOME_CLASS
#include "OtherFile.h" // will eventually lead to #include "SomeClass.h"
#define SOME_CLASS
... boat load of code here...
#endif // SOME_CLASS

Related

Can/should I type whatever I want after #ifndef?

Example:
#ifndef HEADER_h
#define HEADER_h
#endif
Instead of HEADER_h, can I do the following?
#ifndef HEADER
or
#ifndef LIBRARY
or
#ifndef SOMETHING
or
#ifndef ANOTHERTHING
etc.
Header guards are just a convention, a "trick", making use of preprocessor conditions. In using a header guard you are creating a macro with a name, and checking whether that macro was already defined.
There is nothing magical about this macro that binds it to the filename of a header, and as such you can call it whatever you want (within reason).
That doesn't mean that you should write #ifndef URGLEBURGLE, though. You want the name to be useful and unique, otherwise there's not much point.
Typically something like #ifndef [PROJECTNAME]_[FILENAME]_INCLUDED is a good idea.
Yes, you can name the include guard symbol whatever you want, but bear in mind that they are supposed to be unique across headers. You definitely don't want a header
// first.h
#ifndef NON_UNIQUE_H
#define NON_UNIQUE_H
void foo();
#endif
and another one
// second.h
#ifndef NON_UNIQUE_H
#define NON_UNIQUE_H
void bar();
#endif
When you include both in one translation unit, one will "win" and its declarations will be visible, e.g.
// main.cpp
#include "first.h" // now, NON_UNIQUE_H is defined
#include "second.h" // NON_UNIQUE_H already there, doesn't do anything
int main(int, char**)
{
bar(); // error, won't compile, bar() isn't declared
}
Besides the necessity to circumvent such scenarios, it's best to stick to some convention throughout your project. One classical way of doing it is to convert the header file base name to upper case and append _H. If you have header files with the same base name in different directories, you can include the directory name, e.g. SUBDIR_FOO_H and OTHERSUBDIR_FOO_H. But this is up to you.
You can use a construction like
#if !defined(HEADER) || !defined(LIBRARY)
At your question, you are using
#ifndef HEADER_h
#define HEADER_h
#endif
It's the same as "#pragma once"
And yes, you can use different names of defines. In your case, LIBRARY, SOMETHING, HEADER_h - defines, that you can set in code(#define MY_VAR_NAME) or via compiler options(flag -DMY_VAR_NAME).
Your example is a so-called header guard that allows us to ensure the contents of the header are included only once. However, that is not the only use of #ifndef.You can use #ifndef for conditional compilation as in
#ifndef NO_DEBUG
do_some_debug_stuff();
#endif
So it is not only for header guards, but in general you have to carefully choose the name of the symbols you are introducing to prevent they are clashing with symbols defined elsewhere. It is just that header guards are so common that certain conventions exist (eg using FOLDER_FILENAME_H is usually sufficient to ensure uniqueness). And you need to be aware that certain names are reserved (eg starting with two underscores or underscore followed by capital letter).

How to avoid long compilation time for #define in common header

I was wondering if there is an elegant way to solve this problem. Suppose there's a common header eg
// common.h
#ifndef COMMON_H
#define COMMON_H
#define ENABLE_SOMETHING
//#define ENABLE_SOMETHING_ELSE
#define ENABLE_WHATEVER
// many others
#endif
Now this file is included by, let's say 100 other header files and the various #define are used to enable or disable some parts of code which are confined to just 1-2 files.
Everytime a single #define is changed the whole project seems to be rebuilt (I'm working on Xcode 5.1), which makes sense as it must be literally replaced all around the code and the compiler can't know a priori where it's used.
I'm trying to find a better way to manage this, to avoid long compilation times, as these defines are indeed changed many times. Splitting each define in their corresponding file/files could be a solution but I'd like the practical way to have everything packed together.
So I was wondering if there is a pattern which is usually used to solve this problem, I was thinking about having
// common.h
class Enables
{
static const bool feature;
};
// common..cpp
bool Enables::feature = false;
Will this be semantically equivalent when compiling optimized binary? (eg. code inside false enables will totally disappear).
You have two distinct problems here:
Splitting each define in their corresponding file/files could be a solution but I'd like the practical way to have everything packed together.
This is your first problem. If I undestand correctly, if you have more than one functional area, you are not interested in having to include a header for each of them (but a single header for everything).
Apply these steps:
do split the code by functionality, into different headers; Each header should contain (at most) what was enabled by a single #define FEATURESET (and be completely agnostic to the existence of the FEATURESET macro).
ensure each header is only compiled once (add #pragma once at the beginning of each feature header file)
add a convenience header file that performs #if or #ifdef based on your defined features, and includes the feature files as required:
// parsers.h
// this shouldn't be here: #pragma once
#ifdef PARSEQUUX_SAFE
#include <QuuxSafe.h>
#elif defined PARSEQUUX_FAST
#include <QuuxFast.h>
#else
#include <QuuxSafe.h>
#endif
// eventually configure static/global class factory here
// see explanation below for mentions of class factory
Client code:
#include <parsers.h> // use default Quux parser
#define PARSEQUUX_SAFE
#include <parsers.h> // use safe (but slower) Quux parser
So I was wondering if there is a pattern which is usually used to solve this problem
This is your second problem.
The canonical way to enable functionality by feature in C++, is to define feature API, in terms of base classes, class factories and programming to a generic interface.
// common.h
#pragma once
#include <Quux.h> // base Quux class
struct QuuxFactory
{
enum QuuxType { Simple, Feathered };
static std::unique_ptr<Quux> CreateQuux(int arg);
static QuuxType type;
};
// common.cpp:
#include <common.h>
#include <SimpleQuux.h> // SimpleQuux: public Quux
#include <FeatheredQuux.h> // FeatheredQuux: public Quux
std::unique_ptr<Quux> QuuxFactory::CreateQuux(int arg)
{
switch(type) {
case Simple:
return std::unique_ptr<Quux>{new SimpleQuux{arg}};
case Feathered:
return std::unique_ptr<Quux>{new FeatheredQuux{arg}};
};
// TODO: handle errors
}
Client code:
// configure behavior:
QuuxFactory::type = QuuxFactory::FeatheredQuux;
// ...
auto quux = QuuxFactory::CreateQuux(10); // creates a FeatheredQuux in this case
This has the following advantages:
it is straightforward and uses no macros
it is reusable
it provides an adequate level of abstraction
it uses no macros (as in "at all")
the actual implementations of the hypothetical Quux functionality are only included in one file (as an implementation detail, compiled only once). You can include common.h wherever you want and it will not include SimpleQuux.h and FeatheredQuux.h at all.
As a generic guideline, you should write your code, such that it requires no macros to run. If you do, you will find that any macros you want to add over it, are trivial to add. If instead you rely on macros from the start to define your API, the code will be unusable (or close to unusable) without them.
There is a way to split defines but still use one central configuration header.
main_config.h (it must not have an include guard or #pragma once, because that would cause strange results if main_config.h is included more than once in one compilation unit):
#ifdef USES_SOMETHING
#include "something_config.h"
#endif
#ifdef USES_WHATEVER
#include "whatever_config.h"
#endif
something_config.h (must not have include guards for the same reason as main_config.h):
#define ENABLE_SOMETHING
All source and header files would #include only main_config.h, but before the include they must declare what part of it would they be referring to:
some_source.cpp:
#define USES_SOMETHING
#include "main_config.h"
some_other_file.h:
#define USES_WHATEVER
#include "main_config.h"

Undefine already included header

I want to check a header in my source file and if its already defined, I want to undefine it.
Is it possible?
#undef is the opposite of #define and actually cancels it.
It's very useful when you need a macro for some task but does not wish to "pollute" the symbol tables
// referencing FOO is an error (not yet defined)
#define FOO(arg_) ....
FOO(A)
FOO(B)
FOO(C)
#undef FOO
// referencing FOO now is an error (not defined)
There is no way to cancel a #include directive.
'including' something in c/c++ essentially means: make the preprocessor copy and paste the #include <file> directly into your source code (or better: the preprocessed version of your source code).
you can not make that copy and paste action undone. you can however '#undef' stuff introduced by that copy and paste action, but you have to do this for every atom you dislike, you just can't #undef the whole file.
to check, if a given header was already included, you have check if something from that header file is defined already. most headers contain so called 'guards' which look like
#ifndef FOO_H
#define FOO_H
/* lots of code */
#endif
for a file called 'foo.h'. you could check for 'FOO_H' like this:
#ifdef FOO_H
/* do your magic */
#endif
example:
foo.h:
#ifndef FOO_H
#define FOO_H
struct Foo { /* ... * };
#endif
bar.h:
#include "foo.h"
#ifdef FOO_H
#undef Foo
#endif
but that can lead to a lot of headaches if you are not aware of what exactly you are doing. IF your real problem is, that your compiler complaints about 'already declared stuff' then you are not guarding your header files against multiple inclusion...
Do you mean you want to ensure that the header file is included once and only once? The standard solution to this is using an include guard.
I.e. surround the contents of your header with
#ifndef MY_HEADER // a unique identifier for each header
#define MY_HEADER
...
#endif
This is guaranteed to work in all platforms. Some compilers also provide a #pragma to achieve the same effect with less hassle, e.g. in Visual C++ you can add
#pragma once
at the start of your header file.
You can't do what you seem to want to do. If you don't want the header included your choices are
don't include it in the first place
conditionally include it or not via the prepocessor e.g.
#ifdef INC_MY_HEADER
#include "myheader.h"
#endif
or assuming the header has header guards just define the header guard before the header is included e.g. from the commend line -DMY_HEADER or similar
If the real problem is that you have different headers defining things with the same name then you probably need to look into namespaces so you don't get clashes
You could stop the header being included - the simplest way would be to remove or comment out the #include directive.
You could also pre-define your guards so that the include is inneffective.
foo.h
#ifndef __FOO_H
#define __FOO_H
/* contents of foo.h */
#endif /* __FOO_H */
foo.cpp
#define __FOO_H
#include "foo.h"
/* contents of foo.cpp */
Using this strategy you could use the preprocessor to select whether to pre-define the guards, or even have inner guards within you header.
If you really need only to have part of the file included, it would be better to split the file into two.

Circularly declaring Preprocessor directives? Or defines before includes?

I'm a bit new to C++, so bear with me. I'm trying to figure out where exactly to place my #defines and #includes in my project. I have something like this:
main.h
#include "other.h"
#define MAX_GROUPS 100
struct Cat
{
int something[MAX_GROUPS];
}
In other.h I also need to use MAX_GROUPS, so do I also define MAX_GROUPS in other.h like this:
other.h
#define MAX_GROUPS 100
struct Other
{
int taco[MAX_GROUPS];
}
The problem is that I'm defining a constant more than one place. I want to keep it all together.
Alternatively, do I reinclude main.h?
other.h
#include "main.h"
struct Other
{
int taco[MAX_GROUPS];
}
The problem here I think its that is creates like a circular dependancy thing. main.h includes other.h which includes main.h which includes other.h which includes etc...
What is the best way to setup the defines and includes for a project so that things sorta cascade down to other included files? Is it common practice to simply do all your defines before your includes?
Minimising circular dependencies is very important in maintaining your project. For an extended discussion, see "Large Scale C++ Software Design" by John Lakos.
To avoid the specific problem you are having, define values in one header file, and include that header file in every file that needs it. To avoid problems with multiple definitions, use include guards:
#ifndef HEADER_THING_H
#define HEADER_THING_H
/* Rest of the header file goes here. */
#endif
That way if it is already included, it is harmless.
Because main.h #includes other.h, it doesn't need to #define MAX_GROUPS again. It'll pick up that definition from the inclusion.
Generally, you should put the defines inside whichever header is relevant -- or where it is primarily used. In this case, you should put MAX_GROUPS inside other.h. By including other.h in main.h, the define will also be picked up (as mentioned by jwismar).
For bigger projects, however, you'd be better off creating a header file containing only your manifest constants (defines) and just include that where needed.
You could also protect your constants against redefinition case by case...
#ifndef MAX_GROUPS
#define MAX_GROUPS 100
#endif

How are circular #includes resolved?

In c lets say we have 2 files
1.h
#include<2.h>
blah blah
and we have
2.h
#include<1.h>
code
How is this resolved??
Typically you protect your include file with an ifndef/define that corresponds to the file name. This doesn't prevent the file from being included again, but it does prevent the contents (inside the ifndef) from being used and triggering the recursive includes again.
#ifndef HEADER_1_h
#define HEADER_1_h
#include "2.h"
/// rest of 1.h
#endif
#ifndef HEADER_2_h
#define HEADER_2_h
#include "1.h"
// rest of 2.h
#endif
Okay, for the sake of completeness I'll begin by quoting tvanfosson's answer:
You should use include guards:
// header1.hpp
#ifndef MYPROJECT_HEADER1_HPP_INCLUDED
#define MYPROJECT_HEADER1_HPP_INCLUDED
/// Put your stuff here
#endif // MYPROJECT_HEADER1_HPP_INCLUDED
However include guards are not meant to solve circular dependencies issues, they are meant to prevent multiple inclusions, which is quite different.
base.h
/ \
header1.h header2.h
\ /
class.cpp
In this case (quite common), you only want base.h to be included once, and that's what include guards give you.
So this will effectively prevents the double inclusion... but you won't be able to compile!!
The problem can be illustrated by trying to reason as the compiler does:
Include "header1.hpp" > this defines the include guard
Include "header2.hpp
Try to include "header1.hpp" but doesn't because the include guard is already defined
Cannot correctly parse "header2.hpp" because the types coming from "header1.hpp" have not been defined yet (since it was skipped)
Back to "header1.hpp", and the types from "header2.hpp" are still missing since they could not be compiled, and thus it fails here too
In the end, you'll left with a big pile of error messages, but at least the compiler does not crash.
The solution is to somehow remove the need for this circular dependency.
Use forward declarations if possible
Split "header1.h" into 2 parts: the part independent from header2 and the other, you should only need to include the former in header2
And THIS will solve the circular dependency (manually) there is no magic going on in the compiler that will do it for you.
Since you posted your question under the c++ tag as well as c, then I am assuming you are using c++. In c++, you can also use the #pragma once compiler directive:
1.h:
#pragma once
#include "2.h"
/// rest of 1.h
2.h:
#pragma once
#include "1.h"
/// rest of 2.h
The result is the same. But there are some notes:
pragma once will usually compile a little faster since it is a higher level mechanism and doesn't happen at preprocessing like include guards
Some compilers have known bugs "dealing" with #pragma once. Though most if not all modern compilers will handle it correctly. For a detailed list see Wikipedia
Edit: I think the directive is also supported in c compilers but have never tried it, and besides, in most c programs I've seen, include guards are the standard (maybe due to compiler limitations handling the pragma once directive?)
Circular inclusions must be eliminated, not "resolved" with include guards (as the accepted answer suggests). Consider this:
1.h:
#ifndef HEADER_1_h
#define HEADER_1_h
#include "2.h"
#define A 1
#define B 2
#endif // HEADER_1_h
2.h:
#ifndef HEADER_2_h
#define HEADER_2_h
#include "1.h"
#if (A == B)
#error Impossible
#endif
#endif // HEADER_2_h
main.c:
#include "1.h"
This will throw the "Impossible" error at compile time, because "2.h" fails to include "1.h" due to include guards, and both A and B become 0. In practice, this leads to hard to track errors which appear and disappear depending on the order in which header files are included.
The right solution here would be to move A and B to "common.h" which then could be included in both "1.h" and "2.h".