C++ Using MACRO without INCLUDE statement - c++

In DirectX Graphics Samples MiniEngine sample, there is an "inline" source file Functions.inl that uses a macro INLINE that is defined in a header in same folder, Common.h .
What mechanism/declaration permits Functions.inl to use INLINE without an #include "Common.h" statement?
My specific issue is that I have created a VS2019 UWP C++ project, and I am importing a subset of this source and cannot compile the copy of Functions.inl without modifying and adding an #include statement.

"Math/Functions.inl" is not a source file. It is not compiled individually. It appears to be included, just like a header is. Let's take a look at how it is used:
// Core/VectorMath.h
#pragma once
#include "Math/Scalar.h"
...
#include "Math/Functions.inl"
Unlike a header, it is not included into the top of the file, but the bottom. So, I guess it could be called a footer. As you may notice, there are headers included before it. Let's take a look inside one:
// Math/Scalar.h
#pragma once
#include "Common.h"
...
Ah. So, "Common.h" is included before "Math/Functions.inl". That is why "Math/Functions.inl" can use INLINE when included into "Core/VectorMath.h".
Essentially, the file depends on a macro without including it directly and thereby it has an invisible dependency to have that header included before it.
This a bad practice in case of header files which are intended to be included by the user of the library. But this file is presumably intended to not be included except through "Core/VectorMath.h", so the the invisible dependency can even be seen as advantageous. Nevertheless, many IDEs / static code analysers will fail to analyse the file correctly, so I would personally still avoid this practice.

The *.inl extension is usually used to indicate that the file is an inline-definition of something defined in a header. Effectively, the *.inl file is generally treated as an inline-equivalent to a *.cpp file.
In the same way that *.cpp files can use symbols #included in the header for that *.cpp file, *.inl files usually have the same assumption.
In this specific example. it appears that Functions.inl is included after a bunch of other headers are included
#include "Math/Scalar.h"
#include "Math/Vector.h"
#include "Math/Quaternion.h"
#include "Math/Matrix3.h"
#include "Math/Transform.h"
#include "Math/Matrix4.h"
#include "Math/Functions.inl"
Which likely transitively pick up the INLINE macro

Related

Preventing multiple #define when including headers

coming from python and am a bit tripped up on what the proper approach to this is.
I am trying to include this library in my project:
https://github.com/nothings/stb/blob/master/stb_image.h
to do so, i have to #define STB_IMAGE_IMPLEMENTATION exactly once before importing the file (as per that file's doc)
This makes sense, where I am confused is, I have CLASS.h/cpp and in .h I define functions that use typedefs from that file, so I have
#define STB_IMAGE_IMPLEMENTATION
#include <stb_image.h>
in that header file, and can't move these lines to .cpp as headers needs the defs for function def, but as soon as another file includes this header, (#ifndef wont help, i believe), that will be defined twice
I have a structure where TOP creates the CLASS above, but parent also creates OTHER, and OTHER needs to include PARENT, which includes CLASS, which triggers the issue (and prevents me from just moving the #define to PARENT) Note the actual class structure is more complex then this, but this idea seems to be a core issue, and I'm looking for the general best practice.
So, is there some way to ensure these #defines are defined before anything else, and done only once? This seems like a fundamental thing but I can't figure it out - What's the best approach?
This code is a library, and doesn't have a defined entry if that matters
Create a cpp file (or whatever extension you are using for your source files) whose sole purpose is to have
#define STB_IMAGE_IMPLEMENTATION
#include <stb_image.h>
and don't forget to include this cpp file into your project so that it is compiled and the result is linked into your program. In all other places where you need something from this library, just include the stb_image.h header as usual.
These "implementation" macros are a "trick" used by some library authors to make "installing" their library easy. The idea is that when a specific macro (chosen by the library authors) is defined before the header of that library is included, some code with the actual implementation will be added. That is why this must be in a source file instead of a header and only in one source file (otherwise you get multiple definitions for the same functions).
You should have the #define STB_IMAGE_IMPLEMENTATION macro definition in exactly one source file that includes <stb_image.h> (a .cpp file, not in a header).
Since you can also only have one source file that defines main(), it is simple to put the #define in the same file as main() (as long as it also includes <stb_image.h>), but it can be used in any other source file if you prefer. You could even create a source file stb_image_imp.cpp that contains just the two lines shown, and link that into your program (or library) too.
All other source files in the project should only include <stb_image.h> without also defining the macro.
#define is a preprocessor directive and doesn't actually get run everytime the header is accessed so you should
't have any problems
if you are using visual studio you can also do #pragma once to only parse the file once stopping anything from happening twice

Comprehension issue: Precompiled headers & include usage

I got a comprehension issue about precompiled headers, and the usage of the #include directive.
So I got my "stdafx.h" here and include there for example vector, iostream and string. The associated "stdafx.cpp" only includes the "stdafx.h", that's clear.
So if I design my own header file that uses for example "code" that's in vector or iostream, I have to include the header file because the compiler doesn't know the declarations at that time. So why are some posts here (include stdafx.h in header or source file?) saying, it's not good to include the "stdafx.h" in other header files even if this file includes the needed declarations of e.g. vectors? So basically it wouldn't matter to include directly a vector or the precompiled header file, both do the same thing.
I know of course, that I don't have to include a header file in another header file if the associated source file includes the needed header file, because the declarations are known at that time. Well, that only works if the header file is included somewhere.
So my question is: Should I avoid including the precompiled header file in any source file and why? And I am a bit confused, because I'm reading contradictory expressions on the web that I shouldn't include anything in header files anyway, or is it O.K. to include in header files?
So what's right now?
This will be a bit of a blanket statement with intent. The typical setup for PCH in a Visual Studio project follows this general design, and is worth reviewing. That said:
Design your header files as if there is no PCH master-header.
Never build include-order dependencies in your headers that you expect the including source files will fulfill prior to your headers.
The PCH master-header notwithstanding (I'll get to that in a moment), always include your custom headers before standard headers in your source files. This makes your custom header is more likely to be properly defined and not reliant on the including source file's previous inclusion of some standard header file.
Always set up appropriate include guards or pragmas to avoid multiple inclusion. They're critical for this to work correctly.
The PCH master-header is not to be included in your header files. When designing your headers, do so with the intent that everything needed (and only that which is needed) by the header to compile is included. If an including source file needs additional includes for its implementation, it can pull them in as needed after your header.
The following is an example of how I would setup a project that uses multiple standard headers in both the .h and .cpp files.
myobject.h
#ifndef MYAPP_MYOBJECT_H
#define MYAPP_MYOBJECT_H
// we're using std::map and std::string here, so..
#include <map>
#include <string>
class MyObject
{
// some code
private:
std::map<std::string, unsigned int> mymap;
};
#endif
Note the above header should compile in whatever .cpp it is included, with or without PCH being used. On to the source file...
myobject.cpp
// apart from myobject.h, we also need some other standard stuff...
#include "myobject.h"
#include <iostream>
#include <fstream>
#include <algorithm>
#include <numeric>
// code, etc...
Note myobject.h does not expect you to include something it relies on. It isn't using <iostream> or <algorithm>, etc. in the header; we're using it here.
That is a typical setup with no PCH. Now we add the PCH master
Adding the PCH Master Header
So how do we set up the PCH master-header to turbo-charge this thing? For the sake of this answer, I'm only dealing with pulling in standard headers and 3rd-party library headers that will not undergo change with the project development. You're not going to be editing <map> or <iostream> (and if you are, get your head examined). Anyway...
See this answer for how a PCH is typically configured in Visual Studio. It shows how one file (usually stdafx.cpp) is responsible for generating the PCH, the rest of your source files then use said-PCH by including stdafx.h.
Decide what goes in the PCH. As a general rule, that is how your PCH should be configured. Put non-volatile stuff in there, and leave the rest for the regular source includes. We're using a number of system headers, and those are going to be our choices for our PCH master.
Ensure each source file participating in the PCH turbo-mode is including the PCH master-header first, as described in the linked answer from (1).
So, first, the PCH master header:
stdafx.h
#ifndef MYAPP_STDAFX_H
#define MYAPP_STDAFX_H
// MS has other stuff here. keep what is needed
#include <algorithm>
#include <numeric>
#include <iostream>
#include <fstream>
#include <map>
#include <string>
#endif
Finally, the source files configured to use this then do this. The minimal change needed is:
UPDATED: myobject.cpp
#include "stdafx.h" // <=== only addition
#include "myobject.h"
#include <iostream>
#include <fstream>
#include <algorithm>
#include <numeric>
// code, etc...
Note I said minimal. In reality, none of those standard headers need appear in the .cpp anymore, as the PCH master is pulling them in. In other words, you can do this:
UPDATED: myobject.cpp
#include "stdafx.h"
#include "myobject.h"
// code, etc...
Whether you choose to or not is up to you. I prefer to keep them. Yes, it can lengthen the preprocessor phase for the source file as it pulls in the headers, runs into the include-guards, and throws everything away until the final #endif. If your platform supports #pragma once (and VS does) that becomes a near no-op.
But make no mistake: The most important part of all of this is the header myobject.h was not changed at all, and does not include, or know about, the PCH master header. It shouldn't have to, and should not be built so it has to.
Precompiled headers are a method to shorten the build time. The idea is that the compiler could "precompile" declarations and definitions in the header and not have to parse them again.
With the speed of todays computers, the precompilation is only significant for huge projects. These are projects with a minimum of over 50k lines of code. The definition of "signification" is usually tens of minutes to build.
There are many issues surrounding Microsoft's stdafx.h. In my experience, the effort and time spent with discovering and resolving the issues, makes this feature more of a hassle for smaller project sizes. I have my build set up so most of the time, I am compiling only a few files; the files that don't change are not compiled. Thus, I don't see any huge impact or benefit to the precompiled header.
When using the precompiled header feature, every .cpp file must begin by including the stdafx.h header. If it does not, a compiler error results. So there is no point in putting the include in some header file. That header file cannot be included unless the stdafx.h has already been included first.

dealing with includes and using headers

I have "Hello World" code that uses function fhi from another hi.cpp file that has it's header.
Correct my if my understanding is wrong according following:
I can do include cpp file like #include "c:\c\hi.cpp" instead of using header without any problems except that fact that it looks more readable in header file.
If I include header like sample in my main program hi.h, must hi.h include hi.cpp, or it is done automatically according the same file name hi. I'm wondering how compiler knows where is function fhi body.
Is it possible to have different names for header and cpp files?
Programm:
#include "stdafx.h"
#include "c:\c\hi.h"
int _tmain(int argc, _TCHAR* argv[])
{
fhi(1);
return 0;
}
hi.h
#include <cstdlib>
#include <iostream>
int var;
int fhi(int f);
hi.cpp
#include <cstdlib>
#include <iostream>
int fhi(int f)
{
return 0;
}
must hi.h include hi.cpp
No. hi.h contains only declarations, that can be other by other .cpp files.
I'm wondering how compiler knows where is function fhi body.
It doesn't. You need to compile all *.cpp files into the object files. In your case, you will have two object files: program.o and hi.o. The linker can now take these two object files, and spit out the executable. References to other functions(in this case the actual definition of fhi(..)) is resolved in this stage.
Also why are you using absolute paths in #includes? It will break when you move the "c" directory around.
What normally happens is that the build system compiles the .cpp files into object files, that then are used to build the main executable. The means to tell this to the build system vary greatly.
One important point is that your hi.cpp must include hi.h. You should also put an include guard in hi.h, to make it safe to be included more than once in a translation unit.
I can do include cpp file like #include "c:\c\hi.cpp" instead of using
header without any problems except that fact that it looks more
readable in header file.
yes, you can do so but it is not recommended, one of the problems is encapsulation; you are not hiding implementation details. readability as you mention is also a concern, a header is easier to read since it clearly shows what methods are public.
If I include header like sample in my main program hi.h, must hi.h
include hi.cpp, or it is done automatically according the same file
name hi. I'm wondering how compiler knows where is function fhi body.
the header needs to be explicitly included in hi.cpp and any .cpp file that use the class defined in the header.
Is it possible to have different names for header and cpp files?
yes but it is not recommended, it makes it more difficult to find things.
as a general rule: think about that other programmers may want to look in your code so you need to structure it so that it is easy to read and understand as well as making it easier for you 2 years down the road to remember where things are.
In Visual Studio all CPP files included in the project will be compiled to produce OBJ files. These OBJ files will be linked together to form the EXE or DLL.
Including files are similar to pasting the contents of the file at that location. The only difference is that this pasting is done by the pre-compiler during compilation.
Finding out where a function body resides is done by the either the compiler if the function is inline or by the linker when the final binary is created.
First, if the header file is in the same directory as the source file including it, you can use just
#include "hi.h"
In other words, you don't have to use a full path. (See e.g. the inclusion of "stdafx.h".)
Second, in your header file you don't need to include other header files, unless you need types from those. In your header file you don't have anything that needed from the header files you include.
Third, you should protect header files header files from being included more than once in the same source file, this can be done with a so called include guard, on in some compiler via a special directive called #pragma once.
Fourth, in your header file you define a global variable var. This variable will then be defined in every source file you include the header file in, which will lead to errors. You need to declare the variable as extern:
extern int var;
Then in one source file you define the variable like you do now.
Fifth, you should never include source files in header file (with some special exceptions that you don't have to think about yet). Instead you add all source files to the project (I assume you are in MS VisualStudio) and it they will all be built and linked together automatically.
Sixth, since you seem to be using VisualC++, then you are probably using something called precompiled headers. This is something the compiler uses to speed up compilation. However, for this to work you have to include "stdafx.h" in all source files. That include actually has to be the first non-comment line in each source file.

should I include a header that is already included via other headers?

I had only just noticed my programs using the string class were compiling without including the <string> header. It turns out that <iostream> includes <ios_base> which in turn includes <string>.
Is this bad practice and should I explicitly include <string>? Even if it's just a case of clarity?
Is it safe to assume this applies to more than just the <string> header? Perhaps this is implementation specific and or does the standard state the <string> header be included via <ios_base> and <iostream>? Ensuring that any respected and widely used implementation will always include <string> providing the the call to <iostream> exists.
You should explicitly include whatever standard library headers you need.
It is not specified which standard library headers are included by other standard library headers, so such details will differ between compilers.
One case where you can rely on a header being included by another header is if a class in one header derives from a class in another. For example, <iostream> has to include <ios_base> because classes defined in <iostream> are derived from classes defined in <ios_base>.
A good practice is to always include the headers for the classes you'll be using in a given source file, regardless of whether you "know" they're included by already-included files.
If, while refactoring your code, you remove the necessity for one of the higher-level included files (iostream, for example), it could become quite painful to determine why your application no longer compiles.
If you add a proper header (with '#pragma once' or the proper #ifndef) more than once, it only adds a little more time to compiling (just to open, parse and through away the header file contents), but nothing too serious while it makes your files more easy to compile, should the circumstances change (i.e. move them to a different project, make a library out of them, e.t.c.)
If you are really concerned about compile time add the same #ifndef before including the header (though I don't recommend it)
i.e.
// header.h
#ifndef _HEADER_H
#define _HEADER_H
int blahblahblah(int);
#endif
// cppfile.cpp
#ifndef _HEADER_H
#include <header.h>
#endif

#include in .h or .c / .cpp?

When coding in either C or C++, where should I have the #include's?
callback.h:
#ifndef _CALLBACK_H_
#define _CALLBACK_H_
#include <sndfile.h>
#include "main.h"
void on_button_apply_clicked(GtkButton* button, struct user_data_s* data);
void on_button_cancel_clicked(GtkButton* button, struct user_data_s* data);
#endif
callback.c:
#include <stdlib.h>
#include <math.h>
#include "config.h"
#include "callback.h"
#include "play.h"
void on_button_apply_clicked(GtkButton* button, struct user_data_s* data) {
gint page;
page = gtk_notebook_get_current_page(GTK_NOTEBOOK(data->notebook));
...
Should all includes be in either the .h or .c / .cpp, or both like I have done here?
Put as much as you can in the .c and as little as possible in the .h. The includes in the .c are only included when that one file is compiled, but the includes for the .h have to be included by every file that uses it.
The only time you should include a header within another .h file is if you need to access a type definition in that header; for example:
#ifndef MY_HEADER_H
#define MY_HEADER_H
#include <stdio.h>
void doStuffWith(FILE *f); // need the definition of FILE from stdio.h
#endif
If header A depends on header B such as the example above, then header A should include header B directly. Do NOT try to order your includes in the .c file to satisfy dependencies (that is, including header B before header A); that is a big ol' pile of heartburn waiting to happen. I mean it. I've been in that movie several times, and it always ended with Tokyo in flames.
Yes, this can result in files being included multiple times, but if they have proper include guards set up to protect against multiple declaration/definition errors, then a few extra seconds of build time isn't worth worrying about. Trying to manage dependencies manually is a pain in the ass.
Of course, you shouldn't be including files where you don't need to.
Put as many includes in your cpp as possible and only the ones that are needed by the hpp file in the hpp. I believe this will help to speed up compilation, as hpp files will be cross-referenced less.
Also consider using forward declarations in your hpp file to further reduce the include dependency chain.
If I #include <callback.h>, I don't want to have to #include lots of other header files to get my code to compile. In callback.h you should include everything needed to compile against it. But nothing more.
Consider whether using forward declarations in your header file (such as class GtkButton;) will suffice, allowing you to reduce the number of #include directives in the header (and, in turn, my compilation time and complexity).
I propose to simply include an All.h in the project that includes all the headers needed, and every other .h file calls All.h and every .c/.cpp file only includes its own header.