I found the article Where does Visual Studio look for C++ header files? to be a good start, but I have further questions. The general order that VS looks for include files are (1) local directory, (2) those specified with /I, and (3) those specified by the environment (INCLUDE env var or VC++ settings).
Q1. I think that the /X option turns off (3). Right? Or does it also turn off (1)?
Q2. If I have a nested include file (main.c includes inc1.h, which includes inc2.h), where the first included file is found in one of the /I folders, does VS look for the second included file starting in that same /I folder, or just the local folder of the original source file? VS2008 seems to be operating the first way, but I'd like to find it documented somewhere.
The /X option will not disable looking for files in the local directory (assuming you are including them like "this.h" rather than <this.h>). You can easily test this by creating a file
#include "foo.h"
int main() {}
creating an empty foo.h, and compiling using the /X flag.
For Q2, my test with VC2010 showed it behaves the same as VC2008. What I did was had a main:
#include "inc1.h"
int main() {}
with a folder inc that contained an inc1.h that just
#include "inc2.h"
with two different inc2.h files; on in inc and one in the directory with my source file. The one in my source directory was empty, the one in inc had an #error directive. In general, you don't want to rely on this though. Both the C and C++ standards simply say on #include that "the named source file is searched for in an implementation-defined manner".
Q1. The /X option does turn off the INCLUDE var, but not the local directory (see Q2)
Q2. C and C++ compilers, for nested include files, make the location of an include file the current local directory, so you can't turn it off.
eg.
If your inc.h file is found in one of the /I folders and it tries to include a file like this:
#include "foo/foo.h" then it must use its own local directory first, before all the other /I folders as they all might have a foo folder with a foo.h file and your inc.h file would probably not compile as its just included the wrong header file.
I've just found this msdn page for VS2008 which seems to have a complete explanation.
Related
I am trying to include a few libraries in code blocks, however when I add the path of the .h files to the search directory (example C:\Qt\5.1.1\mingw48_32\include\QtNetwork), it only seems to identify the ones in the main file and I think that is due to the fact that in the main file they are included as such (for example) #include "qtcpsocket.h", whereas in the .h file they are included as (for example) #include <QtNetwork/qabstractsocket.h>.
Apart from the fact that one includes the folder in which it is located, what is the major difference? Why it may not work? and what do it need to do to change it?
one more thing I'm sure the files are in the folder.
Here are a few code snippets if this helps
location of file
error
If your source code contains, e.g.
#include <QtNetwork/qabstractsocket.h>
then you are requesting the preprocessor to find and include a file called
QtNetwork/qabstractsocket.h
(or, QtNetwork\qabstractsocket.h, if you're on Windows, as you are)
in one of its search directories.
And if you have specified compiler search directories:
C:\Qt\5.5.1\mingw48_32\include\QtNetwork
C:\Qt\5.5.1\mingw48_32\include\QtCore
then the preprocessor will search the first directory for:
C:\Qt\5.5.1\mingw48_32\include\QtNetwork\QtNetwork\qabstractsocket.h
which does not exist. And it will search the second directory for:
C:\Qt\5.5.1\mingw48_32\include\QtCore\QtNetwork\qabstractsocket.h
which does not exist either.
The usual way would be to specify the compiler search directory:
C:\Qt\5.5.1\mingw48_32\include
and in your code write your #include <...> directives like:
#include <QtNetwork/...>
#include <QtCore/...>
In a "working directory" I have a lot of *.cpp and *.h files that #include each other and files from subdirectories.
For example:
#include "first.h"
#include "second.h"
#include "dir1/third.h"
#include "dir2/fourth.h"
In my own directory (that is different from the "working" directory) I would like to create a new *.cpp and *.h file that includes one of the files from the "working" directory. For example:
#include "/root/workingdirectory/first.h"
However, it does not work. Because "first.h" might include "second.h" and "second.h" is not located in my directory. Is there a way to tell the compiler that it needs to search included files not in the current but in the working directory: /root/workingdirectory/?
To make it even more complex, dir1 and dir2 are not located in my working directory. They are located in /root/workingdirectory2/. So, my second question is if it is possible to resolve this problem by letting compiler know that subdirectories are located somewhere else?
I also need to add, that I do not use any environment for development and compile from the command line (using g++).
As you've already been told, it's useful to read the manual - specifically this chapter - and even more specifically right here.
Specifically, you want
g++ -I/root/workingdirectory -I/root/workingdirectory2
Note also the documentation on #include directive syntax, described here as:
2.1 Include Syntax
Both user and system header files are included using the preprocessing
directive #include. It has two variants:
#include <file>
This variant is used for system header files. It searches for a file named file in a standard list of system directories. You can prepend directories to this list with the -I
option (see Invocation).
#include "file"
This variant is used for header files of your own program. It searches for a file named file first in the directory
containing the current file, then in the quote directories and then
the same directories used for <file>. You can prepend directories to
the list of quote directories with the -iquote option. The argument of
#include, whether delimited with quote marks or angle brackets,
behaves like a string constant in that comments are not recognized,
and macro names are not expanded. Thus,#include <x/*y> specifies
inclusion of a system header file named x/*y.
However, if backslashes occur within file, they are considered
ordinary text characters, not escape characters. None of the character
escape sequences appropriate to string constants in C are processed.
Thus, #include "x\n\\y" specifies a filename containing three
backslashes. (Some systems interpret \ as a pathname separator. All
of these also interpret / the same way. It is most portable to use
only /.)
It is an error if there is anything (other than comments) on the line
after the file name.
So for example
#include "first.h"
will start looking in the same directory as the .cpp file containing this directive (or take a relative path as relative to this directory).
If you want to use the include path (specified by -I) you should use
#include <dir1/third.h>
Usual practice is to use the #include "local.h" form for headers inside a library/package/module (however you've chosen to organize that), and the #include <external.h> form for headers from external/3rd-party or system libraries.
Read The Fine Manual
It's there for everyone to read. You even have a choice of what to use (I'd go with the first):
-Idir
Add the directory dir to the head of the list of directories to be searched for header files. This can be used to override a system header file, substituting your own version, since these directories are searched before the system header file directories. However, you should not use this option to add directories that contain vendor-supplied system header files (use -isystem for that). If you use more than one -I option, the directories are scanned in left-to-right order; the standard system directories come after.
If a standard system include directory, or a directory specified with -isystem, is also specified with -I, the -I option is ignored. The directory is still searched but as a system directory at its normal position in the system include chain. This is to ensure that GCC's procedure to fix buggy system headers and the ordering for the include_next directive are not inadvertently changed. If you really need to change the search order for system directories, use the -nostdinc and/or -isystem options.
-iquotedir
Add the directory dir to the head of the list of directories to be searched for header files only for the case of #include "file"; they are not searched for #include <file>, otherwise just like -I.
For gcc it is the -I option for header includes. For the .cpp files you just need those to appear as an argument to the gcc command.
Every C/C++ compiler (g++, gcc, MinGW, clang, e.t.c.) has a folder called "include" in it's root path where it automatically looks for header files. If you use MinGW, it would be in, for example: "C:\MinGW\include". Just save your header to the include folder like this: C:\MinGW\include\header1.h or C:\MinGW\include\LibGUI\Window.h
If I simply include a file by writing
#include "blah.h"
where exactly does the compiler search for this file? I understand that there are limitations.
What happens if the file is not in the same folder, but much deeper in the structure, how do I tell the compiler to look there? equally if it is above the file in the directory? or maybe deeper in a different branch?
Essentially I don't have a grasp of how you navigate around the structure. I've seen some includes that look something like:
#include ".././foo/whatever/blah.h"
what do the dots mean? they go back up but do different numbers mean different things?
Also is this based on the structure of the files on the computer or their structure in the solution explorer?
Thanks very much for the help on this one, I understand this is a bit of a basic question - just one of those things I never learned.
From the visual studio documentation on #include
The preprocessor searches for include files in the following order:
In the same directory as the file that contains the #include statement.
In the directories of any previously opened include files in the reverse order in which they were opened. The search starts from the directory of the include file that was opened last and continues through the directory of the include file that was opened first.
Along the path specified by each /I compiler option.
Along the paths specified by the INCLUDE environment variable.
You can change the paths passed to the compiler via /I options in the visual studio project settings (for project specific paths) and in the visual studio options for global paths.
2 dots in a path move you up a directory, one dot refers to the current directory. Any other number of dots would not be valid. So your example path of .././foo/whatever/blah.h essentially means "move up one level, look in a folder foo, then look in a folder whatever". The single dot in this case doesn't really do anything.
This navigation is based on the file structure, not the structure in solution explorer
When you write #include "a.h" in a.cpp, then preprocessor searches a.h in the same directory where a.cpp is. If this search is not supported, or if the search fails, then preprocessor searches a sequence of implementation-defined places for this a.h.
"what do the dots mean?"
Lets say you have file with this fullpath: C:\myDir\myProjects\a\a.cpp:
. = C:\myDir\myProjects\a
.. = C:\myDir\myProjects
You should also take a look at What is the difference between #include <filename> and #include "filename"?
This question already has answers here:
What is the difference between #include <filename> and #include "filename"?
(30 answers)
Closed 8 years ago.
I am new to C++.
What is the difference between including the c++ header files using "" and <>
I am trying to use some of the header files form an open source library. All header files in that library are included using <>. Now when I do the same in my header file, its failing at compile time.
<> looks firstly in the header path for the header file whereas "" looks firstly in the current directory of the file for the header.
The distinction is very largely implementation defined; the "..." form
should look first in the place where the file which includes it is
situated; the <...> no. Beyond that, both look in an implementation
defined list of places, with the additional requirement that if the
compiler doesn't find a "..." form in any of the expected places, it
reprocesses the include as if it were a <...> form.
In practice, all of the compilers I know build a list of places using
the -I or /I options, followed by a number of "standard" and
compiler defined places. This list is for <...>; "..." is searched
in the same directory as the including file, then treated as a <...>.
(Some compilers, at least, also have options to add to the list for
"...".)
I'm not sure what's happening with regards to the library. Normally,
when using a third party library, you have to add one or more -I or
/I options to tell the compiler where to find its headers. Once
you've done that, both your code and the library code should find all of
the necessary headers. The one case I can think of where an include
might work in a library header, and not in your own headers, is a
"..." style include in a library header which was included from
another library header, using a path specifier, e.g.:
LibraryFile1.hpp:
#include "Subdir/LibraryFile2.hpp"
LibraryFile2.hpp:
#include "LibraryFile3.hpp"
You will have told the compiler to look for the headers (using a -I
option) in something like LibraryRoot/include, which is where
LibraryFile1.hpp is located; LibraryFile2.hpp is relative to this
location, and in LibraryFile2.hpp, the compiler finds
LibraryFile3.hpp because it is in the same directory as the file which
includes it. If you try to include LibraryFile3.hpp directly,
however, the compiler won't find it.
File includes between <> are looked for in your compiler's path, whereas "" is looking relatively to your current directory (or absolute if you specify a path that starts with / or c:\ but this is not recommended)
On Unix systems, by default the path contains /usr/include. This path may be completed by adding -Isome_directory for it to search in it.
For example, if you have your file test.c and you want to include include/test.h, you have different choices:
Write #include "include/test.h", which will look relatively from the directory of the compiled file.
Write #include <test.h>, but this time you will need to specify -Iinclude to the compiler to add the ./include directory to the compiler's path.
Note, however, that some compilers accept the "" notation for lookups in the path, but that always confused me and is a bad thing to do.
The quotes mean include from local folder and the <> mean to include from another directory specified using a flag to g++ or MSVC or whatever compiler you are using or system headers.
<> looks in the default directory for include files, "" looks in the current directory and than in the default directory
This question is a duplicate of Question 21593. None of the above answers above are totally correct. Like many programmers, I have used the informal convention of using the "myApp.hpp" form for application specific files, and the form for library and compiler system files, i.e. files specified in /I and the INCLUDE environment variable. However, the C standard states that the search order is implementation specific.
Here's the msdn explanation copied here for your convenience).
Quoted form
The preprocessor searches for include files in this order:
1. In the same directory as the file that contains the #include statement.
2. In the directories of the currently opened include files, in the reverse order in which
they were opened. The search begins in the directory of the parent include file and
continues upward through the directories of any grandparent include files.
3. Along the path that's specified by each /I compiler option.
4. Along the paths that are specified by the INCLUDE environment variable.
Angle-bracket form
The preprocessor searches for include files in this order:
1. Along the path that's specified by each /I compiler option.
2. When compiling occurs on the command line, along the paths that are specified by the INCLUDE
environment variable.
Including a file using <> will tell the compiler to look for those files in environment-defined inclusion folders. Those folders can be standard system folders, or defined by your Makefile if you use one, etc. Using "", the compiler will look for inclusion files only in the source file's path.
So you can use "" and use absolute paths or the path which is relative to the source file you try to include in, or you can use <> after defining your inclusion folders, and just specify the name of the header file to include.
IMHO, the second option is better, especially if you use a lot of headers, or multiple libraries, etc...
To define inclusion folders at compilation time : gcc -I ... (man gcc!)
I'm studying C++ right now, coming from a background in Python, and I'm having some trouble understanding how C++ handles multiple source files. In Python, the import statement first checks the current working directory for the module you're trying to import and then it checks the directories in sys.path. In C++, where would I place a custom made .h file? Where would the compiler even look?
For example, I've got a program, foo.exe compiled from a single source file, foo.cpp, both in the same directory. I decide that I want to organize things a little better, so I create a new .h file, bar.h and dump stuff in there. Would I just need to #include to get to the stuff I put there? What if I want to use bar.h with another program (in a completely different directory)?
There are two include variants:
#include "path-spec"
#include <path-spec>
Quote notation:
This form instructs the preprocessor to look for include files in the same directory of the file that contains the #include statement, and then in the directories of any files that include (#include) that file.
The bracket notation looks for header files in certain defined locations.
With gcc you can get some information about these pathes via:
$ echo | gcc -v -x c++ -E -
Compilers accept
-I or /I
options to add additional pathes.
It (generally) looks in the include path if you use #include <foo>, else it uses relative paths if you use #include "../../foo/bar.h".
You set the include path with -I or /I on most compilers. Consult its manual for details.
Don't define any objects in headers though -- you will have multiple definition errors at link time if you do (and include the header in multiple source files).
It works in a similar way. The #include only is used by the compiler. In execution time the file bar.h doesn't gets used. But in compile time it is.
In compile time, the file could be in two places:
1.- The current directory (as in python)
2.- The directories configured in your include path. Where to configure that directories depends of the compiler you are using. Most of them let you define the include directories in the compile command line. And most IDEs let you configure it in some Options menu.
Hope it helps.
If your header files are in the same dir, you can include them like:
#include "bar.h"
If you want to include this header from another dir:
#include "../foobar/bar.h"
Basically quotations mean to search from current directory and brackets like in #include <abc.h> mean to search in standard header file directories. You can add custom directories to the standard search path by adding a -I /path/to/your/custom/headers in the compile command.
#include with angle brackets looks in the system include directories. Like this:
#include <iostream>
With double quotes it looks in the current directory and other directories given to the compiler to search.
#include "foo.h"
g++ -I../include foo.cpp
the thing you are missing in the python model is the linker.
in python you import code and its interpreted right there and then. in c/c++ you compile each source file into an object file. You then tell the linker to collect a bunch of object files into an executable
Typically the includes in c/c++ source files only contain descriptions of whast in the other C files (the names of functions, etc) not the function contents. This is enough for the compiler to compile a given file. Then the linker will combine your object files with libraries of 'well known' functions and make an executable