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
Related
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
There are already many options listed of how to add a path to a C++ compiler so that the #include <...> command works on these paths. However, assume that I have a single file (not an entire project) and I want to add an include path just for this file alone. I'd like to do this via a line of code within the cpp-file (say, as very first line). How is this possible? Why? Because I need to include some header file from another directory which in turn depends also on other header files within that same directory (and I get error messages that these other files could not be found due to the fact that this path was not added to the include-list).
For example:
Let's assume I want to include file_a.h in directory
.../include/extra,
I can do that via
#include <extra/file_a.h>
However, if, e.g., I do not have the extra-directory directly as a sub-directory of include, or the file_a wants to include some other file from somewhere else (maybe even /extra, but it's not a sub directory of include e.g.), then I run into trouble, because then the tracking of directories/dependencies gets hard.
I thought it would be a bad habit to change those directories via compiler, so I thought better solution would be to integrate it into the program, so no matter which compiler I use, it would work anyway without even having to think about afterwards, once specified, which directories I have to add.
Per my understanding you did:
#include <absolute/path/to/header/header.h
or
#include <relative/path/to/header/header.h
But into the header.h some other include are included.
#include <header_1.h>
#include <header_2.h>
[...]
#include <header_n.h>
Those other headers haven't relative/absolute path, so compiler doesn't know how to find them.
To solve this you can use (using gcc) the -I compiler option:
-I dir
Add the directory dir to the list of directories to be searched for header files during preprocessing. [...]
Emphasis mine
So you can use
#include <header.h>
In your file and compile it using
gcc ... -I/path/to/headers ...
When you have to specify one or more include paths in the compile command, you can do it as follows:
g++ -I/path1/to/headers -I/path2/to/headers YourProgram.cpp
Include paths tell the compiler where it finds the files it actually shall include into other files. This is (usually) controlled via compiler options as LPs explained in this answer.
C++ standard does not provide any facilities to do this from within a C++ source file and I am not aware either of any compiler extension of any vendor that would allow doing so, so bad luck...
Now depending on the IDE you are using (hopefully you are using one...), though, you most likely can add include paths for files individually there (would be a strange IDE if it didn't allow...), e. g. with eclipse + GCC, right click the file, select "Properties" -> C/C++ Build -> Tool Settings -> GCC C++ Compiler -> Includes.
Alternatively you could use a make file instead (actually, eclipse in standard settings generates one for you automatically...), which again allows you to set compiler options for each file individually - either written directly by yourself or generated by some other tools facilitating this such as cmake.
I'm using a library written by others which is kinda based on C for C++ usage -I think-. All inclusions used inside the headers or source files are in the form <> instead of "" even though they are not standard library files. My compiler doesn't recognize them and returns error "file not found"
An example of the problem is inside the following header:
#ifndef _ga_ga_h_
#define _ga_ga_h_
// Make sure that we get the configuration into each of the galib components
// that will be used.
#include <ga/gaconfig.h>
// These are the headers for all of the genetic algorithm classes.
#include <ga/GASimpleGA.h>
#include <ga/GASStateGA.h>
#include <ga/GAIncGA.h>
#include <ga/GADemeGA.h>
#include <ga/GADCrowdingGA.h>
// Here we include the headers for all of the various genome types.
#include <ga/GA1DBinStrGenome.h>
#include <ga/GA2DBinStrGenome.h>
#include <ga/GA3DBinStrGenome.h>
#include <ga/GABin2DecGenome.h>
I include that header inside my program using #include "ga.h" but it is very hard to change inside every header/source file in the library.
Is there a way to make the compiler use <> as if they were ""?
I tried adding the paths to "Addition include directories"from Project properties (I'm using Visual Studio), Many inclusions' errors disappeared but around 30 persisted. The strange thing is that they are in a file called "c1xx" but i don't have that file!!
thanks,
The definition is somewhat that <> is used for "system" header files, usually found in locations like /usr/include (on Unix-like systems) and "" is used for local header files. When you compile your code, you can indicate the location of additional directories containing header files e.g. using the -I option when using GCC. Check your compiler's documentation for the setting needed
So, e.g. on Linux and GCC, if your "ga" directory is in /usr/local/include/ga, you would use cc -I /usr/local/include.
This indeed looks like a problem of telling the compiler where to look for the included header files. As mentioned in the other answers, when you do a #include <header.h>, header.h must be in one of the include search paths - either the system includes, or the additional paths that you are telling the compiler to look for headers. In Linux/g++ (as mentioned in the other answers here), you do that by passing in the the additional search paths in the -I flag. The compilation command would look something like:
g++ -I/additional/header/search/path -o a.out your_file.cpp
Since you are using visual studio and the MSVC compiler, the equivalent would be the /I flag, and the compilation command would look something like:
CL /I\additional\header\path your_file.cpp
I am assuming you are using Visual Studio - you can also configure that from the project properties. Go to Configuration Properties > C/C++ > General and modify Additional Include Directories. Refer these for more info:
Setting C++ compiler and build properties
Additional include directories
First about the difference between <header> and <file>. The standards (both C and C++) only specifies that
#include <header>
includes an header named header and that
#include "file"
includes a source file named file, if none is found an header named file is included instead.
What are headers and how they differ from source files is left to the implementation. In practice they are files as well. So #include <header> is looking for a file in some places, #include "file" is looking for a file in some other places and if not found at the same places as for #include <file>.
AFAIK all compilers
are able to search for source files in the directory of the file containing the include directive
are able to be given a list of directories to search for headers before their default search path.
ISTR that Visual C++ is also searching for a source file in the directories of files indirectly including it. (I can't confirm currently if my memory is good; that behavior is AFAIK unachievable with other compilers so I never relied on it and -- by luck? -- it never resulted in a different behavior for my programs).
Obviously that behavior is more or less customizable. For instance with g++ is possible to:
disable the search of a source file in the directory of the file containing the include directive (with -I-, note that -I- is deprecated since gcc 4.0 -- 2005 -- when -iquote has been introduced and there is no non-deprecated way to achieve this)
to add a list of directories to search for source files and not for headers (with -iquote and it's an effect of -I- as well)
to give a list of directories to search for headers after the default list of directories (with -idirafter)
to give a list of directories to search for headers which are processed specially (with -isystem; less warnings are given for constructs in those files which help when using the "treat warnings as errors" flags, they aren't considered as dependencies with -MM and -MMD which usually is a nuisance)
Now for your problem. The library has been visibly designed to be used by adding the directory containing the directory ga to the include path. That should be enough as I'm unaware of any compiler modifying its search path for headers depending on how the file including the header has been included.
Note that c1xx is probably the name of the compiler executable, not the name of a file trying to include another (again, I'm not in position to ensure that's the case now, but compare with cc1plus which is the name of the compiler for GCC -- g++ is a driver handling several things and executing cc1plus to handle the compilation of C++ code)
If you do on the command line:
echo | gcc -v -E -x c++ -
You will get an output with the default include directories for C++. Those are the built in system's include search paths.
If you compile with g++ -I/some/dir -o foo foo.cpp, you are adding an additional include search path (/some/dir) to the compilation.
Headers in the above locations can be found by include directives like #include <header>. #include "header" directives can also find headers in those locations, but they are more relevant for the following case.
When you do #include "header", your compiler will first try to find "header" relative to the directory of foo.cpp if it includes it, despite foo.cpp directory being in search paths or not. If it doesn't find it there, it will try to look up in the include search paths. So this one is more relevant for headers that are more tied to a specific .cpp file and you don't want additional include search paths added to the compilation, or if you prefer to use include directives with relative paths.
So if you use #include <header>, header must be in some of the include search paths, system or /some/dir from -I flag. If header is relative to foo.cpp, but not in search paths, compilation will fail.
If you use #include "header" and header is not in any of the include search paths, it can still be found relative to foo.cpp location.
For example, if I had a file called foo.h, can I always just include it by doing:
#include "foo.h"
or do I sometimes have to do something like:
#include "bar/foobar/foo.h"
EDIT - are they simply used to make the compile time shorter by limiting the search for the file?
Basically, you can pass include paths as options to the C++ compiler, but it won't look for the file recursively. If you pass in the path /opt/include and do "#include "foo.h", it will _ only_ find /opt/include/foo.h, not /opt/include/dummy/foo.h.
In other words, either you pass every possible include path on the command line, or pass the root in and "navigate" by using the #include "dummy/foo.h"
Edit: #MatthieuM. made another good point below, #include "mylibrary/api.h"makes it much clearer which include file you're using than just #include "api.h", especially if you're using multiple libraries.
The compiler isn't going to recursively search your source directory. If you want to use #include "foo.h", you'll need to compile with -Ibar/foobar to tell the compiler it should look in bar/foobar for header files.
Otherwise, you'll have to use the full (relative) path. I tend to prefer this over -I, because it makes the source more independent from compiler flags (which you'll appreciate when changing between build systems).
There may be cases where there are multiple header files with the same name - in this case, a path helps to disambiguate.
I think it depends of your project settings, for example you have section like "Additional Include Directories" in Visual C++ so IDE will also search files in these directories.
The first statement #include "foo.h" will search for foo.h in the current working directory or in the directories mentioned under additional include directories.
If your header file is in some other path you need to explicitly mention it.
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!)