c++ - How to tell g++ compiler where to search for include files?

Question

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++).

Answer 1

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

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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 \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>

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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.