How to include two header files that are contained in the same folder?

[Eclair_de_XII]

TL;DR Summary

Let's say I have a directory called "headers". Contained in that directory are two files, "header_one.h" and "header_two.h". The former defines an "add" function that the latter uses in order to define a "subtract" function. Let's say we have a source file that is one level below "headers", "sample.c" that #include's "header_two.h". How would I compile the program without gcc complaining about not being able to find "headers/header_one.h"?

int add(int x, int y){
    return x+y;
}

#include "headers/header_one.h"

int subtract(int x,int y){
    return add(x,-y);
}

//#include "headers/header_one.h"
#include "headers/header_two.h"

int main(void){
    return 0;
}

If I run this, the compiler tells me that "headers/header_one.h" is not found. I don't understand why this is. Is it assuming that I'm #include-ing "header_one.h" from "headers/header_two.h", instead of from "sample.c"?

The reason I bring this up is because I am having great difficulty linking a C++ library to my programs. The problem presented above is one of the workarounds I came up with. But the true problem is as follows:

"""
I just installed a C++ library, but I'm having trouble linking it to a sample program using the GNU C Compiler. The command I'm using in order to compile my program is "g++ main.cc -L './installed/x64_linux/include' -l 'cpr/cpr'; ". But the compiler doesn't seem to be able to compile the sample program that I copy-pasted from the official website of the library.
This would be the script that I used in order to download the package.

git clone https://github.com/Microsoft/vcpkg.git
cd vcpkg
./bootstrap-vcpkg.sh
./vcpkg integrate install
./vcpkg install cpr

This would be the program that I'm trying to compile from the directory "./vcpkg".

#include <cpr/cpr.h>
//#include "cpr/cpr.h" // Complains about a missing "cpr/api.h" file

int main(int argc, char** argv) {
    cpr::Response r = cpr::Get(cpr::Url{"https://api.github.com/repos/libcpr/cpr/contributors"},
                      cpr::Authentication{"user", "pass", cpr::AuthMode::BASIC},
                      cpr::Parameters{{"anon", "true"}, {"key", "value"}});
    r.status_code;                  // 200
    r.header["content-type"];       // application/json; charset=utf-8
    r.text;                         // JSON text string
}

// https://docs.libcpr.org/

"""

 

[Jarvis323]

The command I'm using in order to compile my program is "g++ main.cc -L './installed/x64_linux/include' -l 'cpr/cpr';

-L is for specifying a location to look for library files in. Usually your libraries are in a folder called lib or lib64 and header files are in a folder called include. To specify the location of your header files, use -<capital i> <path where your headers folder is>.

e.g. it should look something like this:

g++ main.cc -L ./installed/x64_linux/lib/cpr -lcpr -l ./installed/x64_linux/include

Where the first -l is a lowercase L and the second is an upper case i.

This assumes you have your libaray file in ./installed/x64_linux/lib/cpr.

 

[Eclair_de_XII]

Thank you. Your post was most informative. However, for some odd reason, my "ld" program keeps saying that it cannot find the "cpr/cpr" library. I am well-aware that this is not a technical forum for me to ask questions about a specific library, so I think it is reasonable to not ask further about my technical difficulties, which I would be able to resolve if only I would learn C++ properly.

 

[Eclair_de_XII]

Thanks for the help. I found out that I had to link all the dependencies of the "cpr" library/header, in addition to "cpr" itself.

g++ main.cc -L ./installed/x64_linux/lib/ -I ./installed/x64_linux/include/ -lcpr -lcurl -lssl -lcrypto -lz -lpthread

It's unwieldy to have to include all these libraries in order to use just the one. I think there should be a better way. I was just linking the libraries as indicated in the library directory, and skimming through the error messages I received in order to obtain a functioning compilation command. Trial-and-error isn't exactly a good technique in this field I should think, but in my exhaustion, I was getting pretty desperate.

 

[.Scott]

On a second note:
Function declarations are commonly placed in header files (*.h). so:

int add(int x, int y);

#include "headers/header_one.h"
int subtract(int x,int y);

Then the function definitions are placed in a c file:

#include "headers/header_one.h"
#include "headers/header_two.h"

int add(int x, int y)
{
    return x+y;
}
int subtract(int x,int y)
{
    return add(x,-y);
}

One notable exception to this rule are "inline" functions. But I'm guessing you haven't encountered any of those yet.

On a third note:
Your header_two.h includes header_one.h - a common occurrence in c programming.
But it means that had math.c included both of those, header_two.h would have been included twice.
In general, including a header file twice is inviting trouble, so this is how that is handled:

#ifndef _HEADER_TWO_INCLUDED_
#define _HEADER_TWO_INCLUDED_

#include "headers/header_one.h"
int subtract(int x,int y);

#endif // _HEADER_TWO_INCLUDED_

Or, if your compiler supports the #pragma once directive:

#pragma once
#include "headers/header_one.h"
int subtract(int x,int y);