"undefined reference" error in a very very simple c++ program

Question

I have a simple program, which I copied exactly from the example in http://www.learncpp.com/cpp-tutorial/19-header-files/ because I'm learning how to make c++ programs with multiple files.

The program compiles but when building, the following error appears:

/tmp/ccm92rdR.o: In function main: main.cpp:(.text+0x1a): undefined reference to `add(int, int)' collect2: ld returned 1 exit status

Here's the code:

main.cpp

#include <iostream>
#include "add.h" // this brings in the declaration for add()

int main()
{
    using namespace std;
    cout << "The sum of 3 and 4 is " << add(3, 4) << endl;
    return 0;
}

add.h

#ifndef ADD_H
#define ADD_H

int add(int x, int y); // function prototype for add.h

#endif

add.cpp

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

Does anyone knows why this happens?

Thank you very much.

The code is almost perfect. Include add.h in add.cpp. Compile it as g++ main.cpp add.cpp and it will produce a.out — Arun
– Arun, Commented Apr 4, 2013 at 2:54

wallyk · Accepted Answer · 2021-12-10 21:12:09Z

5

The code is almost perfect.

Add a line #include "add.h" in add.cpp.

Compile the files together as g++ main.cpp add.cpp and it will produce an executablea.out

You can run the executable as ./a.out and it will produce the output "The sum of 3 and 4 is 7" (without the quotes)

edited Dec 10, 2021 at 21:12

wallyk

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answered Apr 4, 2013 at 3:03

Arun

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3 Comments

onavarro Over a year ago

Thank you, Arun Saha, it worked perfectly. By the way, will I always have to compile together all the files I have in my program or is there an easier way?

Arun Over a year ago

You are welcome! You can compile each .cpp files separately into .o files respectively and finally link all the .o files to a single executable. Steps are: 1) g++ -c main.cpp 2) g++ -c add.cpp 3) g++ main.o add.o. However, once this quickly becomes tedious. In such situation, a tool is used to manage the compilation and linking of files. The tool is called make (en.wikipedia.org/wiki/Make_(software)) It works based on a configuration file, typically named Makefile, which you have to provide. You can search for and start with a simple Makefile

onavarro Over a year ago

Thanks a lot! I was searching other websites for a simple straight answer but wasn't finding anything useful. I'll try that tool. :)

L30nardo SV. · Accepted Answer · 2013-04-08 16:28:00Z

0

Undefined references may happen when having many .c or .cpp sources and some of them is not compiled.

One good "step-by-step" explanation on how to do it is here

edited Apr 8, 2013 at 16:28

answered Apr 8, 2013 at 11:44

L30nardo SV.

3531 gold badge3 silver badges14 bronze badges

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