C++ beginner: Why doesn't the compiler find my functions defined in header file?

Question

I´m very new to C++ and so, I haven´t got any experience with HeaderFiles. I have the following files (main.cpp):

#include <iostream>
#include "test_file.h"

int main(){
   std::cout << max(3,4) << std::endl;
   return 0;
}

It´s a simple main-function which calls a function from my test_file.h which just includes a prototype: (test_file.h):

#ifndef TEST_FILE_H
#define TEST_FILE_H

int max(int a, int b);

#endif

And here´s my fitting source-file(test_file.cpp) which just includes the final function:

#include "test_file.h"

int max(int a, int b){
   return a>b ? a:b;
}

Every time I run this the same error occurs:

undefined reference to `max(int, int)'
collect2.exe: error: ld returned 1 exit status

I just don´t know why this happens. The only thing I figured out, is that all will work fine if I copy the whole function into the test_file.h.

please include the command you use to compile in the question — 463035818_is_not_an_ai, Dec 15 '17 at 15:14
Compiler **does** find the declaration of your function in the header file. What you get is linker error complaining about not finding a definition for it. Did you link both of your .cpp files together? — Algirdas Preidžius, Dec 15 '17 at 15:15
Because you are trying to compile `main.cpp` separately from `test_file.cpp`? — Sergey Kalinichenko, Dec 15 '17 at 15:15
@JavaProgrammer.002 It seems to be a project file/IDE configuration problem. What IDE are you using? — François Andrieux, Dec 15 '17 at 15:16
I don´t really use an IDE. I just use a text-editor(Atom) with the extension to compile via gpp-compiler. — MeineHTMLCodes, Dec 15 '17 at 15:17
In the long run, you want something to link stuff for you. Most IDEs can do that, but you might want to learn how to write qmake files (or something similar) in the long run. Maybe start reading a qmake tutorial, assuming that you have qmake installed. You can create your first qmake file by `qmake -project` if you want to see how they should look. Also, you should really get an IDE. — Aziuth, Dec 15 '17 at 15:17
@JavaProgrammer.002 -- You should learn exactly what is happening when you build your program. You completely miss the fact that the compiler had no trouble compiling your `main.cpp` program. You will see a `main.o` file as a result of the successful compilation. The problem is that the linker can't find the body of the `max` function anywhere. That is in `test_file.cpp`, and that needs to be compiled separately, creating a `test_file.o` file. Then the linker has to be told to link `main.o` and `test_file.o` to give you the final executable. — PaulMcKenzie, Dec 15 '17 at 15:23
I will :D I desired(is this the right word? :D) a C++ book as christmas present :D — MeineHTMLCodes, Dec 15 '17 at 15:36

C++ beginner: Why doesn't the compiler find my functions defined in header file?

0 Answers0