Includes from file system

Question

I got a problem that I couldn't find an answer to, even though a similar question has been asked here before.

So, in my project, I have the following source code structure:

src
|-main
||-Main.cpp
||-Simulation.h
||-Simulation.cpp
|-inputInterpreter
||-InputInterpreter.h
||-InputInterpreter.cpp
|-utils
||-Utils.h
||-Utils.cpp

In my Main.cpp I have the following source code:

#include <iostream>
#include "../inputInterpreter/InputInterpreter.h"
#include "../utils/Utils.h"

int main() {
    std::cout << "Welcome. For help, type 'help'" << std::endl;
    std::string in;
    in = Utils::getInput();
    while (not Utils::startsWith("quit", in)) {
        InputInterpreter::interpretInput(in);
        in = Utils::getInput();
    };
}

and I get the following error message when I try to compile it with g++:

/cygdrive/c/Users/Acer/AppData/Local/Temp/ccM3znYC.o:Main.cpp:(.text+0x4d): undefined reference to `Utils::getInput()'
/cygdrive/c/Users/Acer/AppData/Local/Temp/ccM3znYC.o:Main.cpp:(.text+0xb6): undefined reference to `Utils::startsWith(std::string, std::string)'
/cygdrive/c/Users/Acer/AppData/Local/Temp/ccM3znYC.o:Main.cpp:(.text+0x104): undefined reference to `InputInterpreter::interpretInput(std::string)'
/cygdrive/c/Users/Acer/AppData/Local/Temp/ccM3znYC.o:Main.cpp:(.text+0x128): undefined reference to `Utils::getInput()'
collect2: error: ld returned 1 exit status

What could be the error? I already included each .h in every source file (as seen above).

Thank you everyone.