Questions about "linkage"

Question

I tried to pass to IDE a code I found in a book and some doubts arose.

Code:

newApplication.cpp

---

#include "stdafx.h"
#include <iostream>
using std::cout;
using std::endl;

#include "GradeBook.h"

int main()
{
    GradeBook gradeBook1("CS101 Introduction to C++ Programming");
    GradeBook gradeBook2("CS102 Data Structures in C++");

    cout << "gradeBook1 created for course: " << gradeBook1.getCourseName()
         << "\ngradeBook2 created for course: " << gradeBook2.getCourseName()
         << endl;
    return 0;
}

Gradebook. h

---

#pragma once
#include <string>
using std::string;

class GradeBook
{
public:
    GradeBook(string name);
    void setCourseName(string name);
    string getCourseName();
    void displayMessage();

private:
    string courseName;
};

Gradebook.cpp

---

#include "stdafx.h"
#include <iostream>
using std::cout;
using std::endl;

#include "GradeBook.h"

GradeBook::GradeBook(string name)
{
    setCourseName(name);
}

void GradeBook::setCourseName(string name)
{
    courseName = name;
}

string GradeBook::getCourseName()
{
    return courseName;
}

void GradeBook::displayMessage()
{
    cout << "Welcome to the grade book for\n" << getCourseName() << "!" << endl;
}

1) How the compiler manages to "know" that the implementation of the method is in GradeBook.cpp whereas in the main() only exists the include of GradeBook.h?

2) The application only worked after I included #include "stdafx.h" in the archive GradeBook.cpp. Why does this happen?

Visual Studio 2017 folder structure

-Dependencias Externas
-Arquivos de Cabeçalho
  *GradeBook.cpp
  *GradeBook.h
  *resource.h
  *stdafx.h
  *targetver.h
-Arquivos de Origem
 *newApplication.cpp
 *stdafx.cpp
-Arquivos de Recurso
 *newApplication.rc

Answer 1

1) How the compiler can "know" that the method implementation is in GradeBook.cpp whereas in the main() only exists the include of GradeBook.h?

He doesn’t know. He doesn’t need to know. The implementation just needs to be there. At the time of compiling you say everything you want it to be compiled. And at the time of link He takes everything that’s available and puts it all together. At that time he sees if everything he needs has been made available, no matter what order, where it comes from, nothing, except if it is there. If it is not there is an error of linkage.

Once compiled generates a code and is available to anyone who wants to use it. And can even nobody use it. Although under normal conditions what is not used is not linked along.

2) The application only worked after I included #include "stdafx.h" in the archive GradeBook.cpp Why does this happen?

Has been answered: What is "stdafx. h" and its importance?.