Why are included files passed down

[PRW56]

This is a very basic thing that I am just wondering about. When I #inlcude "string.h" in my header file for a class, why can I also use strings in my .cpp file for the class. Why/How are included files (i think they are files?) passed down to the .cpp file for the class?

Employee header file:

#ifndef EMPLOYEE_H
#define EMPLOYEE_H

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

using namespace std;

class Employee
{
    public:
        Employee(string empName, int id, string dept, string pos);
    protected:
    private:
        string name;
        int idNumber;
        string department;
        string position;
};

#endif // EMPLOYEE_H 

Employee .cpp file:

#include "Employee.h"

//#include "string.h"

using namespace std;

Employee::Employee(string empName, int id, string dept, string pos)
{
    name = empName;
    idNumber = id;
    department = dept;
    position = pos;
}

Also if I use ANY incorrect terminology, pls let me know. TY for any help!

[MrHutch]

Because you're including your header file, which in turn includes the string header.

If you put a coin in your wallet then put your wallet in a rucksack then the coin is still in the rucksack. :-)

[keskiverto]

Preprocessor reads files recursively, i.e. it does not only process the #include directives of Employee.cpp, but of every included files as well.

[nvrmnd]

and #include "string.h" should be #include <string>

[JLBorges]

The #include directive has the effect of reading in the included file and replacing the #include directive with the contents of that file.

So, after preprocessing,

#ifndef EMPLOYEE_H
#define EMPLOYEE_H

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

using namespace std; // *** this is bad

class Employee
{
    public:
        Employee(string empName, int id, string dept, string pos);
    protected:
    private:
        string name;
        int idNumber;
        string department;
        string position;
};

#endif // EMPLOYEE_H  

would look like:

///// contents of the file iostream are preprocessed and inserted here ////////////////

///// contents of file string.h are preprocessed and inserted here ////////////////

using namespace std;

class Employee
{
    public:
        Employee(string empName, int id, string dept, string pos);
    protected:
    private:
        string name;
        int idNumber;
        string department;
        string position;
};

If we now have #include "Employee.h" , the effect is as if the directive was replaced by the preprocessed contents of employee.h
(Into which the preprocessed contents of "string.h" were already inserted.)

[PRW56]

Thanks for the replies, that makes sense, I am used to having to copy over import or using statements in Java or C#, so this is something I will have to get used to.

[Mathhead200]

There is nothing like include in Java. import in Java is like using in C++. Java "includes" the needed class files automatically via the compiler.

// Java

java.io.BufferedReader br = new java.io.BufferedReader(...);
// --- vs. ---
import java.io.BufferedReader;
BufferedReadder br = new BufferedReader(...);

//C++

std::fstream f(...);
// --- vs. ---
using std::fstream;
fstream f(...);