c++, Connecting Hpp and Cpp

Question

/*Header.h file*/

#include <iostream>
using namespace std;

int main(){
    /*Menu*/
    /*case1:

        int x;
        cout << "Input ammount:";
        cin >> x
        sugarcake(x)*/

    /*case2:

       something else

    */
}

/*sugarcake.cpp file*/

#include <iostream>
#include "Header.h"
using namespace std;

void sugarcake(int x) {
    cout << (x * 0.75) << "st egg" << endl;
    cout << (x * 0.75) << "dl sugar" << endl;
    cout << (x * 0.5) << "tsk vanillasugar" << endl;
    cout << (x * 0.5) << "tsk blabla" << endl;
    cout << (x * 0.75) << "dl wheatflour" << endl;
    cout << (x * 18.75) << "gram butter" << endl;
    cout << (x * 0.25) << "dl water" << endl;
}

How do i make this work or have i understood it completely wrong? (Started out with C++ yesterday so please be kind and i haven't been able to find any clear answer that works for me elsewhere)
TL DR: Call the function sugarcake(int x) in sugarcake.cpp inside the Header.h

I suggest that you keep reading the book that you're using to teach yourself c++. — eerorika, Nov 12 '15 at 12:30
There are suggestions here: http://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list — eerorika, Nov 12 '15 at 12:38
Yes, you have missunderstood how headerfiles are supposed to be used - I'd highly recommend to read a cpp tutorial or book. — MikeMB, Nov 12 '15 at 13:41

score 3 · Accepted Answer · answered Nov 12 '15 at 12:35

You should not put code in the header file, instead use two source (.cpp) files, and let the header file only contain a declaration of (in your case) the sugarcake function.

So the header file should look something like

// These first two line (and the last line of the file) is part of
// a header include guard (see https://en.wikipedia.org/wiki/Include_guard)
#ifndef HEADER_H
#define HEADER_H

void sugarcake(int x);

#endif // HEADER_H

The sugarcake.cpp file:

#include <iostream>
#include "header.h"  // Technically we don't need to include the header
                     // file here, but it's always a good idea to do
                     // anyway

void sugarcake(int x) {
    ...
}

And finally the main.cpp file:

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

int main() {
    int x;
    std::cin >> x;
    sugarcake(x);
}

How to build this depends on your environment, but if you're using GCC from the command line then you do like this

$ g++ -Wall -Wextra main.cpp sugarcake.cpp -o sugarcake

The -Wall and -Wextra options are to enable extra warnings, always a good idea. The -o options tells g++ what to name the executable program it creates. The two source files are the files you just created.

Still don't get it, what's the reason to have the header.h file? Doesn't feel like it's doing something :o Bcuz now you have the menu in a main.cpp file wich does everything? Or does the header.h "connects" all the cpp files? [main.cpp] sugarcake(5); --> [Header.h]void sugarcake(->5) --> [sugarcake.cpp] ... 5 — Widdin, Nov 12 '15 at 12:41
The purpose is to separate roles, in a very big program, it will be easier to find functions and read code, because the program is separated in a lot of header files, which each one is doing one thing only. — ranu, Nov 12 '15 at 12:43
@Widdin In a simple case like this you don't really need the header file, you could just have the `sugarcake` declaration in the `main.cpp` file. However, for larger projects putting common declarations, classes, symbolic constants (`const`, `constexpr` or even preprocessor macros) and other declarations that needs to be shared is really needed to avoid copy-pasting a lot of declarations (and copy-paste coding is prone to introduce subtle (or not so subtle) errors). — Some programmer dude, Nov 12 '15 at 12:45

ranu · Answer 2 · 2015-11-12T12:42:02.763

We use header files to describe functions which will be used after in another occasion per say.

So I will show how I organize my functions in the header files:

/* header.h
 * include guards below, it is very import to use it to be sure you'r not
 * including things twice.
 */
#ifndef HEADER_H
#define HEADER_H

// I write function prototypes in the header files. 
int sum(int &a, int &b);

#endif // HEADER_H

The definition of the function is written in the header.cpp

// header.cpp

#include "header.h"

int sum(int &a, int &b) 
{ 
    int total = a + b; 
    return total; 
}

To use that function in main.cpp you just have to include the header.cpp

// main.cpp
#include <iostream>
#include "header.h"

int main(int argc, char *argv[]) 
    {
        int a = 5;
        int b = 4;

        int total = sum(a, b);

        std::cout << "The total sum of the 4 and 5 numbers is: " << total << std::endl;
    }

See how it is made? Then you can compile the cpp files and it will work!

c++, Connecting Hpp and Cpp

2 Answers2