Creating Header Files

Hello everyone

I've been programming C++ casually for about half a year now, and my programs are starting to grow large. I've been monitoring these boards for some time now, and some times i see some of you gurus including and making your own header files for storing away classes and functions.

There's an entry in the FAQ about making headers, but i just can't get it to work properly, it's quite frustrating, here is my example application:

Code:

---

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

using namespace std;

int main()
{
    char sentence1[] = "Why ain't this working!?";
    int length = strlen(sentence1);
    printout(sentence1, length, 100);
}

---

And the "function.h" header:

Code:

---

#ifndef _FUNCTION_H
#define _FUNCTION_H

inline void printout(char sentence[], int length, int interval)
{
    int x;
    for (x=0; x<length; x++)
    {
        cout<< sentence[x];
        Sleep(interval);
    }
}

#endif

---

All i get is a compiler error saying that printout is undeclared, i have tried everything, but to no prevail.

Thanks in advance :)

I think its because you declare the function before main (it comes before main since it is in a header file)

Tropicalia:

Well that doesn't make sense, because if i make a program like this...

Code:

---

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

using namespace std;

inline void printout(char sentence[], int length, int interval)
{
    int x;
    for (x=0; x<length; x++)
    {
        cout<< sentence[x];
        Sleep(interval);
    }
}

int main()
{
    char sentence1[] = "Why ain't this working!?";
    int length = strlen(sentence1);
    printout(sentence1, length, 100);
}

---

...it works just fine, and this one declares 'printout' before the main function.

Its because you must declare the function prototypes

hmm... for one thing, you might need to #include <windows.h> in functions.h

Aside from that, have you tried testing with a simpler example?

Code:

---

#include "function.h"

int main()
{
    printHelloWorld();
}

---

then for function.h:

Code:

---

#ifndef FUNCTION_H
#define FUNCTION_H

#include <iostream>

void printHelloWorld()
{
    std::cout << "Hello world!" << std::endl;
}

#endif

---

I think Tropicalia means this:

Code:

---

#ifndef _FUNCTION_H
#define _FUNCTION_H

inline void printout(char sentence[], int length, int interval);

inline void printout(char sentence[], int length, int interval)
{
    int x;
    for (x=0; x<length; x++)
    {
        cout<< sentence[x];
        Sleep(interval);
    }
}

#endif

---

Just do it and your code will work =]

If you're using g++, then try something like
g++ -E prog.cpp > temp.txt

The temp.txt file will contain the result of all the #include and #if and #define statements. Look towards the end of the file to see if you're actually getting the file included.

Other compilers have similar flags for just running the pre-processor.

> #ifndef _FUNCTION_H
Pick another name. Everything which begins with _ is reserved, so you should avoid using it yourself.

Putting in prototypes like that and expecting it to work is plain silly. There is absolutely no point in having a prototype and immediately defining the function after that. Where prototypes come into play would be having the prototypes in the header file, and then having an implementation file to implement the functions.

I mean put the prototype before main, not in the header, i tested here and worked

Quote:

---

I mean put the prototype before main, not in the header

---

That's not very useful either. The thing is, the OP's code should work, with a minor adjustment. Somehow, perhaps due to project settings, the header file is not getting included properly.

Laserlight:
Well that one works fine, aside from the fact that i had to throw in a pause to stop it from exiting instantaneously. But why is my bigger version not working then? I tried including windows.h, but it isn't helping.

Maxorator:

The program now looks like this:

Code:

---

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

using namespace std;

int main()
{
    char sentence1[] = "Why ain't this working!?";
    int length = strlen(sentence1);
    printout(sentence1, length, 100);
}

---

and the header:

Code:

---

#ifndef _FUNCTION_H
#define _FUNCTION_H

#include <windows.h>

inline void printout(char sentence[], int length, int interval);

inline void printout(char sentence[], int length, int interval)
{
    int x;
    for (x=0; x<length; x++)
    {
        cout<< sentence[x];
        Sleep(interval);
    }
}

#endif

---

and it still is not working. Im just getting a "Printout undeclared(first use of this function)"?

Tropicalia:
I'm afraid not.

I've never implemented a function in a header file, so I don't know what's the problem in that code exactly. Just don't use header files for implementation, only declare them there. (As laserlight said)

Ah, a test compile of your original code reveals another error: you failed to fully qualify the names. Use std::cout instead of cout.

Quote:

---

Originally Posted by Salem

If you're using g++, then try something like
g++ -E prog.cpp > temp.txt

The temp.txt file will contain the result of all the #include and #if and #define statements. Look towards the end of the file to see if you're actually getting the file included.

Other compilers have similar flags for just running the pre-processor.

> #ifndef _FUNCTION_H
Pick another name. Everything which begins with _ is reserved, so you should avoid using it yourself.

---

I'm assuming G++ is a compiler right? Well i'm using Dev-C++ 4.9.9.2, and i think the compiler is named MingW....

Where is this temp.txt file located?

Quote:

---

Originally Posted by laserlight

Ah, a test compile of your original code reveals another error: you failed to fully qualify the names. Use std::cout instead of cout.

---

Everyone was talking about other things, so we never noticed the real error.

Quote:

---

> #ifndef _FUNCTION_H
Pick another name. Everything which begins with _ is reserved, so you should avoid using it yourself.

---

This helped :) Everything is working as intended now, thanks for the help!

Interesting. I cannot duplicate that error in Dev-C++.

Quote:

---

Originally Posted by laserlight

Interesting. I cannot duplicate that error in Dev-C++.

---

Really? Are we using the same version?

Just for clearance:

This is the faulty version:

Code:

---

#ifndef _FUNCTION_H
#define _FUNCTION_H

#include <windows.h>

inline void printout(char sentence[], int length, int interval)
{
    int x;
    for (x=0; x<length; x++)
    {
        cout<< sentence[x];
        Sleep(interval);
    }
}

#endif

---

And the working one:

Code:

---

#ifndef FUNCTION_H
#define FUNCTION_H

#include <windows.h>
#include <iostream>
using namespace std;

inline void printout(char sentence[], int length, int interval)
{
    int x;
    for (x=0; x<length; x++)
    {
        cout<< sentence[x];
        Sleep(interval);
    }
}

#endif

---

Ah, now I get it. It does look like you have given us a working example where failure to comply with the standard gives you a headache.

Quote:

---

Originally Posted by laserlight

Ah, now I get it. It does look like you have given us a working example where failure to comply with the standard gives you a headache.

---

Indeed, as i said, very frustrating...

In general, placing C++ code inside header files ( or .h files ) is bad practice. Header files should only really contain function prototypes and class delcarations. All class and function bodys should be stored in the attached C++ source file (.cpp ) This increases program matienence and makes it clearer your intentions

Quote:

---

Originally Posted by swgh

In general, placing C++ code inside header files ( or .h files ) is bad practice. Header files should only really contain function prototypes and class delcarations. All class and function bodys should be stored in the attached C++ source file (.cpp ) This increases program matienence and makes it clearer your intentions

---

How do i "attach" a .cpp file to a header file?

You just implement the functions whose prototypes are in the header file, and then compile the source files and link them. If you are using some IDE with some kind of project mechanism, this compilation and linking will all be done for you.

>> Interesting. I cannot duplicate that error in Dev-C++.

Seeing how Dev-C++ supposedly isn't a compiler I don't see how any one could.

Quote:

---

Originally Posted by laserlight

You just implement the functions whose prototypes are in the header file, and then compile the source files and link them. If you are using some IDE with some kind of project mechanism, this compilation and linking will all be done for you.

---

Ah, i get it now. Thanks once again.

Just one clarification though. Identifiers starting by an underscore are reserved for the implementation. Not for the language itself. Hence two compilers behaving differently but still complying to the standard.

And there is an important distinction depending on them being followed by an uppercase letter, another underscore or lowercase letter or digit.

If followed by a lowercase letter or a digit, they are meant to be used by the implementation on the global scope only.

If followed by anything else they can be used by the implementation anywhere.

But since these are guidelines to implementation developers, I think it is perhaps a little too pedantic of some to simply deny access to the declaration of any identifier starting with an underscore.

> Seeing how Dev-C++ supposedly isn't a compiler I don't see how any one could.

Seeing you can't write code with a compiler, I don't see how you could test it otherwise

This is really giving me a headache!

I now separated the function prototype and the body of the function into a header and a .cpp file.

First time around, it worked beautifully, but now im getting all kinds of compiler warnings.

Here are the files:

Main

Code:

---

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

using namespace std;

int main()
{
    char sentence1[] = "Still ain't working!";
    int length = strlen(sentence1);
    printout(sentence1, length, 100);
    std::cin.ignore();
}

---

Header

Code:

---

#ifndef PRINTOUT_H
#define PRINTOUT_H

inline void printout(char sentence[], int length, int interval)

#endif

---

"attached" source file

Code:

---

#include <windows.h>
#include <iostream>

inline void printout(char sentence[], int length, int interval)
{
      int i;
      for (i = 0, i < length, i++)
      std::cout<< sentence[i];
      Sleep(interval);
      }
}

---

expected init-declarator before "using"
expected `,' or `;' before "using"
`printout' undeclared (first use this function)

I just wrote the source files with Dev-C++ in the same project as main and the headers, and as i said, it worked perfectly, but now it just won't compile. I even tried closing Dev-C++ and rewriting the entire application.

Btw: If i want to inline a function, do i have to inline it in the prototype or in the actual declaration? Or both perhaps?

The function cpp file should also include the "printout.h". Otherwise you you include printout.h on your main cpp file, the function definition will not exist.

Also you have one extraneous closing brace in your function cpp file

> Btw: If i want to inline a function, do i have to inline it in the prototype or in the actual declaration? Or both perhaps?

inline functions should actually be defined in the header file not on a cpp file.