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Jul 28th, 2008

C++ Header Help

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Hey, I'm just starting to learn C++ and I need some help with a header file. I'm using a Dummies Guide book and it says that I need to create a header file with the extension .h. SO I created a source file called header.h and put this at the top of the main:

Quote ...

#include "header.h"

But when I compiled it I got an error message stating that all the functions and variables that were set in header.h were unknown to the main. So is there a problem with the way that I created/linked the header file?

This book really doesn't explain how to create header files, it just tells you to do so. And all the other C++ books I could find were outdated, with a copyright of 1993 (even older than me). I would really appreciate some help. Thanks.

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DanDaMan

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Jul 28th, 2008

Re: C++ Header Help

Post the header file and the .cpp file. There's no specific special thing you need to do to create a header file. What you said could be correct, but it's a bit vague.

CoolGamer48

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Jul 28th, 2008

Re: C++ Header Help

Some reading examples here, here and here.

Aia

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Jul 28th, 2008

Re: C++ Header Help

Click to Expand / Collapse Quote originally posted by CoolGamer48 ...

Post the header file and the .cpp file. There's no specific
special thing you need to do to create a header file. What you said could be correct, but it's a bit vague.

main.cpp

C++ Syntax (Toggle Plain Text)
#include <cstdlib>
#include <iostream>
#include "header.h"
using namespace std;
int main(int argc, char *argv[])
{
    variable = 20;
    function();
    system("PAUSE");
    return 0;
}
#include <cstdlib>
#include <iostream>
#include "header.h"
using namespace std;
int main(int argc, char *argv[])
{
    variable = 20;
    function();
    system("PAUSE");
    return 0;
}

header.h

C++ Syntax (Toggle Plain Text)
extern int variable;
void function();
extern int variable;
void function();

Last edited by Ancient Dragon; Jul 28th, 2008 at 2:00 am. Reason: replace quote tags with code tags

DanDaMan

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Jul 28th, 2008

Re: C++ Header Help

You have prototyped the void function(); in the header file, but nowhere you have defined function(). How does the compiler know what it does?

Last edited by Aia; Jul 28th, 2008 at 1:46 am.

Aia

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Jul 28th, 2008

Re: C++ Header Help

Actually that's not the problem. The function was specified in another source file called proto.cpp. Anyway, I was playing around with the source files and somehow figured out how to make it work. But now there's a problem. I keep getting an error message and my console needs to close. Can you please help me out with this?

main.cpp

C++ Syntax (Toggle Plain Text)
#include <cstdlib>
#include <iostream>
#include "header.h"
using namespace std;
int main(int argc, char *argv[])
{
    variable = 20;
    function();
    system("PAUSE");
    return 0;
}
#include <cstdlib>
#include <iostream>
#include "header.h"
using namespace std;
int main(int argc, char *argv[])
{
    variable = 20;
    function();
    system("PAUSE");
    return 0;
}

proto.cpp

C++ Syntax (Toggle Plain Text)
#include <cstdlib>
#include <iostream>
#include "header.h"
using namespace std;
int variable;
void function()
{
    cout << "Words" << endl;
    cout << endl;
    cout << "variable = " << variable << endl;
}
#include <cstdlib>
#include <iostream>
#include "header.h"
using namespace std;
int variable;
void function()
{
    cout << "Words" << endl;
    cout << endl;
    cout << "variable = " << variable << endl;
}

header.h

C++ Syntax (Toggle Plain Text)
using namespace std;
extern int variable;
void function();
using namespace std;
extern int variable;
void function();

When I run this I get the correct output:

Words

variable = 20

But then I get an error message that says that Project.exe (my file) has encountered a problem and needs to close. I sent the error message, but why is this happening and how can I prevent it?

DanDaMan

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Jul 28th, 2008

Re: C++ Header Help

Hi, i don't have a compiler in front of me.... but the program is small and easy so here is something that should work.

main.cpp

c++ Syntax (Toggle Plain Text)
#include <cstdlib>
 
// #include <iostream> 
//don't include things that you don't use...
 
#include "header.h"
 
//using namespace std; 
//you don't use anything from std namespace
 
int main()
{
    variable = 20;
    function();
    return 0;
}
#include <cstdlib>

// #include <iostream> 
//don't include things that you don't use...

#include "header.h"

//using namespace std; 
//you don't use anything from std namespace

int main()
{
    variable = 20;
    function();
    return 0;
}

proto.cpp

c++ Syntax (Toggle Plain Text)
 
//#include <cstdlib> //you don't need cstdlib here...
 
#include <iostream>
#include "header.h"
 
using namespace std;
 
int variable;
 
void function()
{
    cout << "Words" << endl;
    cout << endl;
    cout << "variable = " << variable << endl;
}
//#include <cstdlib> //you don't need cstdlib here...

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

using namespace std;

int variable;

void function()
{
    cout << "Words" << endl;
    cout << endl;
    cout << "variable = " << variable << endl;
}

header.h

c++ Syntax (Toggle Plain Text)
#ifndef HEADER_H
#define HEADER_H
 
//using namespace std;
// don't use using directives inside header files...
// also here you don't need it anywhere....
 
extern int variable;
void function();
 
#endif
#ifndef HEADER_H
#define HEADER_H

//using namespace std;
// don't use using directives inside header files...
// also here you don't need it anywhere....

extern int variable;
void function();

#endif

Quote ...

When I run this I get the correct output:

Words

variable = 20

But then I get an error message that says that Project.exe (my file) has encountered a problem and needs to close. I sent the error message, but why is this happening and how can I prevent it?

you shouldn't get an error message.... the code seems correct...{the corrections i made, were minor...and the program shouldn't crash. Try using a debugger....

Also there is a convention when you have a cpp file like mycode.cpp the corresponding header file should be named: mycode.h..

Last edited by n.aggel; Jul 28th, 2008 at 10:20 am.

n.aggel

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Jul 29th, 2008

Re: C++ Header Help

Now I'm really confused. When I don't include system("PAUSE"); then the console screen just pops up and instantly disappears. What are #ifndef and #define, and what's #endif? It works, thanks. But what are these three lines of code?

DanDaMan

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Jul 29th, 2008

Re: C++ Header Help

Click to Expand / Collapse Quote originally posted by DanDaMan ...

Now I'm really confused. When I don't include system("PAUSE"); then the console screen just pops up and instantly disappears. What are #ifndef and #define, and what's #endif? It works, thanks. But what are these three lines of code?

Why should it (stay open)? Unless you ask for input, the program will do all the code you ask it to, outputting everything, and once it's done (either when it reachers return 0; or the end-bracket of main) it will close.

#define 'new symbol' 'previously known symbol' will make 'new symbol' and 'previously known symbol' look the same to the compiler.

#ifndef 'symbol' checks if 'symbol' is #defined. If so, it will execute all the code after it until it reached something that tells it to stop, like #endif

CoolGamer48

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Jul 29th, 2008

Re: C++ Header Help

Click to Expand / Collapse Quote originally posted by DanDaMan ...

Now I'm really confused. When I don't include system("PAUSE"); then the console screen just pops up and instantly disappears.

Look here

Click to Expand / Collapse Quote originally posted by DanDaMan ...

What are #ifndef and #define, and what's #endif? It works, thanks. But what are these three lines of code?

Those are called preprocessor directives.

Last edited by Aia; Jul 29th, 2008 at 10:23 pm.

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