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How do I use a function of another *.cpp file in my main.cpp file.

Pascal666 (7)
I have been searching in the tutorials for a while, but couldnt find anything.

Basicly I have a form on which I want to draw a circle, but how do I use the drawCircle() function in my other class?/How do I declare this class in the other class?

Error:
56 main.cpp `drawCircle' undeclared (first use this function)

Here is the code of the drawCircle class:
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/*  Make the class name into a global variable  */
char szClassName[ ] = "classDraw";

int drawCircle (HWND Wnd,int X,int Y,int R,int Pen,int Fill,HDC DrawHDC)
{
  int a,b=0;
  if (!DrawHDC) 
  {
    DrawHDC=GetDC(Wnd);
    b=1;
  }
  HPEN   hNPen=CreatePen(PS_SOLID,1,Pen);
  HPEN   hOPen=(HPEN)SelectObject(DrawHDC,hNPen);
  HBRUSH hOldBrush;
  HBRUSH hNewBrush;
  if (Fill)
  {
     hNewBrush=CreateSolidBrush(Pen);
     hOldBrush=(HBRUSH)SelectObject(DrawHDC,hNewBrush);
  }
  else
  {
     hNewBrush=(HBRUSH)GetStockObject(NULL_BRUSH);
     hOldBrush=(HBRUSH)SelectObject(DrawHDC,hNewBrush);
  }
  // Win API function
  a = Ellipse(DrawHDC,X-R,Y+R,X+R,Y-R);
  DeleteObject(SelectObject(DrawHDC,hOPen));
  DeleteObject(SelectObject(DrawHDC,hOldBrush));
  if (b) ReleaseDC(Wnd,DrawHDC);
  return a;
}
chrisname (7395)
If you have a function in one .cpp file that you want to use in another .cpp file, you have to prototype it in the .cpp file you want to use it in.

You have two options:
a. Use a header file
b. Use extern

a.
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#ifndef _HEADER_H_
#define _HEADER_H_

return type function name(parameter list) attributes;

#endif

The #ifndef... #define... #endif stuff is called a header guard. It avoids problems that can be brought about by including a file twice, such as multiple definitions of the same function.

A working example would be

main.h
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#ifndef _MAIN_H_
#define _MAIN_H_

#include <stdio.h>

/* Forward declarations */
int  main(int, char**); /* main: entry point */
void die(const char*); /* die: print error message and then die */
#endif


main.c[pp]
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#include "main.h"

int main(int argc, char** argv) {
    if (argc < 2)
        die("Usage: main arg1 [arg2 .. argn]\n"); /* Compiler knows how to resolve die() */

    return 0;
}

void die(const char* s) {
    fprintf(stderr, "%s\n", s);

    exit(1);
}


b.
Is simpler, but with a header if you want to change a function, you only change it once or twice. With extern, you change it in every file that uses it.

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extern return type function name(parameter list) attributes;

Working code using extern:
main.c[pp]
[code]extern void die(const char*);

int main(int argc, char** argv) {
    if (argc < 2)
        die("Usage: main arg1 [arg2 .. argn]\n"); /* Compiler knows how to resolve die() */

    return 0;
}


die.c[pp]
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extern void exit(int);

void die(const char* s) {
    fprintf(stderr, "%s\n", s);

    exit(1);
}[/


"attributes" is optional, e.g. int* return_an_int_pointer(void) const;.

Omitting parameter names in function declarations is optional. Personally I tend to leave them out because then if I decide a name is not expressive enough (e.g. const char* s doesn't tell you want s does) or I change what a parameter is used for, I only have to rename it in one place.

Omitting parameter names in function definitions is not optional. You can't omit them.

A function declaration is where you declare the function's return type, name and parameter list. A definition is where you write the function body.
Pascal666 (7)
Looks like I solved the problem on my own:

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/*  Declare functions  */
LRESULT CALLBACK WindowProcedure (HWND, UINT, WPARAM, LPARAM);
int drawCircle (HWND, int, int, int, int, int, HDC);


Edit:
I just noticed your reply; thank you for the quick answer.
Last edited on
chrisname (7395)
Yeah, that works. You can declare the function in the same file, but it's usually better to use a header file.
Disch (13742)
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