C / C++ / MFC

// inc.h
#ifndef __INC_H__
#define __INC_H__
int x;
#endif

// src1.cpp
#include "inc.h"

// src2.cpp
#include "inc.h"

1. The preprocessor processes the file src1.cpp file. It will replace the #include-line with the contents of inc.h. The net effect of this would be a single line in src1.cpp, declaring an int variable with name x. No problems so far.
   
2. The preprocessor sends the processed code to the compiler, which sees the variable declaration. No problems so far. An object code file is produced: src1.obj
   
3. The preprocessor processes the file src2.cpp. As with src1.cpp, its contents will be the same (because it includes the same include file). No problems so far.
   
4. Same thing here as in 2). The compiler sees the variable declaration. Since src1.cpp and src2.cpp are compiled independently, there is no name collision. The variable declared in src1.cpp is not seen by the compiler in src2.cpp. No problems here either. src2.obj is produced.
   
5. All source files have been compiled, so it's time for the linker. The linker examines all object files before it attempts to join them into an .exe-file. What it looks for is to make sure that names are not colliding. In this case, the linker will see that both src1.obj and src2.obj declares a variable with the name x. That is an error, because x is no longer unique - which memory location does x refer to? It can be one of two - and that is something the linker does not like! That is also the error you had in your project.
   

// inc.h
#ifndef __INC_H__
#define __INC_H__
extern int x; // Notice the extern keyword
#endif // __INC_H__

// src1.cpp
#include "inc.h"
int x; // this is the actual declaration of the variable

// src2.cpp
#include "inc.h"

#ifndef _MAPHANDLING_H_<br />
#define _MAPHANDLING_H_<br />
<br />
#include "Includes.h" // C++ and SDL headers<br />
<br />
namespace MapHandling<br />
{<br />
    struct TileFile<br />
    { <struct goes here> };<br />
<br />
	extern TileFile Tilesets[1];<br />
	extern void LoadTiles();<br />
};<br />
<br />
#endif // _MAPHANDLING_H_<br />

#include "MapHandling.h"<br />
<br />
MapHandling::TileFile MapHandling::Tilesets[1];<br />
<br />
void MapHandling::LoadTiles()<br />
{ <func goes here> }<br />

Jörgen Sigvardsson wrote:

Good luck with your project!

Windows Calculator told me I will die at 28.

Jörgen Sigvardsson wrote:

That'll work.

1>MapHandling.obj : error LNK2005: "void __cdecl MapHandling::LoadTiles(void)" (?LoadTiles@MapHandling@@YAXXZ) already defined in Game.obj<br />
1>MapHandling.obj : error LNK2005: "struct MapHandling::TileFile * MapHandling::Tilesets" (?Tilesets@MapHandling@@3PAUTileFile@1@A) already defined in Game.obj<br />
1>Main.obj : error LNK2005: "void __cdecl MapHandling::LoadTiles(void)" (?LoadTiles@MapHandling@@YAXXZ) already defined in Game.obj<br />
1>Main.obj : error LNK2005: "struct MapHandling::TileFile * MapHandling::Tilesets" (?Tilesets@MapHandling@@3PAUTileFile@1@A) already defined in Game.obj<br />
1>..\Release/War Game.exe : fatal error LNK1169: one or more multiply defined symbols found

ThatsAlok wrote:

try rebuild all!

Jörgen Sigvardsson wrote:

I'm curious though. Why did you declare an array of only one element?

Windows Calculator told me I will die at 28.

// Includes.h: C++ and SDL headers.<br />
<br />
// C++<br />
#include <iostream><br />
#include <string><br />
#include <stdlib.h><br />
#include <time.h><br />
#include <vector><br />
using namespace std;<br />
<br />
// SDL<br />
#include "SDL.h"<br />
#include "SDL_image.h"<br />

Windows Calculator told me I will die at 28.

bloodwinner wrote:

what is the difference if I had not use it?

void Draw(CDC& dc)
{
    // Create a pen
    CPen pen;    
    pen.CreatePen(PS_SOLID, 2, RGB(255,0,0));
    // Select the pen into a DC
    dc.SelectObject(&pen);
    // Draw
    ...
    // LEAK!  Pen still selected into DC - remember that when a CPen goes out of scope,
    // the underlying pen HANDLE is automatically deleted.
}

void Draw(CDC& dc)
{
    // Save the current DC state
    int nState = dc.SaveDC();
    // Create a pen
    CPen pen;    
    pen.CreatePen(PS_SOLID, 2, RGB(255,0,0));
    // Select the pen into a DC
    dc.SelectObject(&pen);
    // Draw
    ...
    // Restore original DC state (pen, font, brush, etc.).
    dc.RestoreDC(nState);
}

Oliver123 wrote:

5. I think CScrollBar* pScrollBar can tell me which slider is active.

Oliver123 wrote:

IDC_SLIDER_VALUE CString m_SliderValue
IDC_SLIDER_VALUE2 CString m_SliderValue2

Oliver123 wrote:

But how do I determine which slider is active? How do I use CScroll* pScrollBar to determine which of the two sliders is sending the message?

void CExoSliderDlg::OnHScroll( UINT uSBCode, UINT uPos, CScrollBar *pScrollBar )
{
    CString strValue;
    strValue.Format("%u", uPos);
    
    if ((CSliderCtrl *) pScrollBar == &m_slider1)
        m_edit1.SetWindowText(strValue);
    else if ((CSliderCtrl *) pScrollBar == &m_slider2)
        m_edit2.SetWindowText(strValue);
    else
        CDialog::OnHScroll(uSBCode, uPos, pScrollBar);
}