C++ Multiple Definitions

Question

I'm trying to build these files but it's giving me a multiple definition error.

main.cpp:

#include "SDL/SDL.h"
#include "Core.h"
#include "GameStates.h"
#include "globals.h"

int main(int argc, char** args) 
{
if(core.Initilization(640, 480, 32, SDL_SWSURFACE) == -1)
{
    SDL_Quit();
}

while(core.desiredstate != core.quit)
{
    ::currentstate->EventHandling();
    ::currentstate->Logic();
    core.ChangeState();
    ::currentstate->Render();
    ::currentstate->Update();
}

SDL_FreeSurface(core.screen);
SDL_Quit();

}

Core.cpp:

#include "Core.h"
#include "GameStates.h"
#include "SDL/SDL.h"
#include "Intro.h"
#include "globals.h"
#include <string>

/* Starts SDL subsystems and sets screen attributes */
bool Core::Initilization(int SCREEN_WIDTH, int SCREEN_HEIGHT, int SCREEN_BPP, int FLAGS) 
{
    //starts SDL subsystems, returns false upon error
if(SDL_Init(SDL_INIT_EVERYTHING) == -1)
{
    return false;
}

//The screen
screen = SDL_SetVideoMode(SCREEN_WIDTH, SCREEN_HEIGHT, SCREEN_BPP, FLAGS);
//Returns false if there was an error
if(screen == NULL)
{
    return false;
}

SDL_WM_SetCaption("Game", NULL);

return true;
}

/* Loads an image and optimizes it */
SDL_Surface* Core::Load(std::string filename)
{
//original loaded image
SDL_Surface* original = SDL_LoadBMP(filename.c_str());
SDL_Surface* optimized = NULL;

if(original != NULL)
{
    //Sets optimized to optimized version of original
    optimized = SDL_DisplayFormat(original);

    SDL_FreeSurface(original);
}

return optimized;
} 

/* Blits surfaces */
void Core::ApplySurface(int x, int y, SDL_Surface* source, SDL_Surface* destination) 
{
//holds the x y coordinates
SDL_Rect location;
location.x = x;
location.y = y;

if(destination != NULL) 
{
    SDL_BlitSurface(source, NULL, destination, &location);
}
}

/* Sets desiredstate to be used in ChangeState(); */
void Core::SetState(int newstate) 
{
if(desiredstate != state_null && desiredstate != quit)
{
    desiredstate = newstate;
}
}

/* Changes the game state */
void Core::ChangeState()
{
    if(desiredstate != state_null && desiredstate != quit)
    {
        //frees old state memory
    delete ::currentstate;

    switch(desiredstate)
    {
        case intro:
            //allocates new state memory
            ::currentstate = new Intro();
        break;
    }

    stateID = desiredstate;
    desiredstate = state_null;
}
}

GameStates.h:

#ifndef GAMESTATES_H
#define GAMESTATES_H

class GameStates 
{
public:
    virtual void EventHandling() = 0;
    virtual void Logic() = 0;
    virtual void Render() = 0;
    virtual void Update() = 0;
};

#endif

Intro.h:

#ifndef INTRO_H
#define INTRO_H
#include "SDL/SDL.h"
#include "GameStates.h"

class Intro : public GameStates
{
    private:
    SDL_Surface* test;
public:
    Intro();
    void EventHandling();
    void Logic();
    void Render();
    void Update();
    ~Intro();
} intro;

#endif

Intro.cpp:

#include "Intro.h"
#include "GameStates.h"
#include "Core.h"
#include "SDL/SDL.h"

Intro::Intro()
{
test = core.Load("test.bmp");
}

void Intro::EventHandling()
{
SDL_Event event;
while(SDL_PollEvent(&event))
{
    switch(event.type)
    {
        case SDL_QUIT:
            core.SetState(core.quit);
        break;
    }
}
}

void Intro::Logic()
{
//to be coded when the program actually builds...
}
void Intro::Render()
{
core.ApplySurface(30, 30, test, core.screen);
}

void Intro::Update()
{
SDL_Flip(core.screen);
}

Intro::~Intro()
{
SDL_FreeSurface(test);
}

globals.h:

#include "GameStates.h"
#include "SDL/SDL.h"

GameStates* currentstate = NULL;

Sorry if the indentation is off; having to put four spaces for it to be seen as a code block messed with it a bit.

Heres the error message:

/tmp/ccWxKsO5.o:(.bss+0x0): multiple definition of `core'
/tmp/cc13Eqmt.o:(.bss+0x0): first defined here
/tmp/ccWxKsO5.o:(.bss+0x20): multiple definition of `currentstate'
/tmp/cc13Eqmt.o:(.bss+0x10): first defined here
/tmp/ccJXxewI.o:(.bss+0x0): multiple definition of `intro'
/tmp/ccWxKsO5.o:(.bss+0x10): first defined here
/tmp/ccJXxewI.o:(.bss+0x10): multiple definition of `core'
/tmp/cc13Eqmt.o:(.bss+0x0): first defined here
collect2: ld returned 1 exit status

Makefile:

OBJS = main.o Intro.o Core.o
CC = g++
DEBUG = -g
CFLAGS = -Wall -c $(DEBUG)
LIBS = -lSDL

game : $(OBJS)
    $(CC) $(CFLAGS) $(OBJS) -o game $(LIBS)

main.o : Core.h GameStates.h globals.h 
    $(CC) $(CFLAGS) main.cpp $(LIBS)

Core.o : Core.h Core.cpp GameStates.h Intro.h globals.h
    $(CC) $(CFLAGS) Core.cpp $(LIBS)

Intro.o : Intro.cpp GameStates.h Core.h
    $(CC) $(CFLAGS) Intro.cpp $(LIBS)

Robᵩ · Answer 1 · 2012-09-07T19:27:13.250

1

For objects shared between multiple translation units, the rule is: there must be exactly one definition, but you may have multiple declarations.

In practice, this means: put "extern Class object;" in your .h file, and "Class object;" in exactly one of your .CPP files.

For intro, for example, change your Intro.h to:

class Intro : public GameStates
{
  ... // whatever 
};
extern Intro intro;

and add this line to Intro.cpp:

Intro intro;

Similarly for currentstate, in globals.h:

extern GameStates* currentstate;

and in one .CPP (it doesn't matter to the compiler which one):

GateStates* currentstate = NULL;

P.s. Your makefile is broken. You pass -c, which means "don't link" to your link step. Try this:

OBJS = main.o Intro.o Core.o
CC = g++
DEBUG = -g
CFLAGS = -Wall $(DEBUG)
LIBS = -lSDL

game : $(OBJS)
    $(CC) $(CFLAGS) $(OBJS) -o game $(LIBS)

main.o : Core.h GameStates.h globals.h 
    $(CC) -c $(CFLAGS) main.cpp

Core.o : Core.h Core.cpp GameStates.h Intro.h globals.h
    $(CC) -c $(CFLAGS) Core.cpp

Intro.o : Intro.cpp GameStates.h Core.h
    $(CC) -c $(CFLAGS) Intro.cpp

edited Sep 07 '12 at 19:27

answered Sep 07 '12 at 18:19

Robᵩ

163,533
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intro isn't an instance of the Intro class (I see how that can be confusing), but instead is a slot in the states enum. What did I do wrong that's giving me the error, also, what's wrong with core? (which is an instance the Core class). – user1602079 Sep 07 '12 at 18:34
In the code you presented, `intro` **is** an instance of the `Intro` class. If that isn't what you intended, then delete the word `intro` after the class definition of `Intro`. – Robᵩ Sep 07 '12 at 19:20
I don't know what is wrong with `core`. I don't see a declaration in the code you've provided. Regardless, you can likely fix it with the advice I've given: "extern Core core;" goes in "core.h"; "Core core;" goes in "core.cpp". – Robᵩ Sep 07 '12 at 19:25
I've tried your makefile, and now it's given me this: `/usr/lib/gcc/x86_64-linux-gnu/4.6/../../../x86_64-linux-gnu/crt1.o: In function `_start': (.text+0x20): undefined reference to `main' collect2: ld returned 1 exit status make: *** [game] Error 1` – user1602079 Sep 07 '12 at 20:03

score 1 · Answer 2 · answered Sep 07 '12 at 18:33

1

The problem isn't your code, it's your build system.

Any sane build system matches the name of the object files to the name of the source files. But you have ccWxKsO5.o and cc13Eqmt.o. What's worse, the build system appears to be trying to link multiple objects generated from the same source (perhaps some were created by an earlier run of the compiler).

tempnam and globbing *.o is not a reasonable way to build C++ programs.

Well, there may be some code problems also. But those will be a thousand times easier to find and fix once the object names in the error messages correlate to source files.

answered Sep 07 '12 at 18:33

Ben Voigt

277,958
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How would you recommend fixing that? Right now I'm using the Geany IDE with the build command "g++ -Wall -o game main.cpp GameStates.cpp Intro.cpp -lSDL" – user1602079 Sep 07 '12 at 18:38
Personally, I'd use GNU Make. The built-in rule for compiling C++ will be perfectly adequate, you'd just need to set `LDFLAGS` to include `-lSDL`, and list the dependencies of `game`, e.g. `game : main.o GameStates.o Intro.o` – Ben Voigt Sep 07 '12 at 18:42
I've written a proper Makefile now (I was using that command out of sheer laziness) and now I get this error: /tmp/cc1Xv630.o: In function `main': main.cpp:(.text+0x2a): undefined reference to `Core::Initilization(int, int, int, int)' main.cpp:(.text+0x82): undefined reference to `Core::ChangeState()' collect2: ld returned 1 exit status – user1602079 Sep 07 '12 at 18:59
@user1602079: Remember to use the `-c` option when compiling. Undefined reference errors should only happen during the link step, when you need to provide all the object files. – Ben Voigt Sep 07 '12 at 19:01
I've added that and now I get this: g++ -Wall -c -g main.o Intro.o Core.o -o game -lSDL g++: warning: main.o: linker input file unused because linking not done g++: warning: Intro.o: linker input file unused because linking not done g++: warning: Core.o: linker input file unused because linking not done Is there something wrong with my Makefile? – user1602079 Sep 07 '12 at 19:13
@user1602079: Well, you added `-c` to the link line, where it doesn't belong. It should only be on the compile lines. They ought to end up being e.g. `g++ -c -o main.o main.cpp`. But that rule should be built into GNU Make. Can you add your Makefile to the question? – Ben Voigt Sep 07 '12 at 19:17
Ah-hah. Don't pass `$(CFLAGS)` on the link line, but `$(LDFLAGS)` instead. (And it would be conventional to use `CXXFLAGS` in the compile commands for C++ code) And no need to pass `$(LIBS)` when compiling, those are used only during link. – Ben Voigt Sep 07 '12 at 19:27

score 0 · Answer 3 · answered Sep 07 '12 at 18:20

In globals.h, you must declare currentstate extern. Then create globals.cpp, with the definition (GameStates* currentstate = NULL;). I couldn't find any reference for intro or core in your code, but it's probably the same problem: you can declare global variables as often as you want as long as you declare them extern, and only define them once per resulting binary, in only one translation unit.

Also, you probably want to add a header guard (#ifndef GLOBALS_H ...) to globals.h, just in case you add anything else in there.

core was an instance of the Core class and intro was part of the state enum also in the Core class. What's wrong with those two? — user1602079, Sep 07 '12 at 18:29

score 0 · Answer 4 · answered Sep 07 '12 at 18:22

put include guards in globals.h make GameStates* declaration extern

//globals.h
#ifndef _MY_GLOBALS_H_
#define _MY_GLOBALS_H_
#include "GameStates.h"
#include "SDL/SDL.h"

extern GameStates* currentstate;
#endif

//Main.cpp

#include "globals.h"

GameStates* currentState = 0;

C++ Multiple Definitions

4 Answers4

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