Undefined Reference Error - Classes

[analog ether]

Hello !
I'm making a SDL projectand I've got an almost empty "Physic Engine" Class with a static attribute (static SDL_Renderer *) and a basic "Game" class that instantiates a physic engine attribute.

I've included physic engine.cpp in game.cpp but it still says it's undefined...

/usr/bin/ld: /tmp/ccV9PwDN.o: warning: relocation against `_ZN12PhysicEngine8rendererE' in read-only section `.text' /usr/bin/ld: /tmp/ccV9PwDN.o: in function `PhysicEngine::PhysicEngine(SDL_Window*)': main.cpp:(.text+0x1f4): undefined reference to `PhysicEngine::renderer' /usr/bin/ld: /tmp/ccV9PwDN.o: in function `Game::Init(char const*)': main.cpp:(.text+0x39a): undefined reference to `PhysicEngine::renderer' /usr/bin/ld: warning: creating DT_TEXTREL in a PIE collect2: error: ld returned 1 exit status
If you want me to provide code don't hesitate to tell me (you can even ping me)

[thorn igloo]

When your question is answered use !solved to mark the question as resolved.

Remember to ask specific questions, provide necessary details, and reduce your question to its simplest form. For tips on how to ask a good question use !howto ask.

[analog ether]

Relocation error - Classes

Underined Reference Error - Classes

Undefined Reference Error - Classes

[latent summit]

How are you building the project?

[feral atlas]

I've included physic engine.cpp in game.cpp
You're not supposed to do that.

It's going to be difficult to figure out without seeing the code. Maybe you made a typo somewhere, maybe you forgot the definition, maybe you thought the declaration would count as a definition.

Typically you just CTRL+click on PhysicEngine::renderer and try to find the definition.

[slim quest]

did you define PhysicEngine::renderer ?

send us your file structure

[slim quest]

you are not supposed to include cpp fiels either

this is how you share code between translation units (.cpp files) in C++:

You create so called header files (usually with a .h extension). These header files contain all the code that is absolutely required by other translation units to use the code of another translation unit.

This includes:

• class definitions
• function DECLARATIONS
• template definitions
• definitions of inline marked things

So this is how you would create a simple class with a member function with the header + source combination:

//foo.h

class Foo{
  public:
    static int x; //declare the member in foo.h because
                  //the other TUs don't need the definition
                  //at compile time.
    void f() const;
};

//foo.cpp

int Foo::x = 5; //define Foo::x in foo.cpp

void Foo::f() const {} // define Foo::f in foo.cpp

then what will happen is you include foo.h in you other source files and use everything as usual.

When you build your project all translation units are compiled separately and they will produce object files. These files contain the compiled code of the ".cpp" files.

Then a program called a linker links all these files together into one executable.

When compilation happens, the compiler doesn't care if the definition of a symbol (function name, variable name etc.) is available, it only cares if there is a declaration. When linking happens, the linker links the declarations to definitions.

What you see there is a linker error, which basically means that the linker didn't find a definition to link to your static member variable.

and what #include does is it just copy pastes whatever file you included into the place of the include directive

[analog ether]

hmm okay
That's basically what I understood about headers but I have surely done something wrong
If including .cpp file is incorrect, then my file structure is awful...

here's physic_engine.h :

#pragma once
#include <SDL3/SDL.h>

//all my homies hate collision detections

class PhysicEngine {
    public:
        static const int gravity = 100;
        static const int FPS = 60;
        static const int DeltaTime = 1000/FPS;

        static SDL_Renderer * renderer;
    
        PhysicEngine(SDL_Window * window);

        static void MoveDynamicAndCollideWithSolid();
};```
and physic engine.cpp :
```c++
#include "physic_engine.h"

static SDL_Renderer * renderer = nullptr;

PhysicEngine::PhysicEngine(SDL_Window * window) {
    renderer  = SDL_CreateRenderer(window, NULL);
};```

and here's game.h :

#pragma once
#include "physic_engine.h"
#include <SDL3/SDL.h>
#include <vector>

class Game {
    public:
        PhysicEngine * ph;

        const std::vector<int> WINDOW_SIZE = {1080, 960};

        Game();
        ~Game();

        void Init(const char* title);

        [[unrelated stuff]]
};```
and game.cpp ;
```c++
#include "game.h"
#include "texture_manager.cpp"
#include "GameObject.cpp"
#include "physic_engine.cpp"
#include <iostream>

GameObject * player;
GameObject * player2;

PhysicEngine * ph;

Game::Game() {}
Game::~Game() {}

void Game::Init(const char* title) {
    if (SDL_Init(SDL_INIT_VIDEO)) {
        std::cout << "Init worked" << std::endl;

        window = SDL_CreateWindow(title, WINDOW_SIZE[0], WINDOW_SIZE[1], SDL_WINDOW_RESIZABLE);

        ph = new PhysicEngine(window);

        SDL_SetRenderDrawColor(PhysicEngine::renderer, 0, 0, 0, 0);

        is_running = true;

        SDL_SetWindowPosition(window, 0, 100);

        player = new GameObject("../Assets/Image/Pawn/pawn.bmp", renderer, 0, 0);
        player2 = new GameObject("../Assets/Image/Pawn/pawn.bmp", renderer, 500, 0);
    }

    else is_running = false;
}
[[also unrelated stuff]]

(idk if that's what you meant by file structure but here's all of the code, there should be all the needed info)

[analog ether]

Thanks for the explanation, what are the "::" for exactly ? they seemed to be optional sometimes

[slim quest]

you need to do PhysicEngine::renderer

[slim quest]

it is the scope operator

it says "in the scope of"

you define members of the PhysicsEngine class

so you need to tell the compiler

what you did right now with static SDL_Renderer * renderer = nullptr; is define a global variable of type SDL_Renderer* that is only available in that cpp file (because that is what static means when applied to global namespace stuff)

[analog ether]

ooh

okay

it gives me this though

[slim quest]

you dont need to put static

only inside the class definition

not when defining the member outside the class

[slim quest]

you can also just modify the header to do inline static SDL_Renderer* renderer = nullptr; btw

and then no need for the definition in the c++ file if you dont want to

[analog ether]

what's inline ?

is it to give it a value in the header ?

[slim quest]

its a keyword that enables stuff to be defined in header files that usually cant be

its meaning is "allow multiple definitions of this symbol if all the definitions match"

because header files are copy pasted if you put a defintion of lets say a function into a header, without inline, and you include the header in multiple TUs (cpp files), at linking the linker will see multiple definitions of the same function

and it will give you an error

inline fixes this, but it really is just a workaround tool, but sometimes its nice

for example when creating a library, its nice to be able to put static constant members into the header file together with the definition because that way the user of the library can see what the constant is without looking at the full source code

[analog ether]

I see

it works now !
But when I try to replace the #include #include "physic_engine.cpp" by #include "physic_engine.h" it says :
/usr/bin/ld: /tmp/ccep2f9L.o: in function `Game::Init(char const*)': main.cpp:(.text+0x35b): undefined reference to `PhysicEngine::PhysicEngine(SDL_Window*)'
Even though PhysicEngine::PhysicEngine(SDL_Window*) is well defined in the .cpp ?

[thorn igloo]

@analog ether Has your question been resolved? If so, type !solved :)

[slim quest]

how are you compiling?

[slim quest]

how did you change the renderer static member?

did you use inline?

[analog ether]

well I run a bash script
with export LD_LIBRARY_PATH=./SDL/build
then g++ main.cpp -o main -L./SDL/build -lSDL3 -ISDL/include

[analog ether]

yes

and I deleted the declaration in .cpp

[slim quest]

you have to compile all your cpp files

not just main.cpp

[analog ether]

really ?

oh

[slim quest]

g++ main.cpp physics_engine.cpp game.cpp -o main -L./SDL/build -lSDL3 -ISDL/include

[slim quest]

yeah

[analog ether]

okay, that makes sense actually

oh god

the error

is it because I still include .cpps ?

[slim quest]

yes

multiple definition of ...

it basically means what we discussed when defining stuff in headers (or practically anything that you include in other files)

[analog ether]

yeah, it also makes sense

I was just impressed by the length of the error message

[analog ether]

is there an order for compilation ?
Like is g++ main.cpp game.cpp different from g++ game.cpp main.cpp ?

[slim quest]

no

[analog ether]

yeah ok

[slim quest]

they are called translation units for a reason

[analog ether]

that's what I thought but who knows

[slim quest]

they are translated as one unit not dependent on each other

some compilers compile multiple TUs parallel on multiple threads aswell ^^

[analog ether]

nice

it wooooooooorks

there were ~15 errors

I included sdl3/sdl.h in another header

and it all works

thanks

I'll go looking for remnants of illogic code now

your explanations are great, thank you so very much

[slim quest]

im an instructor, they have to be

[analog ether]

sadly it's not always the case y'know
I moved the renderer to the game class and I got it work all clean
thank again

!solved