Whenever I try to compile/build my game then I get these 2 errors:

||=== Build: Debug in TotalWar (compiler: GNU GCC Compiler) ===|
C:\|25|error: invalid use of incomplete type 'class Scene'|
C:\|2|error: forward declaration of 'class Scene'|
||=== Build failed: 2 error(s), 0 warning(s) (0 minute(s), 2 second(s)) ===|

Here's my code:

Where I forward declared Scene class:

// App.hpp
#pragma once
class Scene;

class App
{
    public:
        enum States
        {
            STATE_INTRO,
            STATE_QUIT,
            STATE_MAIN_MENU,
            STATE_PLAYING
        };
        const int Width = 800;
        const int Height = 600;
        static States CurrentState;
        static Scene* CurrentScene;
        sf::RenderWindow Window;
        sf::Clock* Clock;
        float Dt;

        App(std::string &title);
        ~App();
        void SetScene(Scene& scene);
        void Clear();
        void Display();
        void Close();
        sf::RenderWindow* GetWindow();
        float GetDt();
        void ChangeState(States newState);
};

And in Scene.cpp(Where the error happens)

void Gui::Scene::DrawCurrentScene(float dt)
{
    App::CurrentScene->Draw(dt);
}

When I try to call App::CurrentScene->Draw then the error happens, help please.

Without knowing what namespace App is in, it looks to me like Scene is forward-declared in the same namespace as App, but defined in namespace Gui. If I understand what you're doing correctly, the compiler doesn't understand that you intended Scene to mean Gui::Scene.

Try forward-declaring Scene like this:

namespace Gui
{
    class Scene;
} 

#include "Scene.h"
You only forward-declared it. You never included the class definition.


L. Spiro

I did include it.

Edit: Woops, I didn't complete my message when I posted it(Probably 'cause of net problems)

Without knowing what namespace App is in, it looks to me like Scene is forward-declared in the same namespace as App, but defined in namespace Gui. If I understand what you're doing correctly, the compiler doesn't understand that you intended Scene to mean Gui::Scene.

Try forward-declaring Scene like this:

namespace Gui
{
    class Scene;
} 

Thanks, it compiles/builds fine now , though I got a new problem:

void Gui::Scene::Draw(float dt)
{
    for (auto &gui : GuiList)
    {
        app->GetWindow()->draw(gui->TextDisplay);
    }
}

That function in scene.cpp file causes my game to hang and have a white screen, I don't get why it happens, app->GetWindow()->draw causing the problem but I have no idea how to fix it

I did include it.

*From my last edit:

void Gui::Scene::Draw(float dt)
{
    for (auto &gui : GuiList)
    {
        app->GetWindow()->draw(gui->TextDisplay);
    }
}

That function in scene.cpp file causes my game to hang and have a white screen, I don't get why it happens, app->GetWindow()->draw causing the problem but I have no idea how to fix it

I ran the debugger of Code::Blocks, the app->GetWindow()->draw thing really was causing the problem.

I also got an "Program received signal SIGSEGV, Segmentation fault." from the debugger, how can I fix that?

Well here's my code:

app.hpp

#pragma once
namespace Gui
{
    class Scene;
}

class App
{
    public:
        enum States
        {
            STATE_INTRO,
            STATE_QUIT,
            STATE_MAIN_MENU,
            STATE_PLAYING
        };
        const int Width = 800;
        const int Height = 600;
        States CurrentState;
        Gui::Scene* CurrentScene;
        sf::RenderWindow Window;
        sf::Clock* Clock;
        float Dt;

        App(std::string &title);
        ~App();
        void SetScene(Gui::Scene& scene);
        void Clear();
        void Display();
        void Close();
        void DrawScene(float dt);
        sf::RenderWindow* GetWindow();
        float GetDt();
        void ChangeState(States newState);
        const App& operator=(App& app);
};

GetWindow() function:

sf::RenderWindow* App::GetWindow()
{
    return &Window;
}

Isn't the "sf::RenderWindow Window;" enough to make it initialized? Or Am I missing something?

I fixed the problem :D, apparently the Gui::Scene* CurrentScene line(in app.hpp) was the one that is uninitialized, I forgot to call the function SetScene in my main function that's why CurrentScene was left uninitialized.