Coming from Java to C++ I'm attempting to understand abstraction through object orientation.
To put this into a practical example, I am developing a small game using the SFML library for graphics. However this question does not relate to that, simply think of it as background info. Anyway, the way the game works is to process through a number of different states. In this case 2:
The Menu State: The menu of the game is drawn and the game will begin here.
The Game State: This state controls the game, will update entities and draw them.
In order to do this I have created the following classes:
GameStateManager.h
#ifndef GAMESTATEMANAGER_H
#define GAMESTATEMANAGER_H
#include <SFML/Graphics.hpp>
#include <iostream>
#include "GameState.h"
class GameStateManager
{
public:
// Constructor
GameStateManager();
// State variables
static const int NUMGAMESTATES = 2;
static const int MENUSTATE = 0;
static const int GAMESTATE = 1;
// Public Functions
void set_state(int state);
void update();
void draw(sf::RenderWindow &win);
void input(sf::Event event);
private:
// Array of gamestates
GameState game_states[];
// The current state
int current_state;
// Private functions
void load_state(int state);
void unload_state(int state);
};
#endif
GameState.h
#ifndef GAMESTATE_H
#define GAMESTATE_H
#include <iostream>
#include <SFML/Graphics.hpp>
#include "GameStateManager.h"
class GameState
{
protected:
GameStateManager gsm;
public:
virtual void init() = 0;
virtual void update() = 0;
virtual void draw(sf::RenderWindow &win) = 0;
virtual void input(sf::Event event) = 0;
};
#endif
Now you may have noticed the Array of GameStates in Game State Manager? This provides an error to which I do not understand: zero-sized array. Does this mean initialization needs to be made within the header file? Further to this point the compiler mentions an Array of Abstract class isn't allowed?
The second issue is that the field gsm in the abstract GameState class does not recognize and brings up yet another error: Missing type specifier.
Now to complicate things further I have the following class: MenuState. This class is meant to extend GameState.
MenuState.h
#ifndef MENUSTATE_H
#define MENUSTATE_H
#include "GameState.h"
class MenuState: public GameState
{
public:
MenuState(GameStateManager gsm);
void init();
void update();
void draw(sf::RenderWindow &win);
void input(sf::Event event);
private:
sf::Texture title_texture;
sf::Sprite title_sprite;
};
#endif
As mentioned this class will control the menu of the game.
Implementing GameStateManager is done as follows:
GameStateManager.cpp
/*
* GameState Manager will take care of the various states of the game.
* In particular there will be two states: Menu or Ingame. GameStateManager
* will load and unload each state as needed.
*
* Author: Ben Euden
* Date: 2/5/2014
*/
#include "GameStateManager.h"
// Class Constructor
GameStateManager::GameStateManager()
{
game_states = game_states[NUMGAMESTATES];
current_state = MENUSTATE;
load_state(current_state);
}
/*
* Load the current game by creating and initialising the state
* then storing it in the game_states array.
* @Param state The state we wish to load.
*/
void GameStateManager::load_state(int state)
{
if(state == MENUSTATE)
game_states[state] = MenuState(this);
//if(state == GAMESTATE)
//game_states[state] = MainGameState(this); // Not implemented yet.
}
/*
* Unload the state we loaded with load_state
*/
void GameStateManager::unload_state(int state)
{
game_states[state] = NULL;
}
void GameStateManager::set_state(int state)
{
unload_state(state);
current_state = state;
load_state(state);
}
void GameStateManager::update()
{
try{
game_states[current_state].update();
}
catch(int e)
{
std::cout << "Exception occured during update of game state" << e << std::endl;
}
}
void GameStateManager::draw(sf::RenderWindow &win)
{
try{
game_states[current_state].draw(&win);
}
catch(int e)
{
std::cout << "Exception occured when trying to draw gamestate: " << current_state << "Exception number: " << e << std::endl;
}
}
void GameStateManager::input(sf::Event event)
{
game_states[current_state].input(event);
}
And MenuState as follows:
/*
* This class extends the Game State header and will deal with the menu of the game
* this includes drawing the correct text to the screen, moving the selector and
* either exiting, bringing up about or starting the game.
*
* Author: Ben Euden
* Date: 2/5/2014
*/
#include "MenuState.h"
MenuState::MenuState(GameStateManager gsm)
{
gsm = gsm;
init();
}
void MenuState::init()
{
title_texture = sf::Texture();
title_texture.loadFromFile("sprites/Title.png");
title_sprite = sf::Sprite();
title_sprite.setTexture(title_texture);
title_sprite.setPosition(512, 200);
}
void MenuState::update(){}
void MenuState::draw(sf::RenderWindow &win)
{
win.draw(title_sprite);
}
void MenuState::input(sf::Event event)
{
}
Please ignore inplemented methods and positionings. At this point I began to attempt to compile the project (I'm using Visual Studio) when the errors appeared.
Now in understand that the MainGameState hasn't been implemented yet but even with MenuState I'm sure I'm missing something vital here as I am still learning C++. With this in mind also please excuse any breakage of conventions etc again I am learning so feel free to correct me, it is better I learn the right way now rather than develop bad habits.
In Summary I'd like to understand why I am receiving the following errors:
protected:
GameStateManager gsm;
This produces the error: missing ';' before gsm.
GameState game_states[];
Produces the errors of: zero-size array, array of abstract class not allowed.
I believe if I fix these the rest will sort themselves out.
Thank you for your patience, time and assistance with this.
Euden
