SFML objects won't draw when its parent class is reinitialised - c++

Question

I'm working on a new project and an implementing a basic scene change. I have the different scenes setup as their own classes, with the intialisation function being used to create and reposition different SFML objects. I saw this answer and have written my scene switcher similarly:
// Create scene monitoring variable
int scene[2];
scene[0] = 0; // Set current scene to menu
scene[1] = 0; // Set scene change to no
...
// Check for scene change
if(scene[1] == 0) {
// Run tick function based on current scene
switch(scene[0]) {
case 0:
// Main menu - run tick function
menu.tick();
}
}
if(scene[1] == 1) {
// Reset scene that you've changed to
switch(scene[0]) {
case 0:
// Main menu - reset it
menu = Menu(window, scene); // <-- Reinitialise menu here
}
// Set change variable to 0
scene[1] = 0;
}
You can see the full code on the github repository.
However, this doesn't seem to work properly - as soon as a scene change is made, the screen goes blank. The class is reintialised (I added a cout to check), the draw function is still run and mouse clicks are still processed, yet nothing appears in the window.
Am I doing something wrong here?

Answer 1

Doing things that way can lead into leak memory errors. I suggest you a different approach: the StateStack
How this works?
The basics of having a StateStack object is store each possible state of your game/app into a stack. This way, you can process each one in the stack order.
What is an State?
An State is something that can be updated, drawn and handle events. We can make an interface or an abstract class to make our screens behave like a State.
Which are the advantages?
With a stack structure, you can easily control how your different scenes are going to handle the three different processing methods. For instance. If you have a mouse click while you're in a pause menu, you won't that click event to reach the menu state or the "game" state. To achieve this, the solution is really easy, simply return false in your handleEvent method if you don't want the event go further this particular state. Note that this idea is also expandable to draw or update methods. In your pause menu, you won't update your "game" state. In your "game" state you won't draw tour menu state.
Example
With this points in mind, this is one possible way of implementation. First, the State interface:
class State{
public:
virtual bool update() = 0;
virtual bool draw(sf::RenderTarget& target) const = 0;
// We will use a vector instead a stack because we can iterate vectors (for drawing, update, etc)
virtual bool handleEvent(sf::Event e, std::vector<State*> &stack) = 0;
};
Following this interface we can have a example MenuState and PauseState:
MenuState
class MenuState : public State{
public:
MenuState(){
m_count = 0;
m_font.loadFromFile("Roboto-Regular.ttf");
m_text.setFont(m_font);
m_text.setString("MenuState: " + std::to_string(m_count));
m_text.setPosition(10, 10);
m_text.setFillColor(sf::Color::White);
}
virtual bool update() {
m_count++;
m_text.setString("MenuState: " + std::to_string(m_count));
return true;
}
virtual bool draw(sf::RenderTarget &target) const{
target.draw(m_text);
return true;
}
virtual bool handleEvent(sf::Event e, std::vector<State*> &stack){
if (e.type == sf::Event::KeyPressed){
if (e.key.code == sf::Keyboard::P){
stack.push_back(new PauseState());
return true;
}
}
return true;
}
private:
sf::Font m_font;
sf::Text m_text;
unsigned int m_count;
};
PauseState
class PauseState : public State{
public:
PauseState(){
sf::Font f;
m_font.loadFromFile("Roboto-Regular.ttf");
m_text.setFont(m_font);
m_text.setString("PauseState");
m_text.setPosition(10, 10);
m_text.setFillColor(sf::Color::White);
}
virtual bool update() {
// By returning false, we prevent States UNDER Pause to update too
return false;
}
virtual bool draw(sf::RenderTarget &target) const{
target.draw(m_text);
// By returning false, we prevent States UNDER Pause to draw too
return false;
}
virtual bool handleEvent(sf::Event e, std::vector<State*> &stack){
if (e.type == sf::Event::KeyPressed){
if (e.key.code == sf::Keyboard::Escape){
stack.pop_back();
return true;
}
}
return false;
}
private:
sf::Font m_font;
sf::Text m_text;
};
By the way, while I was doing this, I notice that you must have the fonts as an attribute of the class in order to keep the reference. If not, when your text is drawn, its font is lost ant then it fails. Another way to face this is using a resource holder, which is much more efficient and robust.
Said this, our main will look like:
Main
int main() {
// Create window object
sf::RenderWindow window(sf::VideoMode(720, 720), "OpenTMS");
// Set window frame rate
window.setFramerateLimit(60);
std::vector<State*> stack;
// Create menu
stack.push_back(new MenuState());
// Main window loops
while (window.isOpen()) {
// Create events object
sf::Event event;
// Loop through events
while (window.pollEvent(event)) {
// Close window
if (event.type == sf::Event::Closed) {
window.close();
}
handleEventStack(event, stack);
}
updateStack(stack);
// Clear window
window.clear(sf::Color::Black);
drawStack(window, stack);
// Display window contents
window.display();
}
return 0;
}
The stack functions are simple for-loop but, with the detail that iterate the vector backwards. This is the way to imitate that stack behavior, starting from top (size-1 index) and ending at 0.
Stack functions
void handleEventStack(sf::Event e, std::vector<State*> &stack){
for (int i = stack.size()-1; i >=0; --i){
if (!stack[i]->handleEvent(e, stack)){
break;
}
}
}
void updateStack(std::vector<State*> &stack){
for (int i = stack.size() - 1; i >= 0; --i){
if (!stack[i]->update()){
break;
}
}
}
void drawStack(sf::RenderTarget &target, std::vector<State*> &stack){
for (int i = stack.size() - 1; i >= 0; --i){
if (!stack[i]->draw(target)){
break;
}
}
}
You can learn more about StateStacks and gamedev in general with this book