C ++实施状态

Question

My question is: I am trying to implement basic state management in my project and i stuck at changing states. I have all my states in std::stack<State*> container, and push/pop them directly from Application class or from State class. Problem is when i change current state from State class, it can be destroyed before render method called, whitch results in exeption. So how do i avoid this? PS sorry for my english and please say me if something in my problem/code isn clear

Application class:

void Application::pushState(State* state)
{
    this->m_states.push(state);
    this->m_states.top()->open();//enter state
}

void Application::popState()
{
    if (!this->m_states.empty())
    {
        this->m_states.top()->close();//leave state
        delete this->m_states.top();
    }

    if (!this->m_states.empty())
    this->m_states.pop();
}

void Application::changeState(State* state)
{
    if (!this->m_states.empty())
        popState();
    pushState(state);
}

State* Application::peekState()
{
    if (this->m_states.empty()) return nullptr;
    return this->m_states.top();
}

void Application::mainLoop()
{
    sf::Clock clock;

    while (this->m_window.isOpen())
    {
        sf::Time elapsed = clock.restart();
        float delta = elapsed.asSeconds();

        if (this->peekState() == nullptr)
            this->m_window.close();
        this->peekState()->update(delta)//if i change state in State.update(), it may be that code below will now point to not existing state

        if (this->peekState() == nullptr)
            this->m_window.close();
        this->peekState()->render(delta);
    }
}

State class:

void EditorState::update(const float delta)
{
    sf::Event event;
    while (this->m_application->m_window.pollEvent(event))
    {
        if (event.type == sf::Event::Closed)
        {
            this->m_application->popState();
            return;
        }
    }
}

Okay maybe this is not really a problem, but something like "how to" question. As you can see in my code, i update and render states in mainLoop() method. What im tying to figure out is how to manage those updates, asuming that state can be changed from state itself, not only from stateManager (in my case Application class)

Answer 1

Ok, so I'm guessing this is for a game (but it doesn't have to be). Instead of doing what you're doing for switching between states, I use an enum.

enum class GameState {
    MENU, PLAY, PAUSE
}

Then, in your main header

GameState m_gameState = GameState::MENU;

In your main loop, you can check what the current state is by simply doing

if (m_gameState == GameState::MENU)
{
    ...
}

or you can use a switch statement

switch (m_gameState)
{
case GameState::MENU:
    ...
    break;
case GameState::PLAY:
    ...
    break;
case GameState::PAUSE:
    ...
    break;
}

And, if you ever want to switch the state, you can just do

m_gameState = GameState::PAUSE;

Hope this answered your question :D

If not, I must have misunderstood (sorry).