myexample.cpp

// in main
Game game;
game.pushState(std::make_unique<PlayState>(game));
game.gameLoop();

// Game class
using GameStatePtr = std::unique_ptr<IGameState>;
using GameStateStack = std::stack<GameStatePtr>;
using MaybeGameStatePtr = std::optional<IGameState*>;

void Game::pushState(GameStatePtr state)
{
    m_states.push(std::move(state));
}

void Game::popState()
{
    m_states.pop();
}

void Game::changeState(GameStatePtr state)
{
    if (!m_states.empty())
        popState();
    pushState(std::move(state));
}

MaybeGameStatePtr Game::peekState()
{
    if (m_states.empty())
        return std::nullopt;
    return m_states.top().get();
}