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I've posted a simple implementation of this some time ago on StackOverflow. It's written in Python, but maybe it can still help you:
► http://stackoverflow.com/a/7294148/627005

I've posted a simple implementation of this some time ago on StackOverflow. It's written in Python, but maybe it can still help you:
► https://stackoverflow.com/a/7294148/627005

I've come to use attributes and behaviors myself, instead of plain old components. However, from your description of how you'd use the system in a Breakout game, I think you're overdoing it.

@heishe's comment

I've come to use attributes and behaviors myself, instead of plain old components. However, from your description of how you'd use the system in a Breakout game, I think you're overdoing it.

@heishe's comment

class HealthAttribute : public EntityAttribute
{
  public:
    HealthAttribute(Entity* entity, double max, double critical)
        : max_(max), critical_(critical), current_(max)
    { }

    double value() const {
        return current_;
    }    

    void value(double val)
    {
        // Ensure that 0 <= current <= max 
        current_ = std::max(0, val);
        current_ = std::min(current_, max_);

        // Notify other components belonging to this entity that
        // health is too low.
        if (current_ <= critical_) {
            auto ev = std::shared_ptr<Event>(new HealthCriticalEvent())
            entity_->events().post(ev)
        }
    }

  private:
    double current_, max_, critical_;
};

class HealthAttribute : public EntityAttribute
{
  public:
    HealthAttribute(Entity* entity, double max, double critical)
        : max_(max), critical_(critical), current_(max)
    { }

    double value() const {
        return current_;
    }    

    void value(double val)
    {
        // Ensure that 0 <= current <= max 
        if (0 <= val && val <= max_)
            current_ = val;

        // Notify other components belonging to this entity that
        // health is too low.
        if (current_ <= critical_) {
            auto ev = std::shared_ptr<Event>(new HealthCriticalEvent())
            entity_->events().post(ev)
        }
    }

  private:
    double current_, max_, critical_;
};