Is there any kind of “expression class” (C++)

Question

I am creating a game that lets the player enter input, changes some states, then checks if a \"goal value\" is true (obviously this description is muchly simplified), and I

Answer 1

Why not build your own expression classes?

class GoalBase
{
    virtual bool goal() = 0;
};

class Enemies : public GoalBase 
{
   // ..
   private:
      int enemies_;

   public:
      Enemies(int start) : enemies_(start) {}
      void kill() { if (enemies_) --enemies_; }
      bool goal() { return enemies_ == 0; }
};

int main()
{
    Enemies enemiesToKill(5);
    enemiesToKill.kill();    

    // ..
    if (enemiesToKill.goal()) {
        // ..
    }

    return 0;
}

Other classes could have other methods, parameters, operators etc. Use your imagination.