假设有两个无关系的类 Engine 和 Fuel，使用时，怎

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class fuel;
class engine
{
friend class fuel;
private;
int powerlevel;
public;
engine(){ powerLevel = 0;}
void engine_fn(fuel &f);
};
class fuel
{
friend class engine;
private;
int fuelLevel;
public:
fuel(){ fuelLevel = 0;}
void fuel_fn( engine &e);
};