Can anyone check my logic here. I just want to know whether I'm understanding the logic behind this code.

When you pass your Car object to a method declared like this:
void cost(Car f) const throw();
The method cannot modify the Car object because the method is marked by const. Also a copy of the object is passed to the parameter of the cost function.

When you pass your Car object to a method declared like this:
void cost(const Car& f) const throw();
The method cannot modify the Car object because the method is marked by const. This time, a copy is not passed to the parameter of the cost function. Instead, the actual Car object is passed to the parameter list and is marked by const, which means that the cost function cannot modify this Car reference object.

 

I'm not clear on your const throw() syntax, I've not used that before and my c++ compiler doesn't like it.

BUT that said; assuming this is c++, in the first example you're passing a copy of object "f". You can do whatever you want to it inside your method, but you will only modify the local copy. So the caller will not see the changes in its copy of "f".

In the second example you're passing a reference to "f", a pointer to the memory that "f" occupies. But you've declared that you're referencing a const object, so the compiler won't let you change "f" inside of cost() because you told the compiler that it was a reference to a constant (read-only) object.