13.2 Const Arguments

There’s another situation in which constants are used. Just look at this function:

 
float calculateDistance(Vec2 & pos1, Vec2 & pos2);
 

Suppose you can use this function to calculate the distance between two points. In chapter 11 you read that it is often faster to pass a variable by reference as opposed to passing it by value. But there’s a downside on that. In principle, you could alter the values of pos1 and pos2 within this function. And if a value is passed by reference, this would also change those variables in the original location. But imagine you are working in a team and the function calculateDistance is written by someone else. If something unexpected happens in your code, you would have to double check the work of your colleague to verify the values are not changed in there. The name of the function seems to indicate there’s no reason for that, but mistakes happen. A lot.

It would be better if we can now for sure the values we pass to this function will not be changed by it. That way, we can focus on our own code when there’s an error. The solution is simple: we can pass these references as constants:

 
float calculateDistance(C Vec2 & pos1, C Vec2 & pos2);
 

This is much better because:

1. when creating such a function, you will get an error if you try to change pos1 or pos2;
2. the programming which uses this function can be sure the values are not changed in there, without reading your code;
3. you have a strong indication that values will be changed inside a function when the reference is not declared as a constant. (It’s either that, or you need to have a serious talk with your fellow developers.)

Let’s agree that, starting right now, you will pass all function arguments as a const reference, unless you have a good reason not to. Your future colleagues will like you a lot more when you do so.

And what is a good reason not to use a constant? Look at the Esenthel Clamp function:

 
void Clamp(Vec2 & value, C Vec2 & min, C Vec2 & max);
 

This function will change the first argument when its value is lower than the second or higher than the third. It can be used like this:

 
Vec2 pos = Ms.pos(); 
Clamp(pos, Vec2(-0.4,-0.4), Vec2(0.4,0.4)); 
pos.draw(RED);
 

The second and third argument are constant, so the function will never change the minimum or maximum. But the first argument is the one that is meant to change. No const reference there.