Const reference with primitive types?

Hi everybody,
 
I was wondering why a const reference is used in the function below (in rs_ellipse.cpp) for the doubles cx, a0, a1. Given that those are primitive types, what is the benefit of doing this vs say, double cx=data.center.x, or const double cx=data.center.x ?

double RS_Ellipse::areaLineIntegral() const
{
   (...)
    const double& cx=data.center.x;
    const double aE=getAngle();
    const double& a0=data.angle1;
    const double& a1=data.angle2;
   (...)
}

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Hey, thanks for the prompt and complete response! :)

It crossed my mind too that it could be to make the code faster, which I would understand if the variables were arrays or objects, but with a double I wouldn’t know what to expect. Being new to CAD software, to me this looked like the developer was trying to save memory, but I couldn't really see the benefit in this case so I thought I was missing something...

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