operator() in C++

Question

I have an object that has a mathematical function behind it. It seems like a perfect candidate for operator().

Specifically its a light that has a different value for each (phi,theta) position on the sphere.

Now the thing is, when inside the class, accessing the light function has this crunky syntax:

    double operator() ( double phi, double theta )
    {
        // compute light function 
        return sin(2*t) * cos(p) ; // (really this is implemented as a function pointer,
                                   // so the light function can be changed)
    }

    void functionThatUsesLightFunction()
    {
         double val = ( 2.0, 4.0 ) ;  // seems bad // Whoops!  Doesn't work.
         double val2 = (*this)( 2.0, 4.0 ) ; // ok
         double val3 = operator()( 2.0, 4.0 ) ; // no thank you
    }

But from outside the class, it gets this really nice syntax like

    foreach( theta on 0..PI )
        foreach( phi on 0..2*PI )
            val += light( theta, phi ) ;

Do you think I'm misusing operator() here?

Can't you define a (private?) method compute(bla) and call it inside operator()? Then you can call compute inside the class and use the operator() outside. — Vitor Py
I'm not a fan of operator(). Operator() is a method with no name: It is the emperor's new method (as in the emperor's new clothes). operator() looks just as ugly and just as naked from the outside as it does from the inside. What is wrong with a fully clothed function here, such as illumination? — David Hammen
@David: operator() doesn't look at all ugly if you want to create an object that acts like a function. I agree that it makes very little sense if it's only for "internal" use, though. — Oliver Charlesworth

Nawaz Nawaz · Accepted Answer · 2011-08-12T20:04:39

i think you should define yet another function, say calculate, in the private section of the class, and call this function from operator() and other member functions. That way, you wouldn't be calling operator() from member functions, but you can still call it from outside the class. Somthing like this:

class Light
{
  private:
    double calculateLight( double phi, double theta )
    {
        return sin(2*t) * cos(p) ;
    }
  public:
    double operator() ( double phi, double theta )
    {      
         return calculateLight(phi, theta);
    }
  //...
    void functionThatUsesLightFunction()
    {
         double val3 = calculateLight( 2.0, 4.0 );
    }
};

//Outside the class
Light light;
//...
val += light( theta, phi ) ;

There is also a good advantage in adding calculateLight function, as you can choose a good name for this function, which increases readability. operator() adds nothing to the readability.

operator() in C++

3 Answers