可变模板循环，非递归

Question

I have some function, which performs complex drawings. [in pseudocode]

template<typename fields...>       // field names of Brush class
void someFunction(){    
  for(very large loop){

     Brush brush = getBrush();
     int x;

     foreach(field : fields){     // <--- this somehow should be replaced
        x = brush.*field;
        brush.update(x);
     }

  }    
}

[listing 1]

I call it:

someFunction<&Brush::xPos1, &Brush::xPos2, &Brush::xPos3, &Brush::xPos4>()

And I want compiler to generate something like this:

void someFunction(){    
  for(very large loop){

     Brush brush = getBrush();
     int x;

        x = brush.xPos1;
        brush.update(x);

        x = brush.xPos2;
        brush.update(x);

        x = brush.xPos3;
        brush.update(x);

        x = brush.xPos4;
        brush.update(x);

  }    
}

[listing 2]

I mean, I want to get rid of that foreach(field : fields).

---

I found this variadic template loop implementation, but it is recursive. For performance resons this even worst than foreach loop

int a;

template <class T>
void print(const T msg)
{
    a = msg;
}


// And this is the recursive case:
template <class A, class... B>
void print(A head, B... tail)
{
    a = head;
  print(head);
  print(tail...);
}

[listing 3]

---

So the question is.... Is it possible possible to achive result as on [listing 2]? If yes, than how?

Answer 1

I see no real point in doing this. The compiler should optimize both the for loop and recursive template to the same code. In any case, something like this should work:

struct Brush {
    int xPos1, xPos2, xPos3, xPos4;
    void update(int) {}
};

typedef int Brush::* Field;

template<Field...fs>
void Foo()
{
    Brush brush;
    int a[] = { (brush.update( brush.*fs ),0)... };
}

int main()
{
    Foo<&Brush::xPos1, &Brush::xPos2, &Brush::xPos3, &Brush::xPos4>();
}