Question

I was just looking through implementation of non local means algorithm via google (thanks google for code search) and come across this function mirror.

template<typename T,typename U,bool N>
inline int
boundaryExpansion::helperBase<T,U,N>::mirror(const int src,
                                               const int size,
                                               const int last) const {
  const int32 alpha(src%size);
  if (alpha>=0) {
    return (((src/size) & 0x00000001) != 0) ? last-alpha : alpha;
  }
  return (((src/size) & 0x00000001) == 0) ? -alpha-1 : size+alpha;
}

And the line I am interested in is this

const int32 alpha(src%size);

Now what is alpha here? A function or a variable? What this syntax means? Is this a variable declaration?

Answer 1

This is a variable declaration. A declaration of the form:

type variablename = value;

is essentially equivalent to:

type variablename(value);

This is the case regardless of what type is – whether it is a user-defined class or a built-in type. Note that the reverse is not always the case – the = syntax requires that there be an accessible copy constructor.

For similar reasons, you can cast arithmetic types using the constructor syntax, as in: x = int(42.0);