Question

In order to get an “easier-to-remember” interface to the
index-generating function std::distance(a,b), I came up
with the idea of a better distinction of it’s arguments
(when used against the base of a vector: vec.begin() )
by calling a templated function with the vector
and its iterator, like:

std::vector<MyType> vect;
std::vector<MyType>::const_iterator iter;
...
...
size_t id = vectorindex_of(iter, vect);

with the rationale of never confusing the order of
the arguments 😉

The explicit formulation of the above idea would
read sth. like

 template <typename T>
 inline 
 size_t vectorindex_of( 
          typename std::vector<T>::const_iterator iter, 
          const std::vector<T>& vect ) {

  return std::distance( vect.begin(), iter ); 
 }

… which works but looks awkward.

I’d love to have the template mechanism implicitly deduce the types
like (pseudo-code):

 template <typename T>
 inline 
 size_t vectorindex_of(T::const_iterator iter, const T& vect) {
    return std::distance( vect.begin(), iter ); 
 }

… which doesn’t work. But why?

Answer 1

The fix is easy: add typename before T::const_iterator iter. This is needed because class templates may be specialized and using typename tells the compiler a type name is expected at T::const_iterator and not a value or something.

You do the same in your less generic function, too.