Why can I not convert a reverse iterator to a forward iterator?

Question

Well, I know why, it\'s because there isn\'t a conversion, but why isn\'t there a conversion? Why can forward iterators be turned to reverse iterators but not the other way roun

Answer 1

It's very common to have two (reverse) iterators span a range of values (such as in begin(),end() and rbegin(),rend()). For any range described by the two reverse iterators rA,rB, the range rB.base(),rA.base() will span the same range in the forward direction.

#include 
#include 
#include 

int main() {
  std::vector vec{10,11,12,13,14,15};

  // spans the range from 13 to 10
  auto rfirst=std::rbegin(vec)+2;
  auto rlast=std::rend(vec);

  // Loops forward, prints 10 11 12 13
  for(auto it = rlast.base(); it != rfirst.base(); ++it){
    std::cout << *it << " ";
  }
}

If conceptually you are only interested in a single item (such as the result of find_if), then use make_forward by @visitor. Even in this case, the range idea helps to keep track of the validity of the reverse iterator:

#include 
#include 
#include 
#include 

int main() {
  std::vector vec{10,11,12,13,14,15};

  auto rfirst=std::rbegin(vec);
  auto rlast=std::rend(vec);

  auto rfound = std::find_if(rfirst,rlast, [](int v){ return v<13; });

  if(rfound != rlast){
    std::cout << *rfound << " "; // prints 12
    auto forwardFound = make_forward(rfound) ; 
    std::cout << *forwardFound << " "; // prints 12
  }
}