Using const_cast for creating non-const variant of methods

Question

Can the const_cast be used for creating non-const versions of already implemented methods? I think I saw something along these lines (with recommendation for the const method to do the actual work), but I'm not really sure how it's supposed to work.

Value& const search(key) const {
    // find value with key
    return value;
}

Value& search(key) {
    return const_cast<Value&>(search(key));
}

If not this way, what is recommended way of creating non-const functions without code duplicity?

Answer 1

The easiest way to do it is with as_const from C++17:

Value& search(key) {
    return const_cast<Value&>(std::as_const(*this).search(key));
}

Without it you can do this instead (or implement it yourself, it's not very hard)

Value& search(key) {
    return const_cast<Value&>(static_cast<const T&>(*this).search(key));
}

Where T is the type of your class (you can have a generic solution with decltype but it gets really ugly due to decltype(*this) being a reference type).

You can take a look at the as_const implementation here or the generic cast here .

Answer 2

Two approaches.

First:

namespace notstd{ // backported C++17
  template<class T>
  T const& as_const(T& t){return t;}
  template<class T>
  T const&& as_const(T&& t){return t;}      
}
namespace utility { // not ever in std
  template<class T>
  T& remove_const(T const& t){return const_cast<T&>(t);}
  template<class T>
  T&& remove_const(T const&& t){return const_cast<T&&>(t);}
}

then:

Value& const search(Key key) const {
  // find value with key
  return value;
}

Value& search(Key key) {
  return utility::remove_const(notstd::as_const(*this).search(key));
}

or alternatively:

Value& const search(Key key) const {
  return search(*this, key);
}

Value& search(Key key) {
  return search(*this, key);
}
private:
  template<class Self>
  friend decltype(auto) search(Self& self, Key key){
    // find value with key
  }

where we delegate the work to a friend template where self is maybe-const.