Deriving from std::back_insert_iterator ?

Question

I want to derive from std::back_insert_iterator to create a kind of filter for a string type, say back_xml_insert_iterator, that will examine the characters passed through it looking for characters that can not be emitted "naked" into an XML stream, e.g., '"', '&', '<', '>', and '\'', and will on-the-fly insert their character entity references instead, e.g., """ for '"'.

template<class StringType>
class back_xml_insert_iterator : public std::back_insert_iterator<StringType> {
  typedef std::back_insert_iterator<StringType> base_type;
public:
  typedef typename base_type::container_type container_type;
  typedef typename StringType::value_type value_type;

  explicit back_xml_insert_iterator( StringType &s ) : base_type( s ) { }

  back_xml_insert_iterator& operator=( value_type c ) {
    switch ( c ) {
      case '"':
      case '&':
      case '\'':
      case '<':
      case '>':
        char buf[10];
        this->container->append( "&#" );
        this->container->append( itoa( c, buf ) );
        this->container->push_back( ';' );
        break;
      default:
        this->container->push_back( c );
    }
    return *this;
  }
};

This compiles fine. When I create an instance, I confirmed that the constructor is called, but my operator=() is never called. I think it's because the inherited operator*() returns a back_insert_iterator& and not a back_xml_insert_iterator& so back_insert_iterator::operator=() is called rather than mine (since operator=() is not, nor can not be, virtual).

If that's the case, then it seems impossible to derive from back_insert_iterator in a useful way.

If I instead create my own back_insert_iterator_base class like:

template<class ContainerType,class DerivedType>
class back_insert_iterator_base :
  public std::iterator<std::output_iterator_tag,void,void,void,void> {
public:
  typedef ContainerType container_type;

  DerivedType& operator*() {
    return *static_cast<DerivedType*>( this );
  } 

  DerivedType& operator++() {
    return *static_cast<DerivedType*>( this );
  }

  DerivedType& operator++(int) {
    return *static_cast<DerivedType*>( this );
  }

protected:
  back_insert_iterator_base( ContainerType &c ) : container( &c ) {
  }

  ContainerType *container;
};

and derive from that instead like:

template<class StringType>
class back_xml_insert_iterator :
  public back_insert_iterator_base< StringType, back_xml_insert_iterator<StringType> > {
  // ... as before ...
};

then back_xml_insert_iterator works as desired. So is it possible to derive from std::back_insert_iterator and have it work as desired?

Update

Here's how I'd like to use the back_xml_insert_iterator. First, there would be the helper function:

template<class StringType> inline
back_xml_insert_iterator<StringType> back_xml_inserter( StringType &s ) {
  return back_xml_insert_iterator<StringType>( s );
}

Then writing a to_xml() function would be trivial:

template<class InputStringType,class OutputStringType> inline
void to_xml( InputStringType const &in, OutputStringType *out ) {
  std::copy( in.begin(), in.end(), back_xml_inserter( *out ) );
}

Answer 1

You should not try to inherit from classes that were not designed with inheritance in mind. In general you should not inherit from standard containers or iterator. Note that in most cases the usage of iterators is in templates that do not require a type hierarchy in the first place.

It is usually better to add the functionality externally either by providing functors or else implementing your own iterator --which should not be really complex. You can take a look at the iterator adaptor library in boost if you really want to implement your own iterators.

Answer 2

You can derive from back_insert_iterator, but you have to override the operators to return your derived type.

Does that render the strategy pointless? It depends on your coding style. The overrides will simply call the base method and return this, so you end up with a run of copy-paste code, much like what you put in back_insert_iterator_base.

template<class StringType>
class back_xml_insert_iterator : public std::back_insert_iterator<StringType> {
  typedef std::back_insert_iterator<StringType> base_type;
public:
  typedef typename base_type::container_type container_type;
  typedef typename StringType::value_type value_type;

    // add trivial overrides to avoid returning base type:
  back_xml_insert_iterator &operator++() { base::operator++(); return this; }
  back_xml_insert_iterator operator++(int) {
      back_xml_insert_iterator ret( *this );
      base::operator++();
      return ret;
  }

  explicit back_xml_insert_iterator( StringType &s ) : base_type( s ) { }

  ...

Of course, a similar block of code could implement passing operator calls to a member back_insert_iterator, which would eliminate the inheritance as others recommend. This is possibly a better approach.

Note that private inheritance, with : private std::back_insert_iterator, would add safety by preventing users from attempting to use your class as something it's not.

Answer 3

I could get this to work with the call to operator= successfully. Did I get it right?

// back_insert_iterator example

#include <iostream>
#include <iterator>
#include <vector>
using namespace std;

template<class StringType> 
class back_xml_insert_iterator : public std::back_insert_iterator<StringType> { 
   typedef std::back_insert_iterator<StringType> base_type; 
public: 
   typedef typename base_type::container_type container_type; 
   typedef typename StringType::value_type value_type; 

   explicit back_xml_insert_iterator( StringType &s ) : base_type( s ) { } 

   back_xml_insert_iterator& operator=( value_type c ) { 
      switch ( c ) { 
      case '"': 
      case '&': 
      case '\'': 
      case '<': 
      case '>': 
         break; 
      default: 
         this->container->push_back( c ); 
      } 
      return *this; 
   } 
}; 

int main () {
   vector<int> firstvector, secondvector;
   for (int i=1; i<=5; i++)
   { firstvector.push_back(i); secondvector.push_back(i*10); }

   back_xml_insert_iterator< vector<int> > back_it (firstvector);

   back_it = 2;   // It does call operator=

   copy (secondvector.begin(),secondvector.end(),back_it);

   vector<int>::iterator it;
   for ( it = firstvector.begin(); it!= firstvector.end(); ++it )
      cout << *it << " ";
   cout << endl;

   return 0;
}