about erasing an interator of a boost multiindex

Question

I'd like to delete some element out of a boost multi-index container by erasing iterators while visiting the collection.

What I am not sure about is if any iterator invalidation is involved and whether my code below would invalidate firstand last iterators.

If the code below is incorrect, which is the best way considering the specific index (ordered_unique) below?

 #include <iostream>
 #include <stdint.h>
 #include <boost/multi_index_container.hpp>
 #include <boost/multi_index/ordered_index.hpp>
 #include <boost/multi_index/key_extractors.hpp>
 #include <boost/shared_ptr.hpp>

  using namespace std;

  class MyClass{
  public:  
    MyClass(int32_t id) : id_(id) {}
    int32_t id() const
    { return id_; }
  private:  
    int32_t id_;
  };

  typedef boost::shared_ptr<MyClass> MyClass_ptr;

  typedef boost::multi_index_container<
      MyClass_ptr,
      boost::multi_index::indexed_by<
        boost::multi_index::ordered_unique<
          boost::multi_index::const_mem_fun<MyClass,int32_t,&MyClass::id>
        >
      >
    > Coll;

  int main() {

    Coll coll;
    // ..insert some entries 'coll.insert(MyClass_ptr(new MyClass(12)));'

    Coll::iterator first = coll.begin();
    Coll::iterator last  = coll.end();

    while(first != last) {
      if((*first)->id() == 3)
        coll.erase(first++);
      else 
        ++first;    
    }  
 }

score 1 · Accepted Answer · answered Feb 04 '15 at 15:42

1

The reason that erase for containers returns an iterator is to use that result:

first = coll.erase(first);

Then you don't have to worry about how the underlying implementation handles erase or whether it shifts elements around. (In a vector, for instance, your code would've skipped an element in your iteration) However, the documentation does state that:

It is tempting to see random access indices as an analogue of std::vector for use in Boost.MultiIndex, but this metaphor can be misleading, as both constructs, though similar in many respects, show important semantic differences. An advantage of random access indices is that their iterators, as well as references to their elements, are stable, that is, they remain valid after any insertions or deletions.

Still, just seeing coll.erase(first++) is a flag for me, so prefer to do it the other way.

answered Feb 04 '15 at 15:42

Barry

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Thanks Barry. Is 'last' iterator going to remain the same until the while loop ends? I stored coll.end() before the loop begins so I am wondering whether is it going to keep the same value after an 'erase' – Abruzzo Forte e Gentile Feb 04 '15 at 15:44
@AbruzzoForteeGentile answer is yes, `end()` is a valid iterator (just like an iterator pointing to an actual element) and always remains valid after erasure. – Joaquín M López Muñoz Feb 04 '15 at 15:55
@Joaquin. Hi, I need a bit of clarity (sorry to be a bit pedantic here); in my post I meant if the value stored in 'last' remains valid after 'erase'. I am sure that a call t o 'end()' will always be valid. – Abruzzo Forte e Gentile Feb 04 '15 at 16:12
@AbruzzoForteeGentile the value stored in `last` remains valid. – Joaquín M López Muñoz Feb 04 '15 at 18:44

about erasing an interator of a boost multiindex

1 Answers1