CODEWITHSUNDEEP
c++stllambdac++11erase-remove-idiom
fishfood
fishfood

Reputation: 4282

Accessing for_each iterator from lambda

Is it possible to access the std::for_each iterator, so I can erase the current element from an std::list using a lambda (as below)

typedef std::shared_ptr<IEvent>    EventPtr;
std::list<EventPtr> EventQueue;
EventType evt;
...

std::for_each( 
    EventQueue.begin(), EventQueue.end(),

    [&]( EventPtr pEvent )
    {
        if( pEvent->EventType() == evt.EventType() )
            EventQueue.erase( ???Iterator??? );
    }
);

I've read about using [](typename T::value_type x){ delete x; } here on SO, but VS2010 doesn't seem to like this statement (underlines T as error source).

Upvotes: 6

Views: 5340

Answers (3)

Saint-Martin
Saint-Martin

Reputation: 316

Erase is not the only time you may need to know iterator from lambda. To do this in a more general way, I am using & operator (implicit conversion to iterator) like this :

int main (int argc, char* argv []) {
  size_t tmp [6] = {0, 1, 2, 3, 4, 5};
  std::list<size_t> ls ((size_t*)tmp, (size_t*) &tmp [6]);
  //printing next element
  std::for_each ((const size_t*)tmp, (const size_t*) &tmp [5], [] (const size_t& s) {
    std::cout << s << "->";
    std::cout << *(&s +1) << "   ";
  });
  std::cout << std::endl;
}

Upvotes: 0

James McNellis
James McNellis

Reputation: 355157

You are using the wrong algorithm. Use remove_if:

EventQueue.remove_if([&](EventPtr const& pEvent)
{
    return pEvent->EventType() == evt.EventType();
});

The STL algorithms do not give you access to the iterator being used for iteration. This is in most cases a good thing.

(In addition, consider whether you really want to use std::list; it's unlikely that it is the right container for your use case. Consider std::vector, with which you would use the erase/remove idiom to remove elements that satisfy a particular predicate.)

Upvotes: 9

smerlin
smerlin

Reputation: 6566

no, use a regular for instead.

for( auto it = EventQueue.begin(); it != EventQueue.end(); ++it )
{
  auto pEvent = *it;
  if( pEvent->EventType() == evt.EventType() )
      it = EventQueue.erase( it );
);

Upvotes: 1

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