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c++ - Iterator invalidation in boost::unordered_map

问题描述:

I am using boost::unordered_map as follows

typedef boost::shared_ptr<WriterExeciter> PtrWriter;

typedef std::list<PtrWriter> PtrList;

boost::unordered_map<std::pair<unsigned int, unsigned long long>, PtrList> Map

Map instrMap;

Now I am making some changes to the list of type PtrList in a loop

for(auto it = instrMap.begin(); it != instrMap.end(); ++it)

{

auto key = it->first();

auto list& = it->second();

//Make some change to an element in list

if(list.empty())

{

instMap.erase(key);

}

}

  1. Does making changes to the list invalidate the iterator to instrMap?

  2. Erasing the element will invalidate the iterator pointing to the erased element. How do I modify my code so that the this does not cause any problem? Does using it++ instead of ++it help?

Thanks

网友答案:

The erase() operation will invalidate the iterator. However, it also returns a valid iterator to the next element. So you can use something like the following:

for(auto it = instrMap.begin(); it != instrMap.end();)
{
     auto key = it->first();
     auto list& = it->second();    
     //Make some change to an element in list 

      if(list.empty())
      {
            it = instMap.erase(it); 
      }
      else {
            ++it;
      }
}
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