Thread: deleting specific elements from map

Until someone comes out with a library of functions that support SQL on a map, (specifically, a DELETE with a WHERE clause), you'll have to iterate through the map and apply the condition and call the appropriate remove or delete method yourself.

The easiest way would be to read a book that describes the STL map methods and implement the one that suits you. That way, next week, when you want the 2nd easiest way to do it, you'll already now the answer.

Wow, looking at map/algorithm references I don't see anything in it that would let you do that on one line - boost or no boost, notably map doesn't have a remove_if method like std::list.

You might write a generic function that takes a container and applies the erase method correctly on each item that meets a condition (but then it would also take a lot of work to make it for map::value_type - so it might be specifically meant for maps).

I too was surprised not to find any sort of std::erase_if or std::erase_iterator

It seems worse than that: the Dinkumware reference is wrong (map::erase actually returns void) and there doesn't seem to be a way to get a next valid iterator (map::erase can invalidate all iterators, perhaps because of rebalancing, and the iterator in the map is a very complicated thing? - can't find anything in the standard).

All in all it looks that map isn't meant to support something like this in the first place (at least effectively). Perhaps another container might be more suitable...?

I just went through this (the "iterator issue") on another forum. Perhaps the technique shown there for list.erase() will also with for map.erase().

http://forums.macrumors.com/showthread.php?t=573857

I haven't tried remove_if on a map, so I don't know if it works (you would think it would, but who knows edit: apparently not). You can't std::sort a map, since that requires a random access iterator.

Originally Posted by anon

It seems worse than that: the Dinkumware reference is wrong (map::erase actually returns void) and there doesn't seem to be a way to get a next valid iterator (map::erase can invalidate all iterators, perhaps because of rebalancing, and the iterator in the map is a very complicated thing? - can't find anything in the standard).

The 2003 edition of the C++ Standard says quite clearly that "the erase members shall invalidate only iterators and references to the erased elements". Consequently, we just need to increment the iterator, and then pass the previous iterator to erase(). For example:

Code:

map<unsigned int, object>::iterator iter = some_map.begin();
while (iter != some_map.end())
{
    if (iter->second.get_somenum() < 10)
    {
        some_map.erase(iter++);
    }
    else
    {
        ++iter;
    }
}