Question

when I have a std::map, is there an elegant way to at the same time:

1. insert / edit an element given its key
2. get an iterator to the inserted element

The best way I found that prevent doing 2 look-up in the map is:

std::map<int, int> myMap;
//do some stuff with the map
std::map<int,int>::iterator  it = myMap.insert(std::pair<int, int>(0,0)).first;
it->second = 0; //necessary because insert does not overwrite the values

Is it possible to do both of those things in a single statement / line?
thanks

Answer 1

Alas, the STL functions and containers don’t always do what you’d expect. Here are two generic versions, the first more like your code above:

template<class Map>
inline typename Map::iterator ForceInsert1( 
    Map&                           m, 
    const typename Map::key_type&  k, 
    const typename Map::data_type& d )
{
    typename Map::iterator it = m.insert( 
        typename Map::value_type( k, d ) ).first;
    it->second = d; // only necessary if the key already exists
    return it;
}

template<class Map>
inline typename Map::iterator ForceInsert2( 
    Map&                           m, 
    const typename Map::key_type&  k, 
    const typename Map::data_type& d )
{
    typename Map::iterator it = m.find( k );
    if( it != m.end() )
    {
        it->second = d;
    }
    else
    {
        it = m.insert( typename Map::value_type( k, d ) ).first;
    }
    return it;
}

typedef std::map<int, int> MyMap;
void Foo( MyMap& myMap )
{
    ForceInsert1( myMap, 42, 100 );
    ForceInsert2( myMap, 64, 128 );
}