C++ (Qt)
typedef std::pair <int, int> TPair;
typedef std::map<int, int> TMap;
TMap theMap;
TPair elem(1, 2);
 
TMap::iterator it = theMap.find(elem.first);
if (it == theMap.end())
	theMap.insert(elem);
else
	it->second += elem.second;	
 
 

C++ (Qt)
TMap::iterator it = theMap.lower_bound(elem.first);
 
if (it != theMap.end() && !theMap.key_comp()(elem.first, it->first))
  // do update
else
  theMap.insert(it, elem);
 

C++ (Qt)
theMap[ elem.first ] += elem.second;
 

C++ (Qt)
size_t old = theMap.size();
int & value = theMap[elem.first];
if (theMap.size() > old) 
 value = elem.second;
else 
 valie += elem.second;
 

C++ (Qt)
TMap::iterator it = theMap.lower_bound(elem.first);
 
if (it != theMap.end() && !theMap.key_comp()(elem.first, it->first))
  // do update
else
  theMap.insert(it, elem);
 

Вы процитировали практически дословно operator[] из map.