C++ How to find the biggest key in a std::map?

The end:

m.rbegin();

Maps (and sets) are sorted, so the first element is the smallest, and the last element is the largest. By default maps use std::less, but you can switch the comparer and this would of course change the position of the largest element. (For example, using std::greater would place it at begin().

Keep in mind rbegin returns an iterator. To get the actual key, use m.rbegin()->first. You might wrap it up into a function for clarity, though I"m not sure if it's worth it:

template <typename T>
inline const typename T::key_type& last_key(const T& pMap)
{
    return pMap.rbegin()->first;
}

typedef std::map</* types */> map_type;

map_type myMap;
// populate

map_type::key_type k = last_key(myMap);

The entries in a std::map are sorted, so for a std::map m (assuming m.empty() is false), you can get the biggest key easily: (--m.end())->first