C++ Help finding the max value in a map

You can use std::max_element to find the highest map value (the following code requires C++11):

std::map<int, size_t> frequencyCount;
using pair_type = decltype(frequencyCount)::value_type;

for (auto i : v)
    frequencyCount[i]++;

auto pr = std::max_element
(
    std::begin(frequencyCount), std::end(frequencyCount),
    [] (const pair_type & p1, const pair_type & p2) {
        return p1.second < p2.second;
    }
);
std::cout << "A mode of the vector: " << pr->first << '\n';

You never changed currentMax in your code.

map<int,unsigned> frequencyCount;
for(size_t i = 0; i < v.size(); ++i)
    frequencyCount[v[i]]++;

unsigned currentMax = 0;
unsigned arg_max = 0;
for(auto it = frequencyCount.cbegin(); it != frequencyCount.cend(); ++it ) }
    if (it ->second > currentMax) {
        arg_max = it->first;
        currentMax = it->second;
    }
}
cout << "Value " << arg_max << " occurs " << currentMax << " times " << endl;

Another way to find the mode is to sort the vector and loop through it once, keeping track of the indices where the values change.

Here's a templated function based on Rob's excellent answer above.

template<typename KeyType, typename ValueType> 
std::pair<KeyType,ValueType> get_max( const std::map<KeyType,ValueType>& x ) {
  using pairtype=std::pair<KeyType,ValueType>; 
  return *std::max_element(x.begin(), x.end(), [] (const pairtype & p1, const pairtype & p2) {
        return p1.second < p2.second;
  }); 
}

Example:

std::map<char,int> x = { { 'a',1 },{ 'b',2 },{'c',0}}; 
auto max=get_max(x);
std::cout << max.first << "=>" << max.second << std::endl; 

Outputs: b=>2