I would use the operator[].
map <char, int> m1;
m1['G'] ++; // If the element 'G' does not exist then it is created and
// initialized to zero. A reference to the internal value
// is returned. so that the ++ operator can be applied.
// If 'G' did not exist it now exist and is 1.
// If 'G' had a value of 'n' it now has a value of 'n+1'
So using this technique it becomes really easy to read all the character from a stream and count them:
map <char, int> m1;
std::ifstream file("Plop");
std::istreambuf_iterator<char> end;
for(std::istreambuf_iterator<char> loop(file); loop != end; ++loop)
{
++m1[*loop]; // prefer prefix increment out of habbit
}