Sometimes it's necessary to perform a simple validation on a string to ensure, for example, that a string destined to be converted to a float contains only a single '.'.
If only simple validation of some kind is required, it's trivial to use the count algorithm to tally up suspected chars and report their numbers, as in:
using namespace std;
string n = "joebob@bigtrucks@com";
string d = "12.34";
int A = count(n.begin(), n.end(), '@');
int C = count(d.begin(), d.end(), '.');
cout << "There are " << A << " @ signs in JoeBob's email address!" <<
cout << "There's " << C << " decimal point in JoeBob's checking balance."
It's nice to remember that strings are containers in the STL, just like a vector or list, and that their iterators refer to char pointers that can be counted, sorted, or partitioned just like any other collection of values.