STLAlgorithmExtensions/ExistsAndOnlyAlgorithm

    
template<class Container, class Predicate>
        inline bool
        exists_and_only(const Container& cont, Predicate pred)
        {
            typename Container::iterator i = find_if(cont, pred);
            if (i == cont.end()) return false;
            if (find_if(++i, cont.end(), pred) != cont.end()) return false;
            return true;
        }

-- People/Vladimir Prus

Could this be more generic by passing in the number of occurances?

    
template<typename InputIterator, typename T>
        inline bool
        exact_count(InputIterator first, InputIterator last, T const&, int count = 1);

template<typename InputIterator, typename Predicate>
        inline bool
        exact_count_if(InputIterator first, InputIterator last, Predicate pred, int count = 1);

• exact_count(v.begin(), v.end(), some_value) is not intuitively clear expression, I think
• I'm not sure count!=1 is of any use.

<hrule>

I don't think you need an extra algorithm to perform what count(...)==1 (or count(...)==count) already does with the same complexity (in particular, the same number of calls to operator== or to pred).

-- [Herve Bronnimann]