클래스안에서 static 선언된 변수 사용법 좀 가르쳐주세요.

이 문제는 CRTP(curiously recurring template pattern)의 죻은 예가 되기도 하죠
아래 예제는 c++ template complete guide에 나오는 예제입니다.

template<typename CountedType>
class ObjectCounter {
public:
    static size_t Count() {
        return ObjectCounter<CountedType>::count_;
    }
protected:
    ObjectCounter() { 
        ++ ObjectCounter<CountedType>::count_; 
    }
    ObjectCounter(const ObjectCounter<CountedType>&) { 
        ++ObjectCounter<CountedType>::count_; 
    }
    ~ObjectCounter() {
        --ObjectCounter<CountedType>::count_; 
    }
private:
    static size_t count_;
};

template <typename CountedType>
size_t ObjectCounter<CountedType>::count_ = 0;

//---------------------------------------------------------------------
//---------------------------------------------------------------------

template<typename CharT>
class MyString: public ObjectCounter< MyString<CharT> > {
};

int main()
{
    MyString<char>    s1, s2;
    MyString<wchar_t> s3;

    std::cout << MyString<char>::Count()    << std::endl;
    std::cout << MyString<wchar_t>::Count() << std::endl;

    return 0;
}