c++模版的一个好玩的特性

这个代码展示了private继承和模版的一个妙用：

#include <iostream>
#include <memory>

template<typename T>
class Counter {
public:
    Counter() { count++; }
    Counter(const Counter&) { count++; }
    ~Counter() { --count; }
public:
    std::size_t howMany(void) { return count; }
private:
    static std::size_t count;
};

template<typename T>
std::size_t Counter<T>::count = 0;

class B : private Counter<B> {
public:
    B() { }
    ~B() { }
    using Counter<B>::howMany;
private:
    int val;
};

int main(void)
{
    std::auto_ptr<int> pInt(new int(5));

    std::cout << *pInt << std::endl;
    B b1;
    {
        B b2;
        std::cout << b1.howMany() << " " << b2.howMany() << std::endl;
    }

    std::cout << b1.howMany() << std::endl;

    return 0;
}