static_cast问题
在MSDN中看了有关这个操作符的解释，有些地方不太明白，大家给解释一下：
class B {};

class D : public B {};

void f(B* pb, D* pd) {
D* pd2 = static_cast<D*>(pb); // not safe, pb may
// point to just B
B* pb2 = static_cast<B*>(pd); // safe conversion
}


In contrast to dynamic_cast, no run-time check is made on the static_cast conversion of pb. The object pointed to by pb may not be an object of type D, in which case the use of *pd2 could be disastrous. For instance, calling a function that is a member of the D class, but not the B class, could result in an access violation.

红色的地方为什么那样解释呢，既然已经类型装换了，为什么不可以调用D中的呢

[解决办法]
pb是B指针，强转为D后调用一个B没有的D中的函数会出错
第二种就不会，因为pd是D的指针，D继承自B

[解决办法]
子类是有父类的所有东西的
所以向上转型是安全的