dynamic_cast <new_type> (expression)
可以安全的将类的指针或者引用沿着他们的继承层次转换！ 但是指针必须有效，必须为0（可以对值为0的指针应用dynamic_cast，结果仍然是0）或者指向一个对象！ 他只接受基于类对象的指针和引用的类型转换！
在类层次上进行转换的时候 dynamic_cast于static_cast的效果一样！
他返回一个新类型的值，或者会抛出一个异常！
来看代码：
#include<iostream>
using namespace std;
struct V {
virtual void f() {}; // must be polymorphic to use runtime-checked dynamic_cast
};
struct A : virtual V {};
struct B : virtual V {
B(V* v, A* a) {
// casts during construction
dynamic_cast<B*>(v); // well-defined: v of type V*, V base of B, results in B*
dynamic_cast<B*>(a); // undefined behavior: a has type A*, A not a base of B
}
};
struct D : A, B {
D() : B((A*)this, this) { }
};
struct Base {
virtual ~Base() {}
};
struct Derived: Base {
virtual void name() {}
};
struct Some {
virtual ~Some() {}
};
int main(void)
{
D d;
A& a = d; //upcast 这里可以用dynamic_cast
D& new_d = dynamic_cast<D&>(a);//downcast
B& new_b = dynamic_cast<B&>(a);//sidecast
Base* b1 = new Base;
if(Derived* d = dynamic_cast<Derived*>(b1))
{
cout << "downcast from b1 to d succeful" << endl;
d->name();
}
Base* b2 = new Derived;
if(Derived* d = dynamic_cast<Derived*>(b2))
{
cout << "downcast from b2 to d successful" << endl;
d->name();
}
if(Some *d = dynamic_cast<Some*>(b1))
{
cout << "downcast from b1 to Some Successful" << endl;
//d->name();
}
delete b1;
delete b2;
return 0;
}输出结果是：
downcast from b2 to d successful上面看到的是指针的转换，我们也可以用dynamic_cast将基类引用转换成派生类引用！dynamic_cast<Type&>(val);Type是派生类型，val是基类类型， 当val实际引用一个Type类型对象，或者val是一个Type派生类型的对象的时候，dynamic_cast操作才将操作数val转换为想要的Type&类型。还记得我们指针和引用的区别么，引用是不能为空的！总结：1、从子类到基类指针的转换：static_cast和dynamic_cast都是正确地，所谓正确是指方法的调用和数据的访问输出是期望的结果，所谓成功是说转换没有编译错误或者运行异常；2、从基类到子类：static_cast和dynamic_cast都是正确的，其中static_cast的结果是非空指针，dynamic_cast的结果是空指针； 这里，static_cast是错误的；3、两个没有关系的类之间的转换：dynamic_cast返回空指针，而static_cast则是编译不通过；而reinterpret_cast编译通过，返回不是空，但是是错误的；1) If the type of expression is the exactly new_type or a less cv-qualified version of new_type, the result isexpression.2) If the value of expression is the null pointer value, the result is the null pointer value of type new_type3) If new_type is a pointer or reference to Base, and the type of expression is a pointer or reference to Derived, where Base is a unique, accessible base class of Derived, the result is a pointer or reference to the Base class subobject within the Derived object pointed or identified by expression. (note: implicit cast and static_cast can perform this conversion as well)4) If expression is a pointer or reference to a polymorphic type, and new_type is a pointer to void, the result is a pointer to the most derived object pointed or referenced by expression.5) If expression is a pointer or reference to a polymorphic type Base, and new_type is a pointer or reference to the type Derived a run-time check is performed:a) The most derived object pointed/identified by expression is examined. If, in that object, expressionpoints/referes to a public base of Derived, and if only one subobject of Derived type is derived from the subobject pointed/identified by expression, then the result of the cast points/refers to that Derivedsubobject. (this is known as the "downcast")b) Otherwise, if expression points/refers to a public base of the most derived object, and, simultanously, the most derived object has an unambiguous public base class of type Derived, the result of the cast points/refers to that Derived (this is known as the "sidecast")c) Otherwise, the runtime check fails. If the dynamic_cast is used on pointers, the null pointer value of typenew_type is returned. If it was used on references, the exception std::bad_cast is thrown.6) When dynamic_cast is used in a constructor or a destructor (directly or indirectly), and expression refers to the object that's currently under construction/destruction, the object is considered to be the most derived object. Ifnew_type is not a pointer or reference to the construction's/destructor's own class or one of its bases, the behavior is undefined.dynamic_cast却会进行判别，确定源指针所指的内容，是否真的合适被目标指针接受。如果是否定的，那么dynamic_cast则会返回null。
2012/8/10jofranks 于南昌