多态向下强制类型转换有关问题

多态向下强制类型转换问题
Account account(1000.0);//基类
checkingAccount check(2000.0,3.0);//Account派生类
savingAccount saving(3000.0,0.05);//Account派生类

vector<Account *>account1(2);

account1[0] = &check;
account1[1] = &saving;

for(int i = 0; i < 2;i++)
{
account1[i]->credit();
account1[i]->debit();
savingAccount *a = dynamic_cast<savingAccount *>(account1[i]);//出现错误，错误为Account不是多态类型 为什么
}

------解决方案--------------------
坐等结果
------解决方案--------------------
从贴出来的看 Account 没虚函数吧！ 猜的
------解决方案--------------------
得有虚函数才行。
------解决方案--------------------

dynamic_cast

dynamic_cast can be used only with pointers and references to objects. Its purpose is to ensure that the result of the type conversion is a valid complete object of the requested class.

Therefore, dynamic_cast is always successful when we cast a class to one of its base classes:

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class CBase { };
class CDerived: public CBase { };

CBase b; CBase* pb;
CDerived d; CDerived* pd;

pb = dynamic_cast<CBase*>(&d); // ok: derived-to-base
pd = dynamic_cast<CDerived*>(&b); // wrong: base-to-derived 


The second conversion in this piece of code would produce a compilation error since base-to-derived conversions are not allowed with dynamic_cast unless the base class is polymorphic.

When a class is polymorphic, dynamic_cast performs a special checking during runtime to ensure that the expression yields a valid complete object of the requested class:

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// dynamic_cast
#include <iostream>
#include <exception>
using namespace std;

class CBase { virtual void dummy() {} };
class CDerived: public CBase { int a; };

int main () {
try {
CBase * pba = new CDerived;
CBase * pbb = new CBase;
CDerived * pd;

pd = dynamic_cast<CDerived*>(pba);
if (pd==0) cout << "Null pointer on first type-cast" << endl;

pd = dynamic_cast<CDerived*>(pbb);
if (pd==0) cout << "Null pointer on second type-cast" << endl;

} catch (exception& e) {cout << "Exception: " << e.what();}
return 0;
}
Null pointer on second type-cast


Compatibility note: dynamic_cast requires the Run-Time Type Information (RTTI) to keep track of dynamic types. Some compilers support this feature as an option which is disabled by default. This must be enabled for runtime type checking using dynamic_cast to work properly.

The code tries to perform two dynamic casts from pointer objects of type CBase* (pba and pbb) to a pointer object of type CDerived*, but only the first one is successful. Notice their respective initializations:

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CBase * pba = new CDerived;
CBase * pbb = new CBase;


Even though both are pointers of type CBase*, pba points to an object of type CDerived, while pbb points to an object of type CBase. Thus, when their respective type-castings are performed using dynamic_cast, pba is pointing to a full object of class CDerived, whereas pbb is pointing to an object of class CBase, which is an incomplete object of class CDerived.

When dynamic_cast cannot cast a pointer because it is not a complete object of the required class -as in the second conversion in the previous example- it returns a null pointer to indicate the failure. If dynamic_cast is used to convert to a reference type and the conversion is not possible, an exception of type bad_cast is thrown instead.