关于dynamic_cast

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dynamic_cast 进行运行时强制转换。如果在执行时转换非法，则会转换失败。dynamic_cast 的通用形式如下：

dynamic_cast <target-type> (expr)

其中 target-type 指定了强制转换的目标类型，expr 是需要进行强制转换的表达式。

目标类型必须是指针类型或引用类型，也就是说，表达式也必须能够被求值为指针类型或者引用类型的结果。

dynamic_cast 主要被用来执行多态类型之间的强制转换。比如两个多态类 B 和 D，且 D 派生于 B，此时我们可以用 dynamic_cast 将 D* 指针类型转换为 B* 类型，这是因为基类型的指针通常可以指向派生类的对象。然而，只有当指针指向的对象确实是类 D 的对象时，dynamic_cast 才能将指针从类型 D* 转换为类型 B* 。

如果转换指针的类型失败时，经过 dynamic_cast 运算后得到的指针为空值。
如果转换引用的类型失败时，则会抛出 bad_cast 类型异常。

下面示例中，假设 Base 是多态类，Derived 派生于 Base ，那么：

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Base *bp, b_ob; Derived *dp, d_ob;   bp = &d_ob;           // 基类型的指针可以指向派生类的对象   dp = dynamic_cast<Derived *> (bp);   //强制转换基类指针为派生类指针，做法正确

上面，将基类指针 bp 强制转换为派生类指针 dp 是可以的，这是因为 bp 指向的对象确实是一个 Derived 类型的对象。

下面的是强制转换失败的示例：

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1 2	bp = &b_ob; dp = dynamic_cast<Derived *>(bp);    //错误

现在 bp 是一个指向基类对象的指针，这时不能 dynamic_cast 将其强制转换为派生类对象的指针，因为派生类对象的指针不能指向基类的对象。

下面是代码测试多种 dynamic_cast 的情况：

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// dynamicast.cpp : 定义控制台应用程序的入口点。 //   #include "stdafx.h" #include <iostream> using namespace std;   class Base { public:     virtual void f() { cout << "Inside Base.\n"; } };   class Derived : public Base { public:     void f() { cout << "Inside Derived\n"; } };   int _tmain(int argc, _TCHAR* argv[]) {     Base *bp, b_ob;     Derived *dp, d_ob;       dp = dynamic_cast<Derived *>(&d_ob);    //派生类对象的指针强制转换为派生类对象的指针 OK     if (dp) {         cout << "Cast from Derived * to Derived * OK.\n";         dp->f();     } else         cout << "Error\n";     cout << endl;       bp = dynamic_cast<Base *>(&d_ob);  //派生类对象的指针强制转换为基类的指针 OK(基类指针可以指向派生类的对象)     if (bp) {         cout << "Cast from Derived * to Base * OK.\n";             bp->f();     } else         cout << "Error\n";     cout << endl;       bp = dynamic_cast<Base *>(&b_ob);   //基类的指针强制转换为基类的指针 OK     if (bp) {         cout << "Cast from Base * to Base * OK.\n";         bp->f();     } else         cout << "Error\n";     cout << endl;       dp = dynamic_cast<Derived *>(&b_ob);    //基类对象的指针(地址)强制转换为派生类的指针(失败)     if(dp)         cout << "Error\n";     else         cout << "Cast from Base * to Derived * not OK.\n";           cout << endl;       bp = &d_ob;        //bp 指向派生类的对象           dp = dynamic_cast<Derived *> (bp);       //可以，因为 bp 确实已经指向了派生类     if(dp) {         cout << "Casting bp to Derived * OK\n" << "because bp is really pointing\n"              << "to a Derived object.\n";              dp->f();     }else         cout << "Error\n";     cout << endl;       bp = &b_ob;        //bp 指向基类型对象     dp = dynamic_cast<Derived *> (bp);  //失败,因为 bp 现在已经确实指向了基类对象     if(dp)         cout << "Error";     else {         cout << "Now casting bp to a Derived *\n"               << "is not OK because bp is really \n"               << "pointing to a Base object.\n";     }     cout << endl;       dp = &d_ob;        //dp 指向派生类对象     bp = dynamic_cast<Base *> (dp);    //可以,基类指针可以指向派生类对象     if (bp) {         cout << "Casting dp to a Base * is OK.\n";         bp->f();     }else         cout << "Error\n";       return 0; }

运行输出：

Cast from Derived * to Derived * OK.
Inside Derived

Cast from Derived * to Base * OK.
Inside Derived

Cast from Base * to Base * OK.
Inside Base.

Cast from Base * to Derived * not OK.

Casting bp to Derived * OK
because bp is really pointing
to a Derived object.
Inside Derived

Now casting bp to a Derived *
is not OK because bp is really
pointing to a Base object.

Casting dp to a Base * is OK.
Inside Derived