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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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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.
"; }};class Derived : public Base {public: void f() { cout << "Inside Derived
"; }};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.
"; dp->f(); } else cout << "Error
"; 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.
"; bp->f(); } else cout << "Error
"; 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.
"; bp->f(); } else cout << "Error
"; 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
"; else cout << "Cast from Base * to Derived * not OK.
"; 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
" << "because bp is really pointing
" << "to a Derived object.
"; dp->f(); }else cout << "Error
"; 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 *
" << "is not OK because bp is really
" << "pointing to a Base object.
"; } 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.
"; bp->f(); }else cout << "Error
"; 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