C++/面试 - 四种类型转换(cast)的关键字 详解 及 代码

四种类型转换(cast)的关键字 详解 及 代码

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C++中常见的面试题.

题目: 关于C++中, 四种类型转换的关键字, 的详解, 也可以给出代码, 判断输出 或 判断哪些代码有误.

答案及范例如下:

四种关键字: const_cast, 常量性转除;dynamic_cast, 向下安全转型; reinterpret_cast, 重新解释转型; static_cast, 静态转型;

1.  const_cast, 常量性转除:

主要对变量的常量性(const)进行操作, 移除变量的常量性, 即可以被非常量指向和引用, 详见代码;

2. dynamic_cast, 向下安全转型:

主要应用于继承体系, 可以由 "指向派生类的基类部分的指针", 转换"指向派生类"或"指向兄弟类";

static_cast只能转换为"指向派生类";

3. reinterpret_cast, 重新解释转型:

主要是对2进制数据进行重新解释(re-interpret),不改变格式, 而static_cast会改变格式进行解释; 

如由派生类转换基类, 则重新解释转换, 不改变地址, 静态转换改变地址;

4. static_cast, 静态转型:

主要是数据类型的转换, 还可以用于继承;

代码如下:

/*  * cppprimer.cpp  *  *  Created on: 2014.2.10  *      Author: Spike  */  /*eclipse cdt, gcc 4.8.1*/  #include <iostream>  /*常量性移除指针详解*/  struct S { 	S() : value(0) {} 	int value; };  void CastConst (void) { 	const S s; 	std::cout << "s.value = " << s.value << std::endl; 	//S* ps = &s; //error, 指向常量 	S* ps = const_cast<S*>(&s); 	ps->value = 1; 	std::cout << "s.value = " << s.value << std::endl; 	//S& rs = s; //error, 引用常量 	S& rs = const_cast<S&>(s); 	rs.value = 2; 	std::cout << "s.value = " << s.value << std::endl; }  /*安全向下转型*/  struct B /*基类B*/ { 	virtual void f() { std::cout << "Base::f" << std::endl; } 	void thisf() { std::cout << "Base::thisf" << std::endl;} 	virtual ~B() {} };  struct B2 /*基类B2*/ { 	virtual void g() { std::cout << "Base2::g" << std::endl; } 	void thisg() { std::cout << "Base2::thisg" << std::endl;} 	virtual ~B2() {} };  struct D : public B, public B2 /*派生类D*/ { 	virtual void f() { std::cout << "Derived::f" << std::endl; } 	virtual void g() { std::cout << "Derived::g" << std::endl; } 	virtual ~D() {} };  void CastDynamic (void) { 	 B* pB_D = new D; 	 pB_D->f(); 	 //pD->g(); //error, 只包含B部分  	 D *pD_D = dynamic_cast<D*>(pB_D); //转换为派生类 	 pD_D->g(); 	 B2* pB2_D = dynamic_cast<B2*>(pB_D); //转换为兄弟类 	 pB2_D->g();  	 D *pD_Ds = static_cast<D*>(pB_D); 	 pD_Ds->g(); 	 //B2* pB2_Ds = static_cast<B2*>(pB_D); //error, 不能转换为兄弟类 }  /*重新解释转型*/  struct rA { int m_a; }; struct rB { int m_b; }; struct rC : public rA, public rB {};  void CastReinterpret (void) { 	int *i= new int; 	*i = 10; 	std::cout << "*i = " << *i << std::endl; 	std::cout << "i = " << i << std::endl; 	double *d=reinterpret_cast<double*> (i); 	std::cout << "*d = " << *d << std::endl; 	std::cout << "d = " << d << std::endl;  	rC c; 	std::cout << "&c = " << &c << std::endl 			<< "reinterpret_cast<rB*>(&c) = " <<reinterpret_cast<rB*>(&c) << std::endl 			<< "static_cast <rB*>(&c) = " << static_cast <rB*>(&c) << std::endl 			<< "reinterpret_cast<rA*>(&c) = " <<reinterpret_cast<rA*>(&c) << std::endl 			<< "static_cast <rA*>(&c) = " << static_cast <rA*>(&c) << std::endl 			<< std::endl; }  int main (void) { 	std::cout << std::endl << "常量性转除:" << std::endl; 	CastConst(); 	std::cout << std::endl << "安全向下转型:" << std::endl; 	CastDynamic(); 	std::cout << std::endl << "重新解释转型:" << std::endl; 	CastReinterpret(); } 

输出:

常量性转除: s.value = 0 s.value = 1 s.value = 2  安全向下转型: Derived::f Derived::g Derived::g Derived::g  重新解释转型: *i = 10 i = 0x471718 *d = 2.55917e-307 d = 0x471718 &c = 0x22feb0 reinterpret_cast<rB*>(&c) = 0x22feb0 static_cast <rB*>(&c) = 0x22feb4 reinterpret_cast<rA*>(&c) = 0x22feb0 static_cast <rA*>(&c) = 0x22feb0