C++四种类型转换

2019-11-06 06:41:41

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C风格的强制类型转换，格式如下两种： *①Class(exPRession); ②(Class)expression;* 第一：不安全，没有类型检查；第二：可读性差，在代码中不容易发现做了类型转换。

C++的四类转换：reinterpret_cast，const_cast，static_cast，dynamic_cast。分别对应于不同的应用场景。

static_cast 这里写图片描述

void testStatic_Cast(){ // 基本类型的转换 cout << "Begin of basic Type Cast--------------" << endl; char p = 'a'; cout << "p::" << p << endl; int i = static_cast<int>(p); cout << "i::" << i << endl; char q = static_cast<char>(i); cout << "q::" << q << endl; cout << "End of basic Type Cast--------------" << endl; // void*与别的指针类型之间的转换 cout << "Begin of void* Type Cast--------------" << endl; int ii = 2; void* pvii = &ii; int* pii = static_cast<int*>(pvii); cout << ii << endl; cout << *pii << endl; cout << "End of void* Type Cast--------------" << endl; // 向上类型转换（安全） cout << "Begin of Inherit Type Castcout--------------upcast" << endl; Child child2; Parent* parent2 = static_cast<Parent*>(&child2); parent2->sayHello(); parent2->setChildNum(2); cout << "Get Parent member::" << parent2->getChildNum() << endl; cout << "End of Inherit Type Castcout--------------upcast" << endl; // 向下类型转换（不安全） cout << "Begin of Inherit Type Castcout--------------downcast" << endl; Parent parent1; Child* child1 = static_cast<Child*>(&parent1); /*child1->sayHello(); child1->sayDad(); child1->setChildNum(2); child1->setBrotherNum(3);*/ cout << "Get Parent member::" << child1->getChildNum() << endl; cout << "Get Child member::" << child1->getBrotherNum() << endl; cout << "End of Inherit Type Castcout--------------downcast" << endl;}

对于static_cast来讲，对象指针的转换，向上转换是安全的，向下转换则不安全。运行上面的代码，向下转换的时候代码可以执行，但执行完以后会报错如下：这里写图片描述

const_cast 这里写图片描述

#pragma onceclass ConstCastDemo{public: ConstCastDemo(void); ~ConstCastDemo(void); void setNum(int num); int getNum() const;private: int m_nNum;};#include "StdAfx.h"#include "ConstCastDemo.h"ConstCastDemo::ConstCastDemo(void) : m_nNum(0){}ConstCastDemo::~ConstCastDemo(void){}void ConstCastDemo::setNum(int num){ m_nNum = num;}int ConstCastDemo::getNum() const{ return m_nNum;}void testConst_Cast(){ const ConstCastDemo demo; cout << "Num from demo::" << demo.getNum() << endl; // 由于为const对象，所以不可执行非const成员方法 // demo.setNum(100); // 不可以直接进行对象的转换（转换后会成为另一个对象？） /*ConstCastDemo demo1 = const_cast<ConstCastDemo>(demo); demo1.setNum(100); cout << "Num from demo::" << demo.getNum() <<endl; cout << "Num from demo1::" << demo1.getNum() <<endl;*/ // 进行指针的转换，脱去const ConstCastDemo* demo2 = const_cast<ConstCastDemo*>(&demo); demo2->setNum(200); cout << "Num from demo::" << demo.getNum() << endl; cout << "Num from demo2::" << demo2->getNum() << endl; // 进行引用的转换，脱去const ConstCastDemo& demo3 = const_cast<ConstCastDemo&>(demo); demo3.setNum(300); cout << "Num from demo::" << demo.getNum() << endl; cout << "Num from demo3::" << demo3.getNum() << endl;}

dynamic_cast 这里写图片描述

#pragma onceclass Parent{public: Parent(void); virtual ~Parent(void); void sayHello(); void setChildNum(int num); int getChildNum();private: int m_nChildNum;};#include "StdAfx.h"#include "Parent.h"#include <iostream>using namespace std;Parent::Parent(void){}Parent::~Parent(void){}void Parent::sayHello(){ cout << "Parent::say::" << "hello, boy!" << endl;}void Parent::setChildNum(int num){ m_nChildNum = num;}int Parent::getChildNum(){ return m_nChildNum;}#include "Parent.h"#pragma onceclass Child : public Parent{public: Child(void); virtual ~Child(void); void sayDad(); void setBrotherNum(int num); int getBrotherNum();private: int m_nBrotherNum;};#include <iostream>using namespace std;Child::Child(void) : Parent(){}Child::~Child(void){}void Child::sayDad(){ cout << "Child::say::" << "Hello, dad!" << endl;}void Child::setBrotherNum(int num){ m_nBrotherNum = num;}int Child::getBrotherNum(){ return m_nBrotherNum;}void testDynamic_Cast(){ /* ①dynamic_cast作用于类指针或者类引用； ②dynamic_cast用于向上转换或者向下转换； ③指针转换的时候，如果失败，返回NULL指针； ④引用转换的时候，如果失败，则跑出bad_cast异常； */ cout << "Begin to test dynamic_cast--------------Parent to Child" << endl; Parent* parent1 = new Parent(); Child* child1 = dynamic_cast<Child*>(parent1); if (child1 == NULL) { cout << "pointer dynamic_cast downcast failed!" << endl; } else { cout << "pointer dynamic_cast downcast succeed!" << endl; } /*Child& child2 = dynamic_cast<Child&>(*parent1);*/ Parent& parent2 = *parent1; try { Child& child2 = dynamic_cast<Child&>(parent2); cout << "reference dynamic_cast downcast succeed!" << endl; } catch (...) { cout << "reference dynamic_cast throw exception!" << endl; } cout << "End of test dynamic_cast--------------Parent to Child" << endl; cout << "Begin to test dynamic_cast--------------Child to Parent" << endl; Child* child3 = new Child(); Parent* parent3 = dynamic_cast<Parent*>(child3); if (parent3 == NULL) { cout << "pointer dynamic_cast upcast failed!" << endl; } else { cout << "pointer dynamic_cast upcast succeed!" << endl; } Child& child4 = *child3; try { Parent& parent4 = dynamic_cast<Parent&>(child4); cout << "reference dynamic_cast upcast succeed!" << endl; } catch (...) { cout << "reference dynamic_cast throw exception!" << endl; } cout << "End of test dynamic_cast--------------Child to Parent" << endl;}

reinpreter_cast 这里写图片描述