C++四種類型轉換

2019-11-06 06:41:41

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C風格的強制類型轉換，格式如下兩種： *①Class(exPRession); ②(Class)expression;* 第一：不安全，沒有類型檢查；第二：可讀性差，在代碼中不容易發現做了類型轉換。

C++的四類轉換：reinterpret_cast，const_cast，static_cast，dynamic_cast。分別對應于不同的應用場景。

static_cast 這里寫圖片描述

void testStatic_Cast(){ // 基本類型的轉換 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*與別的指針類型之間的轉換 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; // 向上類型轉換（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; // 向下類型轉換（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;}

對于static_cast來講，對象指針的轉換，向上轉換是安全的，向下轉換則不安全。運行上面的代碼，向下轉換的時候代碼可以執行，但執行完以后會報錯如下：這里寫圖片描述

const_cast 這里寫圖片描述

#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; // 由于為const對象，所以不可執行非const成員方法 // demo.setNum(100); // 不可以直接進行對象的轉換（轉換后會成為另一個對象？） /*ConstCastDemo demo1 = const_cast<ConstCastDemo>(demo); demo1.setNum(100); cout << "Num from demo::" << demo.getNum() <<endl; cout << "Num from demo1::" << demo1.getNum() <<endl;*/ // 進行指針的轉換，脫去const ConstCastDemo* demo2 = const_cast<ConstCastDemo*>(&demo); demo2->setNum(200); cout << "Num from demo::" << demo.getNum() << endl; cout << "Num from demo2::" << demo2->getNum() << endl; // 進行引用的轉換，脫去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 這里寫圖片描述

#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作用于類指針或者類引用； ②dynamic_cast用于向上轉換或者向下轉換； ③指針轉換的時候，如果失敗，返回NULL指針； ④引用轉換的時候，如果失敗，則跑出bad_cast異常； */ 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 這里寫圖片描述