（二）C++中的類與建構函式

摘要： C++中的類與建構函式 一、C++中類的常用寫法 1、在標頭檔案中宣告類的屬性和方法 如：MyTeacher.h //防止重複引入 #pragma once class MyTeacher{ private: char* name; int ...

C++中的類與建構函式

一、C++中類的常用寫法

1、在標頭檔案中宣告類的屬性和方法

如：MyTeacher.h

//防止重複引入
#pragma once
class MyTeacher{
private:
char* name;
int age;
public:
void setName(char* name);
char* getName();
void setAge(int age);
int getAge();
};

2、在原始檔中（.cpp）實現對應的方法

如：MyTeacher.cpp

#define _CRT_SECURE_NO_WARNINGS
#include "MyTeacher.h"
#include <iostream>

void MyTeacher::setName(const char* name) {
this->name = new char[100];
strcpy(this->name, name);
}

char* MyTeacher::getName() {
return this->name;
}

void MyTeacher::setAge(int age) {
this->age = age;
}

int MyTeacher::getAge() {
return this->age;
}

3、在具體C++程式碼中使用

如：test.cpp

#include "MyTeacher.h"
using namespace std;

void main() {
MyTeacher t;
t.setName("Jack");
t.setAge(25);
cout << t.getName() << "," << t.getAge() << endl;
getchar();
}

二、C++中類的建構函式、解構函式、拷貝建構函式

1、建構函式

• C++類預設有無參建構函式，重寫無參建構函式會覆蓋預設的無參建構函式
• 有參建構函式會覆蓋預設無參建構函式

class Teacher{
private:
char* name;
int age;
public:
//無參建構函式
Teacher(){
cout << "無參建構函式" << endl;
}

//有參建構函式
Teacher(char *name,int age){
this->name = name;
this->age = age;
cout << "有參建構函式" << endl;
}
};

void main(){
Teacher t1;
Teacher t2("Rose",23);
//有參建構函式另一個呼叫方式
Teacher t3 = Teacher("jack",21);
getchar();
}

2、解構函式

當物件要被系統釋放時，解構函式會被呼叫，一般用作善後處理

class Teacher{
private:
char* name;
int age;
public:
//無參建構函式
Teacher(){
this->name = (char*)malloc(100);
strcpy(name,"Hello,Jack Teacher");
this->age = 25;
cout << "無參建構函式" << endl;
}
//解構函式
~Teacher(){
cout << "解構函式" << endl;
free(this->name);
}

//有參建構函式
Teacher(char *name,int age){
this->name = name;
this->age = age;
cout << "有參建構函式" << endl;
}
};

void func(){
Teacher t1;
}

void main(){
func();

getchar();
}

3、拷貝建構函式

3.1 淺拷貝

• 預設拷貝建構函式，就是值拷貝（淺拷貝）
• 淺拷貝拷貝的是指標的地址（同一塊記憶體區域）

using namespace std;
class Teacher {
private:
char* name;
int age;
public:
//有參建構函式
Teacher(char *name, int age) {
this->name = name;
this->age = age;
cout << "有參建構函式" << endl;
}
//解構函式
~Teacher() {
cout << "解構函式" << endl;
free(this->name);
}

//拷貝建構函式
Teacher(const Teacher &obj) {
this->name = obj.name;
this->age = obj.age;
cout << "拷貝建構函式" << endl;
}

void myprint() {
cout << name << "," << age << endl;
}
};


void main() {
Teacher t1((char*)"rose", 30);
Teacher t2 = t1;
t2.myprint();
getchar();
}

輸出

有參建構函式
拷貝建構函式
rose,30

• 淺拷貝有可能會出現第二次釋放變數導致出現異常的情況

例如：出現異常的情況

using namespace std;
class Teacher {
private:
char* name;
int age;
public:
//有參建構函式
Teacher(char *name, int age) {
int len = strlen(obj.name);
this->name = (char*)malloc(len+1);
strcpy(this->name,name);
this->age = age;
cout << "有參建構函式" << endl;
}
//解構函式
~Teacher() {
cout << "解構函式" << endl;
free(this->name);
}



void myprint() {
cout << name << "," << age << endl;
}
};

void func(){
Teacher t1((char*)"rose", 30);
Teacher t2 = t1;
t2.myprint();
} 

void main() {
func();
getchar();
}

3.2 深拷貝

深拷貝拷貝的是指標指向的資料內容（兩塊記憶體區域）

using namespace std;
class Teacher {
private:
char* name;
int age;
public:
//有參建構函式
Teacher(char *name, int age) {
int len = strlen(obj.name);
this->name = (char*)malloc(len+1);
strcpy(this->name,name);
this->age = age;
cout << "有參建構函式" << endl;
}
//解構函式
~Teacher() {
cout << "解構函式" << endl;
free(this->name);
}

//拷貝建構函式(深拷貝)
Teacher(const Teacher &obj) {
//複製name屬性
int len = strlen(obj.name);
this->name = (char*)malloc(len+1);
strcpy(this->name,obj.name);
this->age = obj.age;
cout << "拷貝建構函式" << endl;
}

void myprint() {
cout << name << "," << age << endl;
}
};

void func(){
Teacher t1((char*)"rose", 30);
//宣告時會被呼叫
Teacher t2 = t1;
//下面這種方式不會被呼叫
//Teacher t1;
//Teacher t2;
//t1 = t2;
t2.myprint();
}

void main() {
func();
getchar();
}

3.3 拷貝建構函式被被呼叫的場景

1. 宣告時賦值
2. 作為引數傳入，實參給形參賦值
3. 作為函式返回值返回，給變數初始化賦值

4、建構函式屬性初始化列表

建構函式屬性初始化列表的格式為:

[建構函式名稱]([本類中屬性列表],[第一個類物件的屬性列表],[第二個類物件的屬性列表]):[第一個類物件]([第一個類物件的屬性列表]),[第二個類物件]([第二個類物件的屬性列表]){

}

示例如下：

class Teacher{
private:
char* name;
public:
Teacher(char* name){
this->name = name;
cout << "Teacher有參建構函式" << endl;
}
~Teacher(){
cout << "Teacher解構函式" << endl;
}
char* getName(){
return this->name;
}
};

class Student{
private:
int id;
Teacher t1;
public:
Student(int id,char* t1_name) : t1(t1_name){
this->id = id;
cout << "Student有參建構函式" << endl;
}
~Student(){
cout << "Student解構函式" << endl;
}
void myprint(){
cout << this->id << "同學的老師是" << t1.getName() << endl;
}
};

void func(){
Student t1(16,(char*)"Jack");
t1.myprint();
}

void main(){
func();

getchar();
}

三、C++中的new 和 delete的使用

C++中通過new和delete來進行動態記憶體分配,new 和delete成對出現

C++中在使用new和delete會對應呼叫建構函式和析構引數，通過C中的malloc方式則不會呼叫建構函式和解構函式

class Teacher{
private:
char* name;
public:
Teacher(char* name){
this->name = name;
cout << "Teacher有參建構函式" << endl;
}
~Teacher(){
cout << "Teacher解構函式" << endl;
}
void setName(char* name){
this->name = name;
}
char* getName(){
return this->name;
}
};

void func(){
//初始化，返回指標地址
Teacher *t1 = new Teacher((char*)"Jack");
//使用
//釋放
delete t1;
int *p2 = new int[10];
p2[0] = 11;
//釋放陣列
delete p2;
}

void main(){
func();

getchar();
}