拷贝构造函数

拷贝构造函数的用途

拷贝构造函数，它由编译器调用来完成一些基于同一类的其他对象的构建及初始化。其唯一的形参必须是引用，但并不限制为const，一般普遍的会加上const限制。

拷贝构造函数发生的时机

1，用一个对象初始化另一个对象
     　　  Cat c1（）;
       　　Cat c2(c1);
2，函数按值传递 （实参--->形参）
3，函数返回对象

默认拷贝构造函数

如果程序员不提供拷贝构造函数，编译器会提供默认的拷贝构造函数，将对应的数据成员逐一赋值。（默认拷贝构造函数不能用于2种特殊情况，程序员必修提供拷贝构造函数）

2种特殊情况

//Aircraft.h
#include<string>
#include<iostream>
class Aircraft {
private:
    static int count;//飞行器的总量
    std::string color;//飞行器的颜色
public:
    Aircraft(const std::string &color);
    Aircraft(const Aircraft &A);
    ~Aircraft();
    static int getCount();//静态成员不允许使用const
};

//Aircraft.cpp
#include<string>
#include<iostream>
#include"Aircraft.h"
using namespace std;

int Aircraft::count = 0;

Aircraft::Aircraft(const string &color) {
    ++count;
    this->color = color;
    cout << color << " is created
";
}
Aircraft::Aircraft(const Aircraft &A) {
    ++count;
    color = A.color;
    cout << color << " copy
";
}
Aircraft::~Aircraft() {
    --count;
    cout << color << " is destroied
";
}
int Aircraft::getCount() {
    return count;
}

//main
#include<cstdlib>
#include<string.h>
#include<iostream>
#include"Aircraft.h"
using namespace std;

int main() {
    cout << "count is " << Aircraft::getCount() << endl;
    Aircraft a("red");
    cout << "count is " << Aircraft::getCount() << endl;
    Aircraft b = a;
    cout << "count is " << Aircraft::getCount() << endl;
    Aircraft *c = new Aircraft("black");
    cout << "count is " << Aircraft::getCount() << endl;
    delete c;
    {
        Aircraft d("orange");
        cout << "count is " << Aircraft::getCount() << endl;
    }
    cout << "count is " << Aircraft::getCount() << endl;
    system("pause");
}

count is 0
red is created
count is 1
red copy
count is 2
black is created
count is 3
black is destroied
orange is created
count is 3
orange is destroied
count is 2
请按任意键继续. . .

//深拷贝
class Store {
private:
    int count;
    int *p;
public:
    Store() {//构造函数
        count = 10;
        p = new int[10];
    }//拷贝构造函数
    Store(const Store &S) {
        count = S.count;
        p = new int[10];
        for (int i = 0; i < 10;++i) {
            p[i] = S.p[i];
        }
    }//析构函数
    ~Store() {
        delete[] p;
    }
};

 拷贝构造函数和重载的“=”

指针的值被复制了，但指针所指内容并未被复制。包含动态分配成员的类除提供拷贝构造函数外，还应该考虑重载"="赋值操作符号。

"="在对象声明语句中，表示初始化，这种初始化也可用圆括号表示。

"="表示赋值操作。将对象theObjone的内容复制到对象theObjthree，这其中涉及到对象theObjthree原有内容的丢弃，新内容的复制。

cat c2 = c1;会调用拷贝构造函数。

c2 = c1；调用的却是重载的“=”运算符。