C++堆与拷贝构造函数

c++创建malloc

• 简单版

#include<iostream>
using namespace std;
class Test
{
public:
    Test()
    {
        cout<<"Test construct"<<endl;
    }
    ~Test()
    {
        cout<<"~~~~~Test"<<endl;
    }
};
int main()
{
    Test *t=new Test;
    cout<<"========="<<endl;
    delete t;
}

//结果为:Test construct
//===========
//~~~~~~~~~Test

#include<iostream>
#include<stdlib.h>
using namespace std;
class Test
{
public:
    Test(int a)
    {
        cout<<"Test construct"<<a<<endl;
    }
    ~Test()
    {
        cout<<"~~~~~Test"<<endl;
    }
};
int main()
{
    Test *t=new Test(5);
    cout<<"========="<<endl;
    delete t;
}

//结果为:Test construct5
//===========
//~~~~~~~~~Test

#include<iostream>
#include<stdlib.h>
using namespace std;
class Test
{
public:
    Test(int a)
    {
        cout<<"Test construct"<<a<<endl;
    }
    Test()
    {
        cout<<"Test construct"<<endl;
    }
    ~Test()
    {
        cout<<"~~~~~Test"<<endl;
    }
};
int main()
{
    Test *t=new Test[5];//只能调用无参的
    cout<<"========="<<endl;
    delete[] t;
}
//运行结果如下
//Test contruct
//Test contruct
//Test contruct
//Test contruct
//Test contruct
//===========
//~~~~~~~~~Test
//~~~~~~~~~Test
//~~~~~~~~~Test
//~~~~~~~~~Test
//~~~~~~~~~Test

使用堆空间的原因

1. 直到运行时才能知道需要多少对象空间；
2. 不知道对象的生存期到底有多长；
3. 直到运行时才知道一个对象需要多少内存空间。

拷贝构造函数

1. 系统默认的拷贝构造函数

#include<iostream>
using namespace std;
class Test
{
    int m_a;
public:
    Test(int a)
    {
        m_a=a;
        cout<<"Test construct"<<a<<endl;
    }
    ~Test()
    {
        cout<<"~~~~Test"<<endl;
    }
    void show()
    {
        cout<<"m_a = "<<m_a<<endl;
    }
    Test(const Test &q)
    {
        m_a=q.m_a;
    }
};

int main()
{
    Test t1(10);
    t1.show();
    Test t2(t1);
    t2.show();
}

//结果为:
//Test contruct10;
//m_a=10;
//m_a=10;
//~~~~~~~~Test
//~~~~~~~Test

2. 系统默认的构造函数

#include<iostream>
using namespace std;
class Test
{
    int m_a;
public:
    Test(int a)
    {
        m_a=a;
        cout<<"Test construct"<<a<<endl;
    }
    ~Test()
    {
        cout<<"~~~~Test"<<endl;
    }
    void show()
    {
        cout<<"m_a = "<<m_a<<endl;
    }
    /*Test(const Test &q)
    {
        m_a=q.m_a;
    }*////默认构造函数只能完成浅拷贝，将形参传给自身

};
void hello(Test temp)
{
    temp.show();
}

int main()
{
    Test t1(10);
    t1.show();
    Test t2(t1)
    t2.show();
    hello(t1);
}


//结果为:
//Test contruct10;
//m_a=10;
//m_a=10;
//~~~~~~~~Test
//~~~~~~~Test

• 深拷贝

#include<iostream>
using namespace std;
class Test
{
    int *m_a;//定义指针类型时，系统默认的拷贝构造函数已经过不去了
public:
    Test(int a)
    {
        m_a=new int;
        *m_a=a;
        cout<<"Test construct"<<*m_a<<endl;
    }
    ~Test()
    {
        cout<<"~~~~Test"<<*m_a<<endl;
        delete m_a;
    }
    void show()
    {
        cout<<"m_a = "<<*m_a<<endl;
    }
    Test(const Test &q)//所以要认为的重新定义
    {
        m_a=new int;
        *m_a=*q.m_a;
    }
};
int main()
{
    Test t1(10);
    t1.show();
    Test t2(t1);
    t2.show();
}


//结果为:
//Test contruct10;
//m_a=10;
//m_a=10;
//~~~~~~~~Test10
//~~~~~~~Test10