C++ 17 翁恺＞拷贝构造

2022-05-05 08:05:13

#include<iostream>
#include<string>
using namespace std;

static int objectCount = 0;

class HowMany
{
public:
	HowMany() { objectCount++; print("HowMany()"); }
	void print(const string& msg="")
	{
		if (msg.size() != 0)cout << msg << ":";
		cout << "objectCount = " << objectCount << endl;
	}
	~HowMany() { objectCount--; print("~HowMany()"); }
};


HowMany f(HowMany x)
{
	cout << "begin of f()" << endl;
	x.print("x argument inside f()");
	cout << "end of f()" << endl;
	return x;
}

int main()
{
	HowMany h;
	h.print("after construction of h");
	HowMany h2 = f(h);//没调用默认构造函数，但f()运行完调用了一次析构，所以f()运行时一定也调用了默认构造函数，只是我们没看到
	//h2的创建也绕过了默认构造函数
	h.print("after call to f()");
	return 0;
}

#include<iostream>
#include<string>
using namespace std;

static int objectCount = 0;

class HowMany
{
public:
	HowMany() { objectCount++; print("HowMany()"); }
	void print(const string& msg="")
	{
		if (msg.size() != 0)cout << msg << ":";
		cout << "objectCount = " << objectCount << endl;
	}
	~HowMany() { objectCount--; print("~HowMany()"); }
};


HowMany f(HowMany x)
{
	cout << "begin of f()" << endl;
	x.print("x argument inside f()");
	cout << "end of f()" << endl;
	return x;
}

int main()
{
	HowMany h;
	h.print("after construction of h");
	//HowMany h2 = f(h);//没调用默认构造函数，但f()运行完调用了一次析构，所以f()运行时一定也调用了默认构造函数，只是我们没看到
	//h2的创建也绕过了默认构造函数

	HowMany h3 = h;//用h构造出一个新的h3，但是构造时没有经过默认构造函数

	h.print("after call to f()");

	//最后又调用了两次析构
	return 0;
}

#include<iostream>
#include<string>
using namespace std;

static int objectCount = 0;

class HowMany
{
public:
	HowMany() { objectCount++; print("HowMany()"); }
	HowMany(int i) { objectCount++; print("HowMany(int)"); }
	void print(const string& msg="")
	{
		if (msg.size() != 0)cout << msg << ":";
		cout << "objectCount = " << objectCount << endl;
	}
	~HowMany() { objectCount--; print("~HowMany()"); }
};


HowMany f(HowMany x)
{
	cout << "begin of f()" << endl;
	x.print("x argument inside f()");
	cout << "end of f()" << endl;
	return x;
}

int main()
{
	HowMany h;
	h.print("after construction of h");
	HowMany h2 = 10;
    //HowMany h2(10);//同上，在c++中初始化对象用 () 或 = 是等价的
	//最后又调用了两次析构
	return 0;
}

#include<iostream>
#include<string>
using namespace std;

static int objectCount = 0;

class HowMany
{
public:
	HowMany() { objectCount++; print("HowMany()"); }
	HowMany(int i) { objectCount++; print("HowMany(int)"); }
	HowMany(const HowMany& o) { objectCount++; print("HowMany(HowMany)"); }
	void print(const string& msg="")
	{
		if (msg.size() != 0)cout << msg << ":";
		cout << "objectCount = " << objectCount << endl;
	}
	~HowMany() { objectCount--; print("~HowMany()"); }
};


HowMany f(HowMany x)
{
	cout << "begin of f()" << endl;
	x.print("x argument inside f()");
	cout << "end of f()" << endl;
	return x;
}

int main()
{
	HowMany h;
	h.print("after construction of h");
	HowMany h2 = h;
	//最后又调用了两次析构
	return 0;
}

#include<iostream>
#include<string>
using namespace std;

static int objectCount = 0;

class HowMany
{
public:
	HowMany() { objectCount++; print("HowMany()"); }
	HowMany(int i) { objectCount++; print("HowMany(int)"); }
	HowMany(const HowMany& o) { objectCount++; print("HowMany(HowMany)"); }
	void print(const string& msg="")
	{
		if (msg.size() != 0)cout << msg << ":";
		cout << "objectCount = " << objectCount << endl;
	}
	~HowMany() { objectCount--; print("~HowMany()"); }
};


HowMany f(HowMany x)
{
	cout << "begin of f()" << endl;
	x.print("x argument inside f()");
	cout << "end of f()" << endl;
	return x;
}

int main()
{
	HowMany h;
	h.print("after construction of h");
	HowMany h2 = f(h);
	//最后又调用了三次析构
	return 0;
}

HowMany（const HowMany& ）叫做 拷贝构造 函数。

拷贝构造 函数：成员级别的拷贝（不是字节对字节的拷贝），成员里有其它类的成员，继续调用其它类的拷贝构造进行拷贝，没有显示给出就会调用默认的。

调用默认拷贝，如果有指针，两个指针指向同一块内存；如果有引用，两个引用绑定同一个变量。

Copy pointer：

#ifndef PERSON_H_
#define PERSON_H_
class Person
{
public:
	Person(const char* s);//构造函数
	~Person();//析构函数
	void print();//函数原型，没有实际body，有一些什么样的函数
	//...
//private:
	char* name ;//数据成员 char* instead of string
	//...
};
#endif // !PERSON_H_

//person.h

#include "person.h"
#include<iostream>
#include<string.h>
using namespace std;

//在.cpp文件中定义在.h文件中声明的那些东西的实体
Person::Person(const char* s)
{
	name = new char[::strlen(s)+1];
	::strcpy(name,s);
}
Person::~Person()
{
	//delete[] name;
}

void Person::print()
{
	cout << "something" << endl;
}
//person.cpp

#include<stdio.h>
#include "person.h"
using namespace std;

int main()
{
	Person p1("Jhon");
	Person p2 = p1;//Person p2(p1);
	printf("p1.name=%p\n", p1.name);
	printf("p2.name=%p\n", p2.name);
	return 0;
}
//main.cpp

指向同一块内存：

p1 和 p2 指向同一块内存，即 Copy pointer，而我们想要 Copy entire block。

Copy entire block ：

#ifndef PERSON_H_
#define PERSON_H_
class Person
{
public:
	Person(const char* s);//构造函数
	Person(const Person& w);//拷贝构造函数
	~Person();//析构函数
	void print();//函数原型，没有实际body，有一些什么样的函数
	//...
//private:
	char* name ;//数据成员 char* instead of string
	//...
};
#endif // !PERSON_H_
//person.h

#include "person.h"
#include<iostream>
#include<string.h>
using namespace std;

//在.cpp文件中定义在.h文件中声明的那些东西的实体
Person::Person(const char* s)
{
	name = new char[::strlen(s)+1];
	::strcpy(name,s);
}
Person::Person(const Person& w)
{
	name = new char[::strlen(w.name) + 1];
	::strcpy(name, w.name);
}
Person::~Person()
{
	//delete[] name;
}

void Person::print()
{
	cout << "something" << endl;
}
//person.cpp

#include<stdio.h>
#include "person.h"
using namespace std;

int main()
{
	Person p1("Jhon");
	Person p2 = p1;//Person p2(p1);
	printf("p1.name=%p\n", p1.name);
	printf("p2.name=%p\n", p2.name);
	return 0;
}
//main.cpp

指向不同内存：

什么时候发生拷贝构造：初始化的时候。

什么时候调用拷贝构造：1.函数参数是一个对象的时候，隐藏调用。2.初始化时。3.函数返回一个对象时。

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