向各位老大请教一个类初始化的问题。为啥不掉用我的拷贝构造函数呢。谢谢。

[yakeezhao]

下面代码的代码，在VC6及VC2008上测试(我这里只有VC这一种编译器）， 结果很奇怪，请各位老大不吝赐教。多谢多谢。

#include <iostream>
using namespace std;
class mystring
{
public:
mystring(char* p = NULL)
{
cout << "mystring(char*) = " << (NULL != p ? p : "NULL") << endl;
if(NULL == p)
{
m_pdata = new char[1];
*m_pdata = 0;
}
else
{
int ilen = strlen(p);
m_pdata = new char[ilen + 1];
strcpy(m_pdata, p);
}
}
~mystring()
{
if(NULL != m_pdata)
{
delete []m_pdata;
m_pdata = NULL;
}
}
mystring(const mystring& obj)
{
cout << "mystring(mystring&) = " << obj.m_pdata << endl;
mystring(obj.m_pdata);
}
const mystring& operator = (const mystring& rhs)
{
cout << "mystring operator = " << rhs.m_pdata << endl;
mystring(rhs.m_pdata);
return *this;
}
public:
char* m_pdata;
};
class mybase
{
public:
virtual ~mybase(){};
mybase() : m_sf8("base")
{
cout << "myfunc8_base : default constructor is necessary in parent class" << endl;
m_sf82 = "base2";
}
private:
mystring m_sf8;
mystring m_sf82;
};
class myderived : public mybase
{
public:
myderived(mystring& sf8) : m_sf8(sf8)
{
cout << "myderived : myderived(mystring&) : m_sf8 = " << m_sf8.m_pdata << endl;
}
private:
const mystring& m_sf8;
};
int main(int argc, char* argv[])
{
mystring ms81("1");
myderived m8d2(ms81);
return 0;
}

运行结果为：
1 -- mystring(char*) = 1
2 -- mystring(char*) = base
3 -- mystring(char*) = NULL
4 -- myfunc8_base : default constructor is necessary in parent class
5 -- mystring(char*) = base2
6 -- mystring operator = base2
7 -- mystring(char*) = base2
8 -- myderived : myderived(mystring&) : m_sf8 = 1

请教：
第 3 行 “mystring(char*) = NULL”是由哪句代码触发的呢？
第 5 行 “mystring(char*) = base2”又是由哪句代码导致的呢？
我觉得那两行结果不该出现啊。倒是代码 ‘myderived(mystring& sf8) : m_sf8(sf8)‘，我觉得应该触发mystring的拷贝构造函数 ’mystring(const mystring& obj)‘，可是这句却没出现。这是怎么一会事啊？类的初始化还真的很晕啊。谢谢各位了。

[一无所有自知之明]

#include <iostream>  
using namespace std;
class mystring
{
public:
mystring(char* p = NULL)
{  
cout << "mystring(char*) = " << (NULL != p ? p : "NULL") << endl;
if(NULL == p)
{
m_pdata = new char[1];
*m_pdata = 0;
}
else
{
int ilen = strlen(p);
m_pdata = new char[ilen + 1];
strcpy(m_pdata, p);
}
}
~mystring()
{
if(NULL != m_pdata)
{
delete []m_pdata;
m_pdata = NULL;
}
}
mystring(const mystring& obj)
{
cout << "mystring(mystring&) = " << obj.m_pdata << endl;
mystring(obj.m_pdata);
}
const mystring& operator = (const mystring& rhs)
{
cout << "mystring operator = " << rhs.m_pdata << endl;
mystring(rhs.m_pdata);
return *this;
}
public:
char* m_pdata;
};
class mybase
{
public:  
virtual ~mybase(){};
mybase() : m_sf8("base")
{
cout << "myfunc8_base : default constructor is necessary in parent class" << endl;
m_sf82 = "base2";
}
private:
mystring m_sf8;
mystring m_sf82;
};
class myderived : public mybase
{
public:  
myderived(mystring& sf8) : m_sf8(sf8)
{
cout << "myderived : myderived(mystring&) : m_sf8 = " << m_sf8.m_pdata << endl;
}
private:
const mystring& m_sf8;
};
int main(int argc, char* argv[])
{
mystring ms81("1");
myderived m8d2(ms81);
return 0;
}

[十八道胡同]

// ConsoleApplication15.cpp : Defines the entry point for the console application.
//

#include "stdafx.h"

#include <iostream>  
using namespace std;
class mystring
{
public:
	mystring(char* p = NULL)
	{  
		cout << "mystring(char*) = " << (NULL != p ? p : "NULL") << "  "<< 1<<endl;
		if(NULL == p)
		{
			m_pdata = new char[1];
			*m_pdata = 0;
		}
		else
		{
			int ilen = strlen(p);
			m_pdata = new char[ilen + 1];
			strcpy(m_pdata, p);
		}
	}
	~mystring()
	{
		if(NULL != m_pdata)
		{
			delete []m_pdata;
			m_pdata = NULL;
		}
	}
	mystring(const mystring& obj)
	{
		cout << "mystring(mystring&) = " << obj.m_pdata <<"  "<< 2<< endl;
		mystring(obj.m_pdata);
	}
	const mystring& operator = (const mystring& rhs)
	{
		cout << "mystring operator = " << rhs.m_pdata << "  "<<3<<endl;
		mystring(rhs.m_pdata);
		return *this;
	}
public:
	char* m_pdata;
};
class mybase
{
public:  
	virtual ~mybase(){};
	mybase() : m_sf8("base")
	{
		cout << "myfunc8_base : default constructor is necessary in parent class" << endl;
		m_sf82 = "base2";
	}
private:
	mystring m_sf8;
	mystring m_sf82;
};
class myderived : public mybase
{
public:  
	myderived(mystring& sf8) : m_sf8(sf8)
	{
		cout << "myderived : myderived(mystring&) : m_sf8 = " << m_sf8.m_pdata << "  "<< 4<<endl;
	}
private:
	const mystring& m_sf8;
};
int main(int argc, char* argv[])
{
	mystring ms81("1");
	myderived m8d2(ms81);
	return 0;
}

mystring(char*) = 1 1
mystring(char*) = base 1
mystring(char*) = NULL 1
myfunc8_base : default constructor is necessary in parent class
mystring(char*) = base2 1
mystring operator = base2 3
mystring(char*) = base2 1
myderived : myderived(mystring&) : m_sf8 = 1 4
Press any key to continue . . .

这样清晰了吧

[十八道胡同]

mystring ms81("1");
cout<<"------------------------------"<<endl;
myderived m8d2(ms81);

->

mystring(char*) = 1  1
------------------------------
mystring(char*) = base  1
mystring(char*) = NULL  1
myfunc8_base : default constructor is necessary in parent class
mystring(char*) = base2  1
mystring operator = base2  3
mystring(char*) = base2  1
myderived : myderived(mystring&) : m_sf8 = 1  4
Press any key to continue . . .

[yakeezhao]

谢谢老大的回复。可是我还是不太明白。老大能说详细点吗？谢谢

[yakeezhao]

我的意思是说，为什么会有第3行“mystring(char*) = NULL”和第5行 “mystring(char*) = base2的输出呢？

我觉得应该出现"mystring(mystring&) = 1"，但是却没有出现？

[十八道胡同]

mystring(char*) = NULL 是在初始化mystring m_sf82;

[十八道胡同]

m_sf82 = "base2";
这句触发了mystring的构造函数mystring(char* p = NULL)

[十八道胡同]

[Quote=引用 7 楼 lcl_data 的回复:]
m_sf82 = "base2";
这句触发了mystring的构造函数mystring(char* p = NULL)
[/Quote]
和=重载函数

[yakeezhao]

[Quote=引用 6 楼 lcl_data 的回复:]

mystring(char*) = NULL 是在初始化mystring m_sf82;
[/Quote]

多谢 LCL_data 老大。老大能不能再指点一下 第5行 “mystring(char*) = base2”是怎么回事呢？多谢。

[十八道胡同]

mystring ms82=ms81;这样可以调用拷贝构造函数
当用一个已经初始化过了的自定义类类型对象去初始化另一个新构造的对象的时候，拷贝构造函数就会被自动调用，

[yakeezhao]

[Quote=引用 8 楼 lcl_data 的回复:]

引用 7 楼 lcl_data 的回复:
m_sf82 = "base2";
这句触发了mystring的构造函数mystring(char* p = NULL)

和=重载函数
[/Quote]

这样的话，会得到第6行和第7行的输出结果。那么第五行是由谁导致的呢？

[十八道胡同]

[Quote=引用 9 楼 yakeezhao 的回复:]
引用 6 楼 lcl_data 的回复:

mystring(char*) = NULL 是在初始化mystring m_sf82;


多谢 LCL_data 老大。老大能不能再指点一下 第5行 “mystring(char*) = base2”是怎么回事呢？多谢。
[/Quote]
要理解这个，你先理解下
mystring ms81="1";
cout<<"------------------------------"<<endl;
mystring ms222;
ms222="bbbbbbbb";
cout<<"------------------------------"<<endl;
为什么2种定义mystring 实例的方法调用的方法会不同。。

[yakeezhao]

终于搞明白了。多谢各位大侠指点，尤其感谢 LCL_data 老大。
m_sf82 = "base2";的确导致输出第 5，6，7 句。
因为我没有定义 const mystring& operator = (const char* p) 的操作。m_sf82 = "base2";在调用const mystring& operator = (const mystring& rhs)的时候，先用"base2"构造出一个临时的mystring实例，才真正调用const mystring& operator = (const mystring& rhs)。

结帖啦。再次感谢大家。 :D