C++对象使用错误之复制构造函数与赋值

先来一段会出错的代码：

#pragma once

#include <iostream>

class StringBad
{
public:
    StringBad(const char *s);
    StringBad();
    ~StringBad();
    //friend
    friend std::ostream &operator <<(std::ostream &os, const StringBad &st);

private:
    char *str;
    int len;
    static int num_strings;
};

#include "StdAfx.h"
#include "StringBad.h"

int StringBad::num_strings = 0;

StringBad::StringBad( const char *s )
{
    len = std::strlen(s);
    str = new char[len + 1];
    std::strcpy(str, s);
    num_strings++;
    std::cout << num_strings << ": \"" << str << "\"object created \n"; 
}

StringBad::StringBad()
{
    len = std::strlen("hello world");
    str = new char[len + 1];
    std::strcpy(str, "hello world");
    num_strings++;
    std::cout << num_strings << ": \"" << str << "\"object created \n"; 
}

StringBad::~StringBad()
{
    std::cout << "\"" << str << "\" object delete, ";
    --num_strings;
    std::cout << num_strings << " left\n";
    delete []str; 
}

std::ostream & operator<<( std::ostream &os, const StringBad &st )
{
    os << st.str;
    return os;
}

// TestObjectThink.cpp : 定义控制台应用程序的入口点。
//

#include "stdafx.h"
#include <iostream>
#include "StringBad.h"

using std::cout;
using std::endl;

void callMe1(StringBad &rsb)
{
    cout << "reference" << endl;
    cout << "\"" << rsb << "\"" << endl;
}

void callMe2(StringBad rsb)
{
    cout << "value" << endl;
    cout << "\"" << rsb << "\"" << endl;
}

int _tmain(int argc, _TCHAR* argv[])
{
    {
        cout << "Starting an inner block \n";
        StringBad hello01("hello_01");
        StringBad hello02("hello_02");
        StringBad hello03("hello_03");

        cout << "hello01: " << hello01 << endl;
        cout << "hello02: " << hello02 << endl;
        cout << "hello03: " << hello03 << endl;    

        callMe1(hello01);
        cout << "hello01: " << hello01 << endl;
        callMe2(hello02);
        cout << "hello02: " << hello02 << endl;
        
        //等效于StringBad hello01Temp = StringBad(hello01);
        //新建一个对象并将其初始化为同类现有对象时，会调用复制构造函数。
        StringBad hello01Temp = hello01;
        cout << "hello01Temp:" << hello01Temp << endl;

        StringBad hello03Temp;
        hello03Temp = hello03;
        cout << "hello03Temp:" << hello03Temp << endl;
    
        //system("pause");
    }
	return 0;
}

 callMe2（）调用完数据出错了，最后在对象释放时出现了崩溃性的错误，解决方法，加入下面两个函数。

StringBad::StringBad( const StringBad &st )
{
    num_strings++;
    len = st.len;
    str = new char[len + 1];
    std::strcpy(str, st.str);
    std::cout << num_strings << ": \"" << str << "\"object created(copy) \n"; 
}

StringBad & StringBad::operator=( const StringBad &st )
{
    if (this == &st)
    {
        return *this;
    }
    delete []str;
    len = st.len;
    str = new char[len + 1];
    std::strcpy(str, st.str);
    std::cout << num_strings << ": \"" << str << "\"(assign) \n"; 
    return *this;
}

  
 

有空了再加上原因， 想急切了解的，参考：C++ primer plus(第六版) P425