写时拷贝（方案四）

方案四

class String
{
   private:
           char* _str;
};

方案四与方案三类似。方案四把用来计数的整形变量放在所开辟的内存空间的首部，

用*((int*)_str)就能来改变计数值

#define_CRT_SECURE_NO_WARNINGS 1

#include<iostream>
using namespace std;
#include<assert.h>

class String
{
public:
           String(char * str = "" )    //不能strlen（NULL）
           {
                    _str = new char[strlen( str) + 5];
                    _str += 4;
                    strcpy(_str, str);
                    GetRefCount(_str) = 1;
           }
           String(const String &s)
           {
                    _str = s._str;
                    ++GetRefCount(_str);
           }

           //要考虑是s1=s2时，s1原先不为空的情况，要先释放原内存
           //如果要释放原内存时，要考虑它的_refCount减1后是否为0，
           //为零再释放，否则其它对象指针无法再访问这片空间
           String& operator=(String& s)
           {
                    if (this != &s )
                    {
                              if (GetRefCount(_str ) == 1)
                              {
                                       delete (_str-4);
                                       _str = s._str;
                                       ++GetRefCount(_str );
                              }
                              else
                              {
                                       --GetRefCount(_str );
                                       _str = s._str;
                                       ++GetRefCount(_str );
                              }
                    }
                    return *this ;
           }
           //如果修改了字符串的内容，那所有指向这块内存的对象指针的内容间接被改变
           //如果还有其它指针指向这块内存，我们可以从堆上重新开辟一块内存空间，
           //把原字符串拷贝过来.
           //再去改变它的内容，就不会产生链式反应
           
         
           char& String ::operator[](const size_t index ) //深拷贝     
           {
                   
                              if (GetRefCount(_str) == 1)
                              {
                                       return _str[index ];
                              }
                              else
                              {
                                        //  1.减引用计数
                                       --GetRefCount(_str );
                                        //  2.拷贝     3.创建新的引用计数
                                       char* tmp = new char [strlen(_str) + 5];   
                                      *((int *)tmp) = 1;
                                       tmp += 4;
                                       strcpy(tmp, _str);
                                       _str = tmp;
                                       return _str[index ];
                              }
           }

           int& GetRefCount(char* ptr)    //获取引用计数(隐式内联函数)
           {
                    return *((int *)(ptr -4));
           }
           ~String()
           {
                    if (--GetRefCount(_str) == 0)
                    {
                              cout << "~String" << endl;
                              delete[] (_str-4);             
                    }
          
           }
           friend ostream& operator<<( ostream& output, const String &s);
           friend istream& operator>>( istream& input, const String &s);
private:
           char* _str;

};


ostream& operator<<(ostream& output, const String &s)
{
           output << s._str;
           return output;
}
istream& operator>>(istream& input, const String &s)
{
           input >> s._str;
           return input;
}

void Test()  //用例测试
{
           String s1("abcdefg" );
           String s2(s1);
           String s3;
           s3 = s2;
           cout << s1 << endl;
           cout << s2 << endl;
           cout << s3 << endl;
           s2[3] = ‘0‘;
           cout << s1 << endl;
           cout << s2 << endl;
           cout << s3 << endl;

           //String s4("opqrst");
           //String s5(s4);
           //String s6 (s5);
           //s6 = s4;
           //cout << s4 << endl;
           //cout << s5 << endl;
           //cout << s6 << endl;

}
int main()
{
           Test();
           system("pause" );
           return 0;
}

以下是其它方案链接地址：

方案一：

http://iynu17.blog.51cto.com/10734157/1755179

方案二：

http://iynu17.blog.51cto.com/10734157/1755185

方案三：

http://iynu17.blog.51cto.com/10734157/1755208

本文出自 “言安阳” 博客，谢绝转载！

原文：http://iynu17.blog.51cto.com/10734157/1755213