自定义的string

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/****************************************************************************************************************
* string class. 仿真std::string
* SDU. All Rights Reserved.
* Author: Guangcong Liu
****************************************************************************************************************
*/

#include <cstddef>
#include <cassert>
#include <cstring>

class string;

string operator+(const string&, const string&);
bool operator==(const string&, const string&);

class string{
      public:
             string(const char* = 0);
             string(const string&);
             ~string();

             string& operator=(const string&);
             string& operator=(const char*);

             string& operator+=(const string&);
             string& operator+=(const char*);

             size_t size() const;

             char&       operator[](size_t);
             const char& operator[](size_t) const;

             const char* c_str() const;

      private:
             char*          _str;
};

string::string(const char* str)
{
   if(str == 0)
   {
       _str = new char[1];
       _str[1] = '/0';
   }
   else
   {
       _str = new char[strlen(str) + 1];
       strcpy(_str, str);
   }
}

string::string(const string& other)
{
_str = new char[strlen(other._str) + 1];
strcpy(_str, other._str);
}

string::~string()
{
delete [] _str;
}

string& string::operator=(const string& rhs)
{
    if(&rhs != this)
    {
       delete [] _str;
       _str = new char[strlen(rhs._str) + 1];
       strcpy(_str, rhs._str);
    }
    return *this;
}

string& string::operator=(const char* rhs)
{
    if(rhs != _str)
    {
       delete [] _str;
       _str = new char[strlen(rhs) + 1];
       strcpy(_str, rhs);
    }
    return *this;
}

string& string::operator+=(const string& rhs)
{
   char * ori = _str;
   _str = new char[strlen(ori) + strlen(rhs._str) + 1];
   strcpy(_str, ori);
   strcat(_str, rhs._str);
   delete [] ori;
   return *this;
}

string& string::operator+=(const char* rhs)
{
   if(rhs)
   {
       char * ori = _str;
       _str = new char[strlen(ori) + strlen(rhs) + 1];
       strcpy(_str, ori);
       strcat(_str, rhs);
       delete [] ori;
    }
     return *this;
}

string operator+(const string& lhs, const string& rhs)
{
string temp = lhs;
temp += rhs;
return temp;
}

bool operator==(const string& lhs, const string& rhs)
{
if(strcmp(lhs.c_str(), rhs.c_str()) == 0) return true;
return false;
}

size_t string::size() const
{
return strlen(_str);
}

char& string::operator[](size_t i)
{
return _str[i];
}

const char& string::operator[](size_t i) const
{
return _str[i];
}

const char* string::c_str() const
{
return _str;
}