String的寫時拷貝

//test.h
#pragma once#include <iostream>
#include <string.h>
#include <cstdio>
#include <assert.h>
using namespace std;#define TESTHEADER printf("\n================%s=============\n", __FUNCTION__)
class String
{
public:String(const char* str);String(const String& s);String& operator = (const String s);~String();char* c_str();void CopyOnWrite();char& operator [] (size_t pos);const char& operator [] (size_t pos)const;String& Insert(size_t pos, char ch);String& Insert(size_t pos, const char* str);String& Append(const char* str);String& Erase(size_t pos, size_t n);void Expand(size_t n);
private:char* _str;int* _pcount;size_t _size;size_t _capacity;
};//test.cc
#include "test.h"
String::String(const char* str):_size(strlen(str))
,_capacity(_size)
{_str = new char [strlen(str) + 1];_pcount = new int(1);strcpy(_str, str);
}//s(s1)
String::String(const String& s)
{_str = s._str;_pcount = s._pcount;++(*_pcount);_size = s._size;_capacity = s._capacity;
}//s1 = s2
String& String:: operator = (const String s) 
{if(_str != s._str){_str = s._str;--(*_pcount);if(*_pcount == 0){delete [] _str;delete _pcount;}_size = s._size;_capacity = s._capacity;}return *this;
}String::~String()
{delete [] _str;delete _pcount;_size = 0;_capacity = 0;
}
char* String::c_str()
{return _str;
}void String::CopyOnWrite()
{if(*_pcount > 1){char* tmp = new char[_size + 1];strcpy(tmp, _str);--(*_pcount);_str = tmp;_pcount = new int(1);}
}//s[1]
char& String::operator [] (size_t pos)
{CopyOnWrite();return _str[pos];
}const char& String:: operator [] (size_t pos)const
{return _str[pos];
}void String::Expand(size_t n)
{if( *_pcount  == 1){char* tmp = new char [n];strcpy(tmp, _str);delete [] _str;--(*_pcount);delete _pcount;_pcount = new int(1);_str = tmp;}else{char* tmp = new char [n];strcpy(tmp, _str);_str = tmp;--(*_pcount);_pcount = new int(1);}
}String& String::Insert(size_t pos, char ch)
{assert(pos < _size);if(_size == _capacity){Expand(2 * _capacity);}int end = _size;for(; end >= (int)pos; end--){_str[end] = _str[end - 1];}_str[pos] = ch;return *this;
}String& String::Insert(size_t pos, const char* str)
{assert(pos <= _size);int len = strlen(str);if(_size == _capacity){Expand(_size + len);}int end = _size;for(; end >= (int)pos; end--){_str[end + len] = _str[end];}strncpy(_str + pos, str, len);_size += len;/* _str[_size] = '\0'; */return *this;
}String& String::Append(const char* str)
{int len_ = strlen(_str);int len = strlen(str);if(_size == _capacity){Expand(_size + len);}strcpy(_str + len_, str);_size += len;return *this;
}String& String::Erase(size_t pos, size_t n)
{assert(pos <= _size);CopyOnWrite();if(pos + _size > _capacity){_str[pos] = '\0';}strcpy(_str + pos, _str + pos + n);_size -= n;return *this;
}

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