C++ string类的隐式共享写时拷贝实现代码

#include <ostream> #include <istream> #include <string.h> #include "_stringv2.h" using std::ostream; using std::istream; String::String(): _cstr(NULL), _used(new size_t(1)), _length(0), _capacity(0) { } String::String(const String &s): _cstr(s._cstr), _used(s._used), _length(s._length), _capacity(s._capacity) { ++*_used; } String::String(const char *cstr) { if(cstr == NULL) return; size_t len = strlen(cstr); _initString(cstr, len); } String::String(const char *cstr, size_t len) { if(cstr == NULL) return; size_t str_len = strlen(cstr); if(len <= str_len) { _initString(cstr, len); } } String::~String() { _decUsed(); } String& String::operator=(const String &s) { ++*(s._used); _decUsed(); _cstr = s._cstr; _length = s._length; _capacity = s._capacity; return *this; } String& String::operator=(const char *cstr) { if(cstr != NULL) { _decUsed(); size_t len = strlen(cstr); _initString(cstr, len); } return *this; } String& String::operator+=(const String &s) { _addAssignOpt(s._cstr, s._length); return *this; } String& String::operator+=(const char *cstr) { if(cstr != NULL) _addAssignOpt(cstr, strlen(cstr)); return *this; } void String::clear() { _decUsed(); _cstr = NULL; _used = new size_t(1); _length = 0; _capacity = 0; } void String::_initString(const char *cstr, size_t len) { if(cstr == NULL) return; _cstr = new char[len + 1]; memcpy(_cstr, cstr, len); _cstr[len] = 0; _used = new size_t(1); _length = len; _capacity = len; } void String::_decUsed() { --*_used; if(*_used == 0) { if(_cstr != NULL) { delete[] _cstr; _cstr = NULL; _length = 0; _capacity = 0; } delete _used; _used = NULL; } } char* String::_renewAndCat(const char *cstr, size_t len) { size_t new_len = len + _length; size_t capacity = new_len; capacity += (capacity >> 1); char *ptr = new char[capacity+1]; if(_cstr != NULL) memcpy(ptr, _cstr, _length); ptr[_length] = 0; _length = new_len; _capacity = capacity; strncat(ptr, cstr, len); return ptr; } void String::_addString(const char *cstr, size_t len) { //本函数，只有在引用计数为1时，才可用 if(*_used != 1) return; if(len + _length > _capacity) { char *ptr = _renewAndCat(cstr, len); delete[] _cstr; _cstr = ptr; } else { strncat(_cstr, cstr, len); _length += len; } } void String::_addAssignOpt(const char *cstr, size_t len) { if(*_used == 1) _addString(cstr, len); else { _decUsed(); _cstr = _renewAndCat(cstr, len); _used = new size_t(1); } } String operator +(const String &lhs, const String &rhs) { String stemp(lhs); stemp += rhs; return stemp; } ostream& operator << (ostream &os, const String &s) { os<<s.cstr(); return os; } istream& operator >> (istream &in, String &s) { const int BUFFER_SIZE = 256; char buffer[BUFFER_SIZE]; char *end = buffer + BUFFER_SIZE -1; s.clear(); do { //用于判断是否读完输入内容，因为如果还未读取的输入字符数大于buffer //的容量，则buffer的最后一个字符会被get函数置为'\0' *end = '#'; in.get(buffer, BUFFER_SIZE); s += buffer; }while(*end == '\0'); in.get(); return in; }