1.經典的string類問題
在面試中,面試官總喜歡讓學生自己來模拟實作string類,最主要是實作String類的構造、拷貝構造、指派運算符重載以及析構函數。大家看下以下string類的實作是否有問題?
class String
{
public:
String(const char* str = "")
{
// 構造string類對象時,如果傳遞nullptr指針,認為程式非法,此處斷言下
if(nullptr == str)
{
assert(false);
return;
}
_str = new char[strlen(str) + 1];
strcpy(_str, str);
}
~String()
{
if(_str)
{
delete[] _str;
_str = nullptr;
}
}
private:
char* _str;
};
// 測試
void TestString()
{
String s1("hello bit!!!");
String s2(s1);
}
說明:上述String類沒有顯式定義其拷貝構造函數與指派運算符重載,此時編譯器會合成預設的,當用s1構
造s2時,編譯器會調用預設的拷貝構造。最終導緻的問題是,s1、s2共用同一塊記憶體空間,在釋放時同一塊
空間被釋放多次而引起程式崩潰,這種拷貝方式,稱為淺拷貝。
淺拷貝:也稱位拷貝,編譯器隻是将對象中的值拷貝過來。如果對象中管理資源,最後就會**導緻多個對象共享同一份資源,當一個對象銷毀時就會将該資源釋放掉,而此時另一些對象不知道該資源已經被釋放,以為還有效,是以 當繼續對資源進項操作時,就會發生發生了通路違規。**要解決淺拷貝問題,C++中引入了深拷
貝。
深拷貝 :如果一個類中涉及到資源的管理,其拷貝構造函數、指派運算符重載以及析構函數必須要顯式給出。一般情況都是按照深拷貝方式提供。
寫時拷貝: 寫時拷貝就是一種拖延症,是在淺拷貝的基礎之上增加了引用計數的方式來實作的。
引用計數:用來記錄資源使用者的個數。在構造時,将資源的計數給成1,每增加一個對象使用該資源,就給
計數增加1,當某個對象被銷毀時,先給該計數減1,然後再檢查是否需要釋放資源,如果計數為1,說明該
對象時資源的最後一個使用者,将該資源釋放;否則就不能釋放,因為還有其他對象在使用該資源。
寫時拷貝
寫時拷貝在讀取時的缺陷
string的模拟實作代碼:
#pragma once
#include <string.h>
#include <stdlib.h>
#include <iostream>
using namespace std;
#define DEFAULTCAPA 16
#define COUNTCAPA(size) (((size) / DEFAULTCAPA + 1) * DEFAULTCAPA)
class String
{
char * m_data;
size_t m_size;
size_t m_capacity;
public:
String(const char * str = "") :
m_capacity(DEFAULTCAPA)
{
if (nullptr == str)
{
str = "";
}
m_size = strlen(str);
m_capacity = COUNTCAPA(m_size);
m_data = new char[m_capacity];
strncpy(m_data, str, m_size);
}
String(String &s)
{
m_size = s.m_size;
m_capacity = s.m_capacity;
m_data = new char[m_capacity];
strcpy(m_data, s.m_data);
}
String & operator = (const char * str)
{
m_size = strlen(str);
reserve(m_size);
strcpy(m_data, str);
return *this;
}
String & operator = (String &s)
{
m_size = s.m_size;
reserve(m_size);
strcpy(m_data, s.m_data);
return *this;
}
String(size_t n, char ch):
m_size(n),
m_capacity(COUNTCAPA(n))
{
m_data = new char[m_capacity];
memset(m_data, ch, m_size);
}
~String()
{
if (m_data)
{
delete[]m_data;
m_data = NULL;
}
m_size = m_capacity = 0;
}
size_t size()
{
return m_size;
}
size_t length()
{
return m_size;
}
size_t capacity()
{
return m_capacity - 1;
}
bool empty()
{
return m_size == 0;
}
void clear()
{
m_size = 0;
}
void reserve(size_t size)
{
if (size >= m_capacity)
{
m_capacity = COUNTCAPA(size);
m_data = (char *)realloc(m_data, m_capacity);
}
}
void resize(size_t size, char ch = '\0')
{
reserve(size);
if (m_size < size)
{
memset(m_data + m_size, ch, size - m_size);
}
m_size = size;
}
char &operator[](int i)
{
return m_data[i];
}
typedef char * iterator;
iterator begin()
{
return m_data;
}
iterator end()
{
return m_data + m_size;
}
void push_back(char ch)
{
reserve(m_size);
m_data[m_size] = ch;
m_size++;
}
void pop_back(char ch)
{
if (!empty())
{
m_size--;
}
}
String & operator += (const char * str)
{
int tmp = m_size;
m_size += strlen(str);
reserve(m_size);
strcpy(m_data + tmp, str);
return *this;
}
String & operator += (String &s)
{
int tmp = m_size;
m_size += s.m_size;
reserve(m_size);
strcpy(m_data + tmp, s.m_data);
return *this;
}
/*作業:append 兩種,一種n個ch的,一種直接續字元串或容器的*/
const char * c_str()
{
return m_data;
}
size_t find(char ch, int pos = 0)
{
if (pos < 0 || pos >= m_size)
{
return -1;
}
char * tmp = strchr(m_data + pos, ch);
if (tmp)
{
return tmp - m_data;
}
return -1;
}
size_t find(const char *str, int pos = 0)
{
if (pos < 0 || pos >= m_size)
{
return -1;
}
char * tmp = strstr(m_data + pos, str);
if (tmp)
{
return tmp - m_data;
}
return -1;
}
size_t find(const String &s, int pos = 0)
{
if (pos < 0 || pos >= m_size)
{
return -1;
}
char * tmp = strstr(m_data + pos, s.m_data);
if (tmp)
{
return tmp - m_data;
}
return -1;
}
String substr(size_t start, size_t num)
{
String tmp;
tmp.resize(num);
strncpy(tmp.m_data, m_data + start, num);
return tmp;
}
String operator + (const char * str) const;
String operator + (const String &s) const;
friend String operator +(const char * str, const String &s);
bool operator > (const char * str) const;
bool operator < (const char * str) const;
bool operator >= (const char * str) const;
bool operator <= (const char * str) const;
bool operator == (const char * str) const;
bool operator != (const char * str) const;
bool operator > (const String &s) const;
bool operator < (const String &s) const;
bool operator >= (const String &s) const;
bool operator <= (const String &s) const;
bool operator == (const String &s) const;
bool operator != (const String &s) const;
friend bool operator > (const char * str, const String &s);
friend bool operator < (const char * str, const String &s);
friend bool operator >= (const char * str, const String &s);
friend bool operator <= (const char * str, const String &s);
friend bool operator == (const char * str, const String &s);
friend bool operator != (const char * str, const String &s);
friend ostream & operator << (ostream & os, const String &s);
friend istream & operator >> (istream & is, String &s);
};
ostream & operator << (ostream & os, const String &s)
{
int i;
for (i = 0; i < s.m_size; i++)
{
os << s.m_data[i];
}
return os;
}
istream & operator >> (istream & is, String &s)
{
char * tmp = new char[1024];
is.getline(tmp, 1024);
s.m_size = strlen(tmp);
s.m_capacity = COUNTCAPA(s.m_size);
strcpy(s.m_data, tmp);
delete[]tmp;
/*delete[]s.m_data;
s.m_data = tmp;*/
return is;
}
String String::operator + (const char * str) const
{
String res;
res.m_size = m_size + strlen(str);
res.reserve(res.m_size);
strncpy(res.m_data, m_data, m_size);
strcpy(res.m_data + m_size, str);
return res;
}
String String::operator + (const String &s) const
{
String res;
res.m_size = m_size + s.m_size;
res.reserve(res.m_size);
strncpy(res.m_data, m_data, m_size);
strncpy(res.m_data + m_size, s.m_data, s.m_size);
return res;
}
String operator +(const char * str, const String &s)
{
String res;
res.m_size = strlen(str) + s.m_size;
res.reserve(res.m_size);
strcpy(res.m_data, str);
strncat(res.m_data, s.m_data, s.m_size);
return res;
}
bool String::operator > (const char * str) const
{
return strncmp(m_data, str, m_size) > 0;
}
bool String::operator < (const char * str) const
{
int tmp = strncmp(m_data, str, m_size);
if (!tmp && strlen(str) > m_size)
{
return true;
}
return tmp < 0;
}
bool String::operator >= (const char * str) const
{
return !(*this < str);
}
bool String::operator <= (const char * str) const
{
return !(*this > str);
}
bool String::operator == (const char * str) const
{
int tmp = strncmp(m_data, str, m_size);
if (!tmp && strlen(str) == m_size)
{
return true;
}
return false;
}
bool String::operator != (const char * str) const
{
return !(*this == str);
}
bool String::operator > (const String &s) const
{
int i;
int minsize = m_size < s.m_size ? m_size : s.m_size;
for (i = 0; i < minsize; i++)
{
if (m_data[i] > s.m_data[i])
{
return true;
}
else if (m_data[i] < s.m_data[i])
{
return false;
}
}
return m_size > s.m_size;
}
bool String::operator < (const String &s) const
{
int i;
int minsize = m_size < s.m_size ? m_size : s.m_size;
for (i = 0; i < minsize; i++)
{
if (m_data[i] < s.m_data[i])
{
return true;
}
else if (m_data[i] > s.m_data[i])
{
return false;
}
}
return m_size < s.m_size;
}
bool String::operator >= (const String &s) const
{
return !(*this < s);
}
bool String::operator <= (const String &s) const
{
return !(*this > s);
}
bool String::operator == (const String &s) const
{
int i;
int minsize = m_size < s.m_size ? m_size : s.m_size;
for (i = 0; i < minsize; i++)
{
if (m_data[i] != s.m_data[i])
{
return false;
}
}
return m_size == s.m_size;
}
bool String::operator != (const String &s) const
{
return !(*this == s);
}
bool operator > (const char * str, const String &s)
{
int tmp = strncmp(str, s.m_data, s.m_size);
if (!tmp && strlen(str) > s.m_size)
{
return true;
}
return tmp > 0;
}
bool operator < (const char * str, const String &s)
{
return strncmp(str, s.m_data, s.m_size) < 0;
}
bool operator >= (const char * str, const String &s)
{
return !(str < s);
}
bool operator <= (const char * str, const String &s)
{
return !(str > s);
}
bool operator == (const char * str, const String &s)
{
int tmp = strncmp(s.m_data, str, s.m_size);
if (!tmp && strlen(str) == s.m_size)
{
return true;
}
return false;
}
bool operator != (const char * str, const String &s)
{
return !(str == s);
}
這個實作的比較詳細,其中最主要是實作String類的構造、拷貝構造、指派運算符重載以及析構函數。