string类详解（下）
4. string类的模仿实现首先，我们先补充一下关于编码的知识：
int main()
{
char buff1[] = "abcd";
char buff2[] = "比特";
cout << sizeof(buff1) << endl;
cout << sizeof(buff2) << endl;
cout << buff1 << endl;
cout << buff2 << endl;
return 0;
}
https://i-blog.csdnimg.cn/direct/5390d2315b754ac9be0f50d22be7a384.png
https://i-blog.csdnimg.cn/direct/7f4f37f5f872401e938fad14b9624cce.png
假如严格按照标准的话，我们应该要实现成 basic_string ，但是这样难度太大，要思量各种编码的拷贝，以是我们就实现的轻微简单一些，不要实现成模板了。
4.1 构造 + 析构
//string::string()
//{
// _str = new char{ '\0' };
// _size = 0;
// _capacity = 0;
//}
string::string(const char* str)
:_size(strlen(str))
{
_str = new char;
_capacity = _size;
strcpy(_str, str);
}
//s2(s1)
string::string(const string& s)
{
_str = new char;
strcpy(_str, s._str);
_size = s._size;
_capacity = s._capacity;
}
string::~string()
{
delete[] _str;
_str = nullptr;
_size = _capacity = 0;
}
4.2 c_str
const char* string::c_str() const
{
return _str;
}
4.3 下标遍历
size_t string::size() const
{
return _size;
}
char& string::operator[](size_t pos)
{
assert(pos < _size);
return _str;
}
const char& string::operator[](size_t pos) const
{
assert(pos < _size);
return _str;
}
4.4 迭代器
string::iterator string::begin()
{
return _str;
}
string::iterator string::end()
{
return _str + _size;
}
https://i-blog.csdnimg.cn/direct/aed6a83d78894c1f973b7644bc52ee30.png
string::const_iterator string::begin() const
{
return _str;
}
string::const_iterator string::end() const
{
return _str + _size;
}
4.5 插入
void string::reserve(size_t n)
{
if (n > _capacity)
{
char* tmp = new char;
strcpy(tmp, _str);
delete[] _str;
_str = tmp;
_capacity = n;
}
}
void string::push_back(char ch)
{
if (_size == _capacity)
{
size_t newcapacity = 0 == _capacity ? 4 : _capacity * 2;
reserve(newcapacity);
}
_str = ch;
_str = '\0';
++_size;
}
//"hello""xxxxxxxxxxxxx"
void string::append(const char* str)
{
size_t len = strlen(str);
if (_size + len > _capacity)
{
reserve(_size + len);
}
strcpy(_str + _size, str);
_size += len;
}
string& string::operator+=(char ch)
{
push_back(ch);
return *this;
}
string& string::operator+=(const char* str)
{
append(str);
return *this;
}
https://i-blog.csdnimg.cn/direct/8c84df6372ed4d799a773ac98df79bcb.png
void string::insert(size_t pos, char ch)
{
assert(pos <= _size);
if (_size == _capacity)
{
size_t newcapacity = 0 == _capacity ? 4 : _capacity * 2;
reserve(newcapacity);
}
/*int end = _size;
while (end >= (int)pos)
{
_str = _str;
--end;
}*/
size_t end = _size + 1;
while (end > pos)
{
_str = _str;
--end;
}
_str = ch;
++_size;
}
https://i-blog.csdnimg.cn/direct/278b1c448b0a4fc5aa592d1a883f968a.png
void string::insert(size_t pos, const char* str)
{
assert(pos <= _size);
size_t len = strlen(str);
if (_size + len > _capacity)
{
reserve(_size + len);
}
/*int end = _size;
while (end >= (int)pos)
{
_str = _str;
--end;
}*/
size_t end = _size + len;
while (end > pos + len - 1)
{
_str = _str;
--end;
}
memcpy(_str + pos, str, len);
_size += len;
}
4.6 删除
https://i-blog.csdnimg.cn/direct/0ebc8d0a95b54ca7a669f3e730404f22.png
const size_t string::npos = -1;
void string::erase(size_t pos, size_t len)
{
assert(pos < _size);
//len 大于等于后面字符个数时，有多少删多少
if (len >= _size - pos)
{
_str = '\0';
_size = pos;
}
else
{
strcpy(_str + pos, _str + pos + len);
_size -= len;
}
}
4.7 查找
size_t string::find(char ch, size_t pos)
{
for (size_t i = pos; i < _size; i++)
{
if (_str == ch)
{
return i;
}
}
return npos;
}
size_t string::find(const char* str, size_t pos)
{
char* p = strstr(_str + pos, str);
return p - _str;
}
4.8 赋值
https://i-blog.csdnimg.cn/direct/d57d639ec54a42aa9a67994b02ac33fa.png
//s1 = s3
//s1 = s1
string& string::operator=(const string& s)
{
if (this != &s)
{
char* tmp = new char;
strcpy(tmp, s._str);
delete[] _str;
_str = tmp;
_size = s._size;
_capacity = s._capacity;
}
return *this;
}
4.9 交换
https://i-blog.csdnimg.cn/direct/ad97209973c24ddfa00bcb02827400d1.png
//s1.swap(s3)
void string::swap(string& s)
{
std::swap(_str, s._str);
std::swap(_size, s._size);
std::swap(_capacity, s._capacity);
}
4.10 提取子串
https://i-blog.csdnimg.cn/direct/7bdb943fcf114ed99507836394e7f854.png
string string::substr(size_t pos, size_t len)
{
//len大于后面剩余字符，有多少取多少
if (len > _size - pos)
{
string sub(_str + pos);
return sub;
}
else
{
string sub;
sub.reserve(len);
for (size_t i = 0; i < len; i++)
{
sub += _str;
}
return sub;
}
}
4.11 比力巨细
bool string::operator<(const string& s) const
{
return strcmp(_str, s._str) < 0;
}
bool string::operator>(const string& s) const
{
return !(*this <= s);
}
bool string::operator<=(const string& s) const
{
return *this < s || *this == s;
}
bool string::operator>=(const string& s) const
{
return !(*this < s);
}
bool string::operator==(const string& s) const
{
return strcmp(_str, s._str) == 0;
}
bool string::operator!=(const string& s) const
{
return !(*this == s);
}
4.12 流插入 && 流提取
void string::clear()
{
_str = '\0';
_size = 0;
}
//一个字符一个字符放入str里，会有很多次扩容，可以优化
//istream& operator>> (istream& is, string& str)
//{
// str.clear();
// //流提取（>>）提取不了空格和换行，istream里的函数get()可以
// char ch = is.get();
// while (ch != ' ' && ch != '\n')
// {
// str += ch;
// ch = is.get();
// }
// return is;
//}
istream& operator>> (istream& is, string& str)
{
str.clear();
char buff;
int i = 0;
char ch = is.get();
while (ch != ' ' && ch != '\n')
{
buff = ch;
// 0 - 126
if (i == 127)
{
buff = '\0';
str += buff;
i = 0;
}
ch = is.get();
}
if (i != 0)
{
buff = '\0';
str += buff;
}
return is;
}
ostream& operator<< (ostream& os, const string& str)
{
for (size_t i = 0; i < str.size(); i++)
{
os << str;
}
return os;
}
5. 现代版写法的String类
我们之前写的拷贝构造和赋值运算符重载是传统写法，其实另有现代写法：
https://i-blog.csdnimg.cn/direct/60ad9c7a575f4e36999cca63ed239318.png
//现代写法（让别人干活，交换）
//s2(s1)
string::string(const string& s)
{
string tmp(s._str);
/*std::swap(tmp._str, _str);
std::swap(tmp._size, _size);
std::swap(tmp._capacity, _capacity);*/
//这个是我们写的string类里的交换函数
swap(tmp);
}
https://i-blog.csdnimg.cn/direct/846161402ebe408fb09c451c16e0df11.png
/*string& string::operator=(const string& s)
{
if (this != &s)
{
string tmp(s._str);
swap(tmp);
}
return *this;
}*/
//s1 = s3
string& string::operator=(string tmp)
{
swap(tmp);
return *this;
}
5.1 完整代码
//string.h
#include <iostream>
#include <assert.h>
using namespace std;
namespace bit
{
class string
{
public:
typedef char* iterator;
typedef const char* const_iterator;
iterator begin();
iterator end();
const_iterator begin() const;
const_iterator end() const;
//string();
string(const char* str = "");
string(const string& s);
//string& operator=(const string& s);
string& operator=(string tmp);
~string();
const char* c_str() const;
size_t size() const;
char& operator[](size_t pos);
const char& operator[](size_t pos) const;
void reserve(size_t n);
void push_back(char ch);
void append(const char* str);
string& operator+=(char ch);
string& operator+=(const char* str);
void insert(size_t pos, char ch);
void insert(size_t pos, const char* str);
void erase(size_t pos = 0, size_t len = npos);
size_t find(char ch, size_t pos = 0);
size_t find(const char* str, size_t pos =0);
void swap(string& s);
string substr(size_t pos = 0, size_t len = npos);
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;
void clear();
private:
//char _buff;
char* _str = nullptr;
size_t _size = 0;
size_t _capacity = 0;
const static size_t npos;
//特例，const静态成员变量只有整型可以这样声明定义（了解即可，不建议这样写）
//const static size_t npos = -1;
//不支持
//const static double N = 2.2;
};
istream& operator>> (istream& is, string& str);
ostream& operator<< (ostream& os, const string& str);
}
//string.cpp
#include "string.h"
namespace bit
{
const size_t string::npos = -1;
//string::string()
//{
// _str = new char{ '\0' };
// _size = 0;
// _capacity = 0;
//}
string::iterator string::begin()
{
return _str;
}
string::iterator string::end()
{
return _str + _size;
}
string::const_iterator string::begin() const
{
return _str;
}
string::const_iterator string::end() const
{
return _str + _size;
}
string::string(const char* str)
:_size(strlen(str))
{
_str = new char;
_capacity = _size;
strcpy(_str, str);
}
//传统写法（实在人）
//s2(s1)
/*string::string(const string& s)
{
_str = new char;
strcpy(_str, s._str);
_size = s._size;
_capacity = s._capacity;
}*/
//现代写法（让别人干活，交换）
//s2(s1)
string::string(const string& s)
{
string tmp(s._str);
/*std::swap(tmp._str, _str);
std::swap(tmp._size, _size);
std::swap(tmp._capacity, _capacity);*/
//这个是我们写的string类里的交换函数
swap(tmp);
}
//s1 = s3
//s1 = s1
/*string& string::operator=(const string& s)
{
if (this != &s)
{
char* tmp = new char;
strcpy(tmp, s._str);
delete[] _str;
_str = tmp;
_size = s._size;
_capacity = s._capacity;
}
return *this;
}*/
/*string& string::operator=(const string& s)
{
if (this != &s)
{
string tmp(s._str);
swap(tmp);
}
return *this;
}*/
//s1 = s3
string& string::operator=(string tmp)
{
swap(tmp);
return *this;
}
string::~string()
{
delete[] _str;
_str = nullptr;
_size = _capacity = 0;
}
const char* string::c_str() const
{
return _str;
}
size_t string::size() const
{
return _size;
}
char& string::operator[](size_t pos)
{
assert(pos < _size);
return _str;
}
const char& string::operator[](size_t pos) const
{
assert(pos < _size);
return _str;
}
void string::reserve(size_t n)
{
if (n > _capacity)
{
char* tmp = new char;
strcpy(tmp, _str);
delete[] _str;
_str = tmp;
_capacity = n;
}
}
void string::push_back(char ch)
{
/*if (_size == _capacity)
{
size_t newcapacity = 0 == _capacity ? 4 : _capacity * 2;
reserve(newcapacity);
}
_str = ch;
_str = '\0';
++_size;*/
insert(_size, ch);
}
//"hello""xxxxxxxxxxxxx"
void string::append(const char* str)
{
/*size_t len = strlen(str);
if (_size + len > _capacity)
{
reserve(_size + len);
}
strcpy(_str + _size, str);
_size += len;*/
insert(_size, str);
}
string& string::operator+=(char ch)
{
push_back(ch);
return *this;
}
string& string::operator+=(const char* str)
{
append(str);
return *this;
}
void string::insert(size_t pos, char ch)
{
assert(pos <= _size);
if (_size == _capacity)
{
size_t newcapacity = 0 == _capacity ? 4 : _capacity * 2;
reserve(newcapacity);
}
/*int end = _size;
while (end >= (int)pos)
{
_str = _str;
--end;
}*/
size_t end = _size + 1;
while (end > pos)
{
_str = _str;
--end;
}
_str = ch;
++_size;
}
void string::insert(size_t pos, const char* str)
{
assert(pos <= _size);
size_t len = strlen(str);
if (_size + len > _capacity)
{
reserve(_size + len);
}
/*int end = _size;
while (end >= (int)pos)
{
_str = _str;
--end;
}*/
size_t end = _size + len;
while (end > pos + len - 1)
{
_str = _str;
--end;
}
memcpy(_str + pos, str, len);
_size += len;
}
void string::erase(size_t pos, size_t len)
{
assert(pos < _size);
//len 大于等于后面字符个数时，有多少删多少
if (len >= _size - pos)
{
_str = '\0';
_size = pos;
}
else
{
strcpy(_str + pos, _str + pos + len);
_size -= len;
}
}
size_t string::find(char ch, size_t pos)
{
for (size_t i = pos; i < _size; i++)
{
if (_str == ch)
{
return i;
}
}
return npos;
}
size_t string::find(const char* str, size_t pos)
{
char* p = strstr(_str + pos, str);
return p - _str;
}
//s1.swap(s3)
void string::swap(string& s)
{
std::swap(_str, s._str);
std::swap(_size, s._size);
std::swap(_capacity, s._capacity);
}
string string::substr(size_t pos, size_t len)
{
//len大于后面剩余字符，有多少取多少
if (len > _size - pos)
{
string sub(_str + pos);
return sub;
}
else
{
string sub;
sub.reserve(len);
for (size_t i = 0; i < len; i++)
{
sub += _str;
}
return sub;
}
}
bool string::operator<(const string& s) const
{
return strcmp(_str, s._str) < 0;
}
bool string::operator>(const string& s) const
{
return !(*this <= s);
}
bool string::operator<=(const string& s) const
{
return *this < s || *this == s;
}
bool string::operator>=(const string& s) const
{
return !(*this < s);
}
bool string::operator==(const string& s) const
{
return strcmp(_str, s._str) == 0;
}
bool string::operator!=(const string& s) const
{
return !(*this == s);
}
void string::clear()
{
_str = '\0';
_size = 0;
}
//一个字符一个字符放入str里，会有很多次扩容，可以优化
//istream& operator>> (istream& is, string& str)
//{
// str.clear();
// //流提取（>>）提取不了空格和换行，istream里的函数get()可以
// char ch = is.get();
// while (ch != ' ' && ch != '\n')
// {
// str += ch;
// ch = is.get();
// }
// return is;
//}
istream& operator>> (istream& is, string& str)
{
str.clear();
char buff;
int i = 0;
char ch = is.get();
while (ch != ' ' && ch != '\n')
{
buff = ch;
// 0 - 126
if (i == 127)
{
buff = '\0';
str += buff;
i = 0;
}
ch = is.get();
}
if (i != 0)
{
buff = '\0';
str += buff;
}
return is;
}
ostream& operator<< (ostream& os, const string& str)
{
for (size_t i = 0; i < str.size(); i++)
{
os << str;
}
return os;
}
}
//Test.cpp
#include "string.h"
namespace bit
{
void test_string1()
{
string s1("hello world");
cout << s1.c_str() << endl;
for (size_t i = 0; i < s1.size(); i++)
{
s1++;
}
for (size_t i = 0; i < s1.size(); i++)
{
cout << s1 << " ";
}
cout << endl;
//封装：统一屏蔽了底层实现细节，提供了一种简单通用的访问容器的方式
string::iterator it1 = s1.begin();
while (it1 != s1.end())
{
cout << *it1 << " ";
++it1;
}
cout << endl;
for (auto e : s1)
{
cout << e << " ";
}
cout << endl;
string s2;
cout << s2.c_str() << endl;
const string s3("xxxxxxx");
string::const_iterator it3 = s3.begin();
while (it3 != s3.end())
{
//*it3 = 'y';//err
cout << *it3 << " ";
++it3;
}
cout << endl;
for (size_t i = 0; i < s3.size(); i++)
{
//s3++;
cout << s3 << " ";
}
cout << endl;
}
void test_string2()
{
string s1("hello world");
cout << s1.c_str() << endl;
s1.push_back('x');
cout << s1.c_str() << endl;
s1.append("yyyyy");
cout << s1.c_str() << endl;
s1 += 'z';
s1 += "mmmmmm";
cout << s1.c_str() << endl;
}
void test_string3()
{
string s1("hello world");
cout << s1.c_str() << endl;
s1.insert(6, 'x');
cout << s1.c_str() << endl;
s1.insert(0, 'x');
cout << s1.c_str() << endl;
string s2("hello world");
cout << s2.c_str() << endl;
s2.insert(6, "yyy");
cout << s2.c_str() << endl;
s2.insert(0, "yyy");
cout << s2.c_str() << endl;
string s3("hello world");
cout << s3.c_str() << endl;
//s3.erase(6, 10);
s3.erase(6);
cout << s3.c_str() << endl;
string s4("hello world");
cout << s4.c_str() << endl;
s4.erase(6, 3);
cout << s4.c_str() << endl;
}
void test_string4()
{
string s1("hello world");
cout << s1.find('o') << endl;
cout << s1.find("wor") << endl;
}
void test_string5()
{
string s1("hello world");
string s2(s1);
s1 = 'x';
cout << s1.c_str() << endl;
cout << s2.c_str() << endl;
string s3("yyyy");
s1 = s3;
cout << s1.c_str() << endl;
cout << s3.c_str() << endl;
string s4("zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz");
s1 = s4;
cout << s1.c_str() << endl;
cout << s4.c_str() << endl;
s1 = s1;
cout << s1.c_str() << endl;
cout << s3.c_str() << endl;
std::swap(s1, s3);
cout << s1.c_str() << endl;
cout << s3.c_str() << endl;
s1.swap(s3);
cout << s1.c_str() << endl;
cout << s3.c_str() << endl;
}
void test_string6()
{
string url("https://gitee.com/ailiangshilove/cpp-class/blob/master/%E8%AF%BE%E4%BB%B6%E4%BB%A3%E7%A0%81/C++%E8%AF%BE%E4%BB%B6V6/string%E7%9A%84%E6%8E%A5%E5%8F%A3%E6%B5%8B%E8%AF%95%E5%8F%8A%E4%BD%BF%E7%94%A8/TestString.cpp");
size_t pos1 = url.find(':');
string url1 = url.substr(0, pos1 - 0);
cout << url1.c_str() << endl;
size_t pos2 = url.find('/', pos1 + 3);
string url2 = url.substr(pos1 + 3, pos2 - (pos1 + 3));
cout << url2.c_str() << endl;
string url3 = url.substr(pos2 + 1);
cout << url3.c_str() << endl;
}
void test_string7()
{
//string s1("hello world");
//cout << s1 << endl;
string s1;
cin >> s1;
cout << s1 << endl;
}
void test_string8()
{
string s1("hello world");
string s2(s1);
cout << s1 << endl;
cout << s2 << endl;
string s3("xxxxxxxxxxxxxxxxxxxxxxxxx");
s1 = s3;
cout << s1 << endl;
cout << s3 << endl;
}
}
int main()
{
bit::test_string8();
return 0;
}
6. 写时拷贝（了解）
写时拷贝就是一种拖延症，是在浅拷贝的基础之上增长了引用计数的方式来实现的。
引用计数：用来记载资源使用者的个数。在构造时，将资源的计数给成1，每增长一个对象使用该资源，就给计数增长1，当某个对象被销毁时，先给该计数减1，然后再检查是否需要开释资源，假如计数为1，阐明该对象时资源的末了一个使用者，将该资源开释；否则就不能开释，因为另有其他对象在使用该资源。
https://i-blog.csdnimg.cn/direct/6c6f0195636f408eba7f1c1a801140a7.png
举个例子：
void copy_on_write()
{
//如果计数不等于1，再去做深拷贝
}
void string::push_back(char ch)
{
//写时拷贝
copy_on_write();
/*if (_size == _capacity)
{
size_t newcapacity = 0 == _capacity ? 4 : _capacity * 2;
reserve(newcapacity);
}
_str = ch;
_str = '\0';
++_size;*/
insert(_size, ch);
}
可以用下面的代码来看是否是用了写时拷贝：
void test_string9()
{
std::string s1("hello world");
std::string s2(s1);
cout << (void*)s1.c_str() << endl;
cout << (void*)s2.c_str() << endl;
}
Windows的VS下没有用写时拷贝，而Linux的g++就使用了写时拷贝。
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