C++初阶 | [七] string类（上）

摘要：标准库中的string类的常用函数

C语言中，字符串是以'\0'结尾的一些字符的集合，为了操作方便，C标准库中提供了一些str系列的库函数，但是这些库函数与字符串是分离开的，不太符合OOP(面向对象)的思想，而且底层空间需要用户自己管理，稍不留神可能还会越界访问。

在OJ中，有关字符串的题目基本以string类的形式出现，而且在常规工作中，为了简单、方便、快捷，基本都使用string类，很少有人去使用C库中的字符串操作函数。

（string 类不属于 STL 【C++】-7- STL简介，属于标准库）下面介绍 string类中比较常用、重要的函数。string类的接口设计繁多，需要时查一下文档即可。cplusplus.com/reference/string/string/

1. Constructor

关于构造函数不多赘述，参考文档可以很清楚的了解这些构造函数。→ https://cplusplus.com/reference/string/string/string/

补充说明： npos 为 string类中的静态成员变量，类型为无符号整型。static const size_t npos = -1 ，-1表示为无符号整型的最大值1111 1111 1111 1111 1111 1111 1111 1111 → 4,294,967,295。

2. 遍历string_Element Access

operator[]	Get character of string (public member function)
at	Get character in string (public member function)
back	Access last character (public member function)
front	Access first character (public member function)

1）operator[]

像普通数组一样，以[下标]的方式访问string类对象中的成员是最常用、便捷的一种方式。同样的，这种访问方式也支持修改：

#include<iostream>
#include<string>int main()
{std::string s1("Hello!");for (int i = 0; i < s1.size(); ++i){std::cout << s1[i] << " ";//访问}std::cout << std::endl;for (int i = 0; i < s1.size(); ++i){std::cout << ++s1[i] << " ";//修改}return 0;
}

注意： operator[] 越界，程序直接终止（assert断言处理）

2）at

at：越界抛异常

#include<iostream>
#include<string>int main()
{std::string s1("Hello!");for (int i = 0; i < s1.size(); ++i){std::cout << s1.at(i) << " ";}std::cout << std::endl; return 0;
}

3. Iterator_迭代器

迭代器是更通用、主流的遍历方式——不是所有的容器都适用operator[]，譬如链表——空间按地址不连续。为了方便理解，可以把迭代器看作指针（虽然实际底层实现可能是指针也可能不是）。

begin	Return iterator to beginning (public member function)
end	Return iterator to end (public member function)
rbegin	Return reverse iterator to reverse beginning (public member function)
rend	Return reverse iterator to reverse end (public member function)
cbegin	Return const_iterator to beginning (public member function)
cend	Return const_iterator to end (public member function)
crbegin	Return const_reverse_iterator to reverse beginning (public member function)
crend	Return const_reverse_iterator to reverse end (public member function)

#include<iostream>
#include<string>int main()
{std::string s2("Hello World!");std::string::iterator it = s2.begin();while (it != s2.end()){std::cout << *it << " ";++it;}std::cout << std::endl;return 0;
}

1）reverse_iterator
2）范围for
```
#include<iostream>
#include<string>int main()
{for (auto e : s2){std::cout << e << " ";}return 0;
}
```
范围for 本质上就是迭代器，编译器会在编译的时候替换成迭代器。

3）const_iterator

int main()
{const std::string s3("RoundBottle");std::string::const_iterator c_it = s3.cbegin();while (c_it != s3.cend()){std::cout << *c_it << " ";++c_it;}std::cout << std::endl;std::string::const_reverse_iterator cr_it = s3.crbegin();while (cr_it != s3.crend()){std::cout << *cr_it << " ";++cr_it;}std::cout << std::endl;return 0;
}

如上代码，const 对象调用 std::string::const_iterator 和 std::string::const_reverse_iterator 。

ps.可以用 auto 自动识别类型——auto cr_it = s3.crbegin();

4. Capacity

size	Return length of string (public member function)
length	Return length of string (public member function)
max_size	Return maximum size of string (public member function)
resize	Resize string (public member function)
capacity	Return size of allocated storage (public member function)
reserve	Request a change in capacity (public member function)
clear	Clear string (public member function)
empty	Test if string is empty (public member function)
shrink_to_fit	Shrink to fit (public member function)

clear()：一般只清理空间，不释放空间
size() and length()：string 类对象结尾以 '\0' 为结束标志——为了兼容C语言 。size() and length() 都不把结尾的 '\0' 算在内，先有 length 后有 size，是为了和其他容器保持一致，“size” 这种表达更具有通用性。功能一样，都是返回 string类 的对象的长度（不包括结尾的 '\0' ）。
reserve：提前预留空间，因为频繁的扩容是有代价的，提前预留空间可以提高效率（一般不缩容）。另外，不同平台下实际实现方案有所不同，譬如 vs编译器下有一些对齐的规则，最终开出来的空间会比 reserve 指定的空间大小要大一点；g++平台下一般是按指定的空间大小开空间。
resize：改变 size 的大小。（ps.如果指定的 size 大小过大，改变 size 的大小会导致改变 capacity 的大小。

关于不同平台扩容的不同规则：vs平台一般是1.5倍扩容，g++平台一般是2倍扩容。

5. Modifiers

operator+=	Append to string (public member function)
append	Append to string (public member function)
push_back	Append character to string (public member function)
assign	Assign content to string (public member function)
insert	Insert into string (public member function)
erase	Erase characters from string (public member function)
replace	Replace portion of string (public member function)
swap	Swap string values (public member function)
pop_back	Delete last character (public member function)

operator+=：常用，推荐使用，可以插入字符或字符串
push_back：插入字符
append：插入一串指定字符
insert , erase , replace：能不用就不用。因为挪动数据，影响效率

使用示例：

int main()
{std::string s2("Hello,Round Bottle");s2 += 'x';s2 += "llllll";s2 += "321";s2 += '!';std::cout << s2;s2.push_back('7');std::cout << s2;s2.append("aaaaaa");s2.append(3, '0');s2.append("alison", 2);std::cout << s2;return 0;
}

Non-member function overloads

operator+ ：全局函数（尽量减少用，代价很大）

int main()
{std::string s2("Hello,Round Bottle");std::string s3 = s2 + "777";std::cout << s3 << std::endl;return 0;
}

关于Swap函数：

①std库中提供了 swap 函数模板：（3次深拷贝——1次拷贝构造+两次赋值——效率低）

②害怕成本太高，std库中又提供了现成的针对 string类对象的：（就是 Non-member function overloads 表格中所展示的 swap 函数）

③ string类中自己有 swap 成员函数：（就是 Modifiers 表格中所展示的 swap 函数）

swap 成员函数使用示例：

int main()
{std::string s1("nothing");std::string s2("Hello,Round Bottle");std::cout << "s1:" << s1 << std::endl;std::cout << "s2:" << s2 << std::endl;s1.swap(s2);std::cout << "--------------swap---------------" << std::endl;std::cout << "s1:" << s1 << std::endl;std::cout << "s2:" << s2 << std::endl;return 0;
}

综上，针对交换 string类的对象，建议使用 string类自己的成员函数 swap 进行交换——效率更高。

6. String operations

c_str	Get C string equivalent (public member function)
data	Get string data (public member function)
get_allocator	Get allocator (public member function)
copy	Copy sequence of characters from string (public member function)
find	Find content in string (public member function)
rfind	Find last occurrence of content in string (public member function)
find_first_of	Find character in string (public member function)
find_last_of	Find character in string from the end (public member function)
find_first_not_of	Find absence of character in string (public member function)
find_last_not_of	Find non-matching character in string from the end (public member function)
substr	Generate substring (public member function)
compare	Compare strings (public member function)

c_str：与C语言接口兼容。使用示例如下：

int main()
{std::string s1("nothing");printf("%s", s1.c_str());return 0;
}

7. 设计string类的意义——编码

编码：值和符号一一映射对应的关系 → 编码表（e.g. ASCII）

Unicode：万国码 ⇨ UTF

UTF-8 ⇢ 兼容 ASCII（Linux → UTF-8）（Windows —— 针对中国用户 → gbk ——参考了UTF-8）
UTF-16
UTF-32

string	String class (class) ⇨ UTF-8
u16string	String of 16-bit characters (class)
u32string	String of 32-bit characters (class)
wstring	Wide string (class)

👆适应不同的编码，为了更好的表示世界上的各种语言。