C++初阶 | [七] (上) string类

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

C++初阶 | [七] (上) string类_第1张图片

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
#include

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
#include

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
#include

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;
}

C++初阶 | [七] (上) string类_第2张图片

  • 1)reverse_iterator

    C++初阶 | [七] (上) string类_第3张图片

    C++初阶 | [七] (上) string类_第4张图片

  • 2)范围for

    #include
    #include
    
    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。

    C++初阶 | [七] (上) string类_第5张图片

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 

C++初阶 | [七] (上) string类_第6张图片

  • 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。

  • wchar_t:2 byte(宽字符)⇨ wstring
  • char16_t:16 bit → 2 byte (UTF-16)
  • char32_t:32 bit → 4 byte (UTF-32)

乱码:(数)值 通过不同的编码表 得出了不同的符号——存储方式与解释方式不匹配。

GBK:GBK字库_百度百科 (baidu.com)


END

你可能感兴趣的:(#,C++初阶,c++,开发语言,c语言)