#include <iostream>
#include <string>

std::string str;

std::string str = "Hello, World!";

std::string str1 = "Hello, ";
std::string str2 = "World!";
std::string result = str1 + str2;

#include <iostream>
#include <string>

int main() {
    // 声明并初始化字符串
    std::string greeting = "Hello, World!";
    std::cout << "Greeting: " << greeting << std::endl;

    // 使用 size() 获取字符串长度
    std::cout << "Length of the greeting: " << greeting.size() << std::endl;

    // 使用 empty() 检查字符串是否为空
    std::cout << "Is the greeting empty? " << (greeting.empty() ? "Yes" : "No") << std::endl;

    // 使用 operator[] 访问特定位置的字符
    std::cout << "Character at position 7: " << greeting[7] << std::endl;

    // 使用 substr() 获取子字符串
    std::string sub = greeting.substr(7, 5);
    std::cout << "Substring from position 7 with length 5: " << sub << std::endl;

    // 使用 find() 查找子字符串
    std::cout << "Position of 'World' in the greeting: " << greeting.find("World") << std::endl;

    // 使用 replace() 替换字符串中的部分内容
    // 替换 'World' 为 'C++'
    std::string modified = greeting;
    std::string::size_type pos = modified.find("World");
    if (pos != std::string::npos) {
        modified.replace(pos, 5, "C++"); // 从位置 pos 开始，替换 5 个字符为 "C++"
    }
    std::cout << "Modified greeting: " << modified << std::endl;

    return 0;
}

Length of the greeting: 13
Is the greeting empty? No
Character at position 7: W
Substring from position 7 with length 5: World
Position of 'World' in the greeting: 7
Modified greeting: Hello, C++!

#include <iostream>
#include <string>

int main() {
    std::string str = "Hello, World!";
    
    // size()
    std::cout << "Length: " << str.size() << std::endl;

    // empty()
    std::cout << "Is empty? " << (str.empty() ? "Yes" : "No") << std::endl;

    // operator[]
    std::cout << "First character: " << str[0] << std::endl;

    // at()
    std::cout << "Character at position 7: " << str.at(7) << std::endl;

    // substr()
    std::string sub = str.substr(7, 5);
    std::cout << "Substring from position 7 with length 5: " << sub << std::endl;

    // find()
    size_t pos = str.find("World");
    std::cout << "Position of 'World': " << pos << std::endl;

    // replace()
    str.replace(pos, 5, "C++");
    std::cout << "Modified string: " << str << std::endl;

    // append()
    str.append(" How are you?");
    std::cout << "Appended string: " << str << std::endl;

    // insert()
    str.insert(7, " Beautiful");
    std::cout << "String after insert: " << str << std::endl;

    // erase()
    str.erase(7, 10);
    std::cout << "String after erase: " << str << std::endl;

    // clear()
    str.clear();
    std::cout << "String after clear: " << (str.empty() ? "Empty" : "Not empty") << std::endl;

    // c_str()
    str = "Hello, C++!";
    const char* cstr = str.c_str();
    std::cout << "C-style string: " << cstr << std::endl;

    // compare()
    int cmp = str.compare("Hello, C++!");
    std::cout << "Comparison result: " << cmp << std::endl;

    // find_first_of()
    size_t pos_first_vowel = str.find_first_of("aeiou");
    std::cout << "First vowel at position: " << pos_first_vowel << std::endl;

    // find_last_of()
    size_t pos_last_vowel = str.find_last_of("aeiou");
    std::cout << "Last vowel at position: " << pos_last_vowel << std::endl;

    // find_first_not_of()
    size_t pos_first_non_vowel = str.find_first_not_of("aeiou");
    std::cout << "First non-vowel at position: " << pos_first_non_vowel << std::endl;

    // find_last_not_of()
    size_t pos_last_non_vowel = str.find_last_not_of("aeiou");
    std::cout << "Last non-vowel at position: " << pos_last_non_vowel << std::endl;

    return 0;
}

Length: 13
Is empty? No
First character: H
Character at position 7: W
Substring from position 7 with length 5: World
Position of 'World': 7
Modified string: Hello, C++!
Appended string: Hello, C++! How are you?
String after insert: Hello,  BeautifulC++! How are you?
String after erase: Hello, C++! How are you?
String after clear: Empty
C-style string: Hello, C++!
Comparison result: 0
First vowel at position: 1
Last vowel at position: 4
First non-vowel at position: 0
Last non-vowel at position: 10