什么是C++中的RAII原则？

#include <iostream>
#include <fstream>

class FileHandler {
private:
    std::fstream file;

public:
    // 构造函数获取资源
    FileHandler(const std::string& filename, std::ios_base::openmode mode = std::ios_base::in) 
        : file(filename, mode) {
        if (!file.is_open()) {
            throw std::runtime_error("Unable to open file");
        }
        std::cout << "File opened: " << filename << std::endl;
    }

    // 析构函数释放资源
    ~FileHandler() {
        if (file.is_open()) {
            file.close();
            std::cout << "File closed" << std::endl;
        }
    }

    // 读取文件内容
    std::string readLine() {
        std::string line;
        std::getline(file, line);
        return line;
    }
};

int main() {
    try {
        FileHandler file("example.txt");
        std::cout << file.readLine() << std::endl;
    } catch (const std::exception& e) {
        std::cerr << "Error: " << e.what() << std::endl;
    }
    return 0;
}