VS2019 C++ 通过 filesystem 对外部普通的配置文件中字符串读写功能的实现，不需要键值对的实现方法并可以读取指定行的信息

在Visual Studio 2019的C++中，如果你想要通过`filesystem`库直接读写外部普通文本配置文件（如`.txt`），而无需使用键值对，你可以使用文件流（fstream）来操作。以下是基本的步骤：

1. 包含头文件：

#include <iostream>
#include <filesystem> // for filesystem operations

2. 创建文件读写函数：

std::string readLineFromFile(const std::string& filePath, size_t lineNumber) {
    std::ifstream file(filePath);
    if (!file.is_open()) {
        throw std::runtime_error("Failed to open the file.");
    }
    
    std::streampos position = lineNumber - 1; // Subtract one because file indexing starts from 0
    file.seekg(position, std::ios_base::beg); // Seek to the specified line

    std::string line;
    std::getline(file, line);
    file.close();

    return line;
}

void writeLineToFile(const std::string& filePath, const std::string& content, size_t lineNumber) {
    std::ofstream file(filePath, std::ios::app); // 'a' for appending
    if (!file.is_open()) {
        throw std::runtime_error("Failed to open the file for writing.");
    }

    file << "\n" << content; // Add newline before writing
    file.seekp(lineNumber * '\n', std::ios_base::cur); // Go back to the start of the line
    file << content; // Write the content

    file.close();
}

3. 使用示例：

int main() {
    try {
        std::string configFile = "config.txt";
        std::string lineToRead = readLineFromFile(configFile, 2); // Read second line
        std::cout << "Line 2 content: " << lineToRead << std::endl;

        std::string newLine = "New configuration value";
        writeLineToFile(configFile, newLine, 5); // Append to line 5
    } catch (const std::exception& e) {
        std::cerr << "Error: " << e.what() << std::endl;
    }

    return 0;
}