如何在一个实时数据的ofstream文件中始终有N行

#include <iostream>
#include <fstream>
#include <string>
#include <vector>

void writeData(const std::string& filename, const std::string& data, int N) {
    std::ofstream ofs(filename, std::ios::app); // 打开文件，追加模式
    if (!ofs) {
        std::cerr << "Failed to open file for writing." << std::endl;
        return;
    }

    std::ifstream ifs(filename); // 打开文件，读取行数
    if (!ifs) {
        std::cerr << "Failed to open file for reading." << std::endl;
        return;
    }

    int lineCount = 0;
    std::string line;
    while (std::getline(ifs, line)) {
        lineCount++;
    }

    ifs.close();

    if (lineCount < N) {
        ofs << data << std::endl; // 直接写入数据
    } else {
        std::vector<std::string> lines;
        ifs.open(filename); // 重新打开文件，读取除最早一行外的数据
        if (!ifs) {
            std::cerr << "Failed to open file for reading." << std::endl;
            return;
        }

        while (std::getline(ifs, line)) {
            lines.push_back(line);
        }

        ifs.close();

        ofs.close();
        ofs.open(filename, std::ios::trunc); // 打开文件，截断模式

        for (int i = 1; i < lineCount; i++) {
            ofs << lines[i] << std::endl; // 重新写入除最早一行外的数据
        }

        ofs << data << std::endl; // 写入新数据
    }

    ofs.close();
}

int main() {
    std::string filename = "data.txt";
    std::string data = "New data";
    int N = 5;

    writeData(filename, data, N);

    return 0;
}