create & append string into a file

FileAppender.cpp

struct FileAppender {
    std::ofstream _file;
    std::mutex _lock;

    explicit FileAppend(const std::string& filename) {
        _file.open(filename.c_str(), std::ios_base::app);
    }

    virtual ~FileAppend() {
        _file.flush();
        _file.close();
    }

    void append(const std::string& content) {
        std::lock_guard<std::mutex> lock(_lock);
        _file << content;
    }

    void appendNow(const std::string& content) {
        append(content);
        _file.flush();
    }

    void flush() {
        _file.flush();
    }

    void append(const char* fmt, ...) {
        va_list args;
        va_start(args, fmt);
        std::ostringstream ss;
        while (*fmt != '\0') {
            if (*fmt == '%') {
                fmt++;
                switch (*fmt) {
                    case 'd':
                        ss << va_arg(args, int);
                        break;
                    case 'f':
                        ss << va_arg(args, float);
                    case 'l':
                        if (*(fmt+1) == 'd') {fmt++;}
                        ss << va_arg(args, long);
                    default:
                        //not supported yet
                        assert(false);
                }
            } else {
                ss << *fmt;
            }
            fmt++;
        }
        va_end(args);
        std::lock_guard<std::mutex> lock(_lock);
        _file << ss.str();
    }


    void appendLine(const std::string& content) {
        std::lock_guard<std::mutex> lock(_lock);
        _file << content;
        _file << "\n";
    }

};