如何确定std :: ofstream打开的文件的当前大小？

Question

我有一个类型的文件流ofstream.构造函数以追加模式打开文件,所有消息总是写在文件末尾.

我需要写入一些固定大小的输出文件,例如1Mb,然后我需要关闭,重命名和压缩它,然后打开一个同名的新文件.

这需要在达到特定大小的文件时完成.

我尝试使用tellg()但是在网上阅读了东西(和这个)后,我明白这不是正确的方法.

由于我是C++的新手,我正在尝试找出最优化和正确的方法来获得准确的文件当前大小ofstream？

class Logger {
    std::ofstream outputFile;
    int curr_size;
    Logger (const std::string logfile) : outputFile(FILENAME,
                                                     std::ios::app)
    {
        curr_size = 0;
    }
};

在程序的某个地方,我正在写入数据:

    // ??? Determine the size of current file ???

    if (curr_size >= MAX_FILE_SIZE) {
        outputFile.close();
        //Code to rename and compress file
        // ...
        outputFile.open(FILENAME, std::ios::app);
        curr_size = 0;
    }

    outputFile << message << std::endl;
    outputFile.flush();

Answer 1

fstreams 既可以是输入流，也可以是输出流。tellg()将返回输入位置并tellp()告诉您输出位置。tellp()附加到文件后会告诉你它的大小。

考虑像这样初始化您的记录器（编辑：添加输出流运算符的示例）：

#include <iostream>
#include <fstream>

class Logger {
    std::string m_filename;
    std::ofstream m_os;
    std::ofstream::pos_type m_curr_size;
    std::ofstream::pos_type m_max_size;
public:
    Logger(const std::string& logfile, std::ofstream::pos_type max_size) :
        m_filename(logfile),
        m_os(m_filename, std::ios::app),
        m_curr_size(m_os.tellp()),
        m_max_size(max_size)
    {}

    template<typename T>
    friend Logger& operator<<(Logger&, const T&);
};

template<typename T>
Logger& operator<<(Logger& log, const T& msg) {
    log.m_curr_size = (log.m_os << msg << std::flush).tellp();

    if(log.m_curr_size>log.m_max_size) {
        log.m_os.close();
        //rename & compress
        log.m_os = std::ofstream(log.m_filename, std::ios::app);
        log.m_curr_size = log.m_os.tellp();
    }
    return log;
}

int main()
{
    Logger test("log", 4LL*1024*1024*1024*1024);
    test << "hello " << 10 << "\n";
    return 0;
}

如果您使用 C++17 或有可用的实验版本<filesystem>，您还可以使用它来获取绝对文件大小，如下所示：

#include <iostream>
#include <fstream>
#include <filesystem>

namespace fs = std::filesystem;

class Logger {
    fs::directory_entry m_logfile;
    std::ofstream m_os;
    std::uintmax_t m_max_size;

    void rotate_if_needed() {
        if(max_size_reached()) {
            m_os.close();
            //rename & compress
            m_os = std::ofstream(m_logfile.path(), std::ios::app);
        }
    }
public:
    Logger(const std::string& logfile, std::uintmax_t max_size) :
        m_logfile(logfile),
        m_os(m_logfile.path(), std::ios::app),
        m_max_size(max_size)
    {
        // make sure the path is absolute in case the process
        // have changed current directory when we need to rotate the log
        if(m_logfile.path().is_relative())
            m_logfile = fs::directory_entry(fs::absolute(m_logfile.path()));
    }

    std::uintmax_t size() const { return m_logfile.file_size(); }
    bool max_size_reached() const { return size()>m_max_size; }

    template<typename T>
    friend Logger& operator<<(Logger&, const T&);
};

template<typename T>
Logger& operator<<(Logger& log, const T& msg) {
    log.m_os << msg << std::flush;
    log.rotate_if_needed();
    return log;
}

int main()
{
    Logger test("log", 4LL*1024*1024*1024*1024);
    std::cout << test.size() << "\n";
    test << "hello " << 10 << "\n";
    std::cout << test.size() << "\n";
    test << "some more " << 3.14159 << "\n";
    std::cout << test.size() << "\n";
    return 0;
}