Singleton ofstram：创建但未写入的文件

Question

我创建了一个单例class来包装一些ofstream

struct OutputFiles {
 public: 
  std::ofstream minus_total, plus_total, HoughMappingMinus_cc, HoughMappingPlus_cc;

  template <typename T>
  static void init(OutputFilesName<T>& files_name) {
    instance_impl(files_name);
  }

  static OutputFiles& instance() {
    return instance_impl();
  }

  template <typename T>
  OutputFiles(OutputFilesName<T>& files_name) :
    minus_total(files_name.resut_subdir_name + "/" + files_name.result_file_name_data_M),
    plus_total(files_name.resut_subdir_name + "/" + files_name.result_file_name_data_P),
    HoughMappingMinus_cc(files_name.resut_subdir_name + "/" + files_name.result_file_name_M_CC),
    HoughMappingPlus_cc(files_name.resut_subdir_name + "/" + files_name.result_file_name_P_CC)
  { }
  OutputFiles() = default;
  OutputFiles(const OutputFiles&) = delete;
  OutputFiles& operator=(const OutputFiles&) = delete;

 private:
  template <typename... Args>
  static OutputFiles& instance_impl(Args&&... args) {
    static OutputFiles singleton( std::forward<Args&&>(args)... );
    return singleton;
  }
};

这个 class 必须用 OutputFilesName<T> class

初始化

template <typename T>
struct OutputFilesName {
  OutputFilesName(
    const T&& i_resut_subdir_name,
    const T&& i_result_file_name_M_CC,
    const T&& i_result_file_name_P_CC,
    const T&& i_result_file_name_data_M,
    const T&& i_result_file_name_data_P
  ) : 
    resut_subdir_name(i_result_file_name_M_CC),
    result_file_name_M_CC(i_result_file_name_M_CC),
    result_file_name_P_CC(i_result_file_name_P_CC),
    result_file_name_data_M(i_result_file_name_data_M),
    result_file_name_data_P(i_result_file_name_data_P)
  { }
  const T resut_subdir_name;
  const T result_file_name_M_CC;
  const T result_file_name_P_CC;
  const T result_file_name_data_M;
  const T result_file_name_data_P;
};

我写主要代码调用它：

int main(){
    OutputFilesName<std::string> fn{"fa_name","fb_name","aa","bb","cc"};
    fs::create_directory(fn.resut_subdir_name);
    OutputFiles::init(fn);
    OutputFiles::instance().minus_total << "1" << std::endl;
    OutputFiles::instance().plus_total << "2" << std::endl;
    OutputFiles::instance().HoughMappingMinus_cc << "3"<< std::endl;
    OutputFiles::instance().HoughMappingPlus_cc << "4"<< std::endl;
    bool open1 = OutputFiles::instance().minus_total.is_open();
    bool open2 = OutputFiles::instance().plus_total.is_open();
    bool open3 = OutputFiles::instance().HoughMappingMinus_cc.is_open();
    bool open4 = OutputFiles::instance().HoughMappingPlus_cc.is_open();
    std::cout << open1 << open2 << open3 << open4 << std::endl;
}

结果和完整代码here,

ofstream好像打不开了

文件和目录已创建，

但是内容是空的

我很困惑。

Answer 1

这是一种管理对象的奇怪方式。

您的 OutputFiles::init(fn) 调用 instance_impl(fn)，一个函数模板特化。该专业化有一个名为 singleton 的 static 局部变量，使用 ctor args fn 实例化，您 return 对其进行引用。一切顺利。

然后，您 OutputFiles::instance() 得到一个 OutputFiles&。问题是，该函数调用 instance_impl()，一个不同的函数模板特化。该特化也有一个名为 singleton 的 static 局部变量，默认构造。因此，它的所有 ofstream 成员也都是默认构造的，流式传输给它们什么也做不了。

关键是instance_impl的两个特化是不同的函数。所以这不是绕过静态初始化的有效方法。

为什么不以正常方式实例化一个 OutputFiles 并将其存储在某个地方？

Singleton ofstram：创建但未写入的文件

Singleton ofstram: Files created but did not be written

c++

singleton

ofstream