C ++ ifstream类型不完整 [英] C++ ifstream has incomplete type

问题描述

我正在修改一个项目，我想将输出写入文件.在我的 .hpp 文件中，我声明了一个文件流，如下所示:

I'm modifying an project where I want to write an output to a file. In my .hpp file I declared a file stream like so:

// fname.hpp file
#include <fstream>

std::ifstream m_ifs = ("path_to_file");

然后在我的 .cpp 文件中

// fname.cpp
if (!m_ifs.is_open()) {
      std::cerr << "Not writing to file" << std::endl;
      return;
}
m_ifs << "Write something"

但是当我编译时，出现以下错误:

But when I compile I get the following error:

error: field ‘m_ifs’ has incomplete type ‘std::ifstream {aka std::basic_ifstream<char>}’

/usr/include/c++/5/iosfwd:116:11: note: declaration of ‘std::ifstream {aka class std::basic_ifstream<char>}’
     class basic_ifstream;

错误指向我在 hpp 文件上的声明.

The error points to my declaration on the hpp file.

我在网上找到的解决方案无法解决问题.

The solutions I found on the web didn't solve the problem.

以下工作有效:

// fname.hpp file
#include <fstream>

const std::string m_file = ("path_to_file");
std::fstream m_fs;

和

// fname.cpp
m_fs.open(m_file.c_str());
if (!m_fs.is_open()) {
      std::cerr << "Not writing to file" << std::endl;
      return;
}
m_fs << "Write something"

推荐答案

对我来说，它似乎是一个全局变量.如果确实如此，请通过以下方式声明它们:

It looks like a global variable to me. If it indeed is, declare them the following way:

//fname.hpp:
const std::string m_file = ("path_to_file");
std::fstream m_fs;

和

// fname.cpp
m_fs.open(m_file.c_str());
if (!m_fs.is_open()) {
      std::cerr << "Not writing to file" << std::endl;
      return;
}
m_fs << "Write something"

此外，您无法写入ifstream.将其更改为ofstream或fstream.

Also, you can't write to ifstream. Change it to ofstream or fstream.

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