为什么我不能在Mac

为什么我不能在Mac

本文介绍了为什么我不能在Mac OS X上阅读和追加std :: fstream?的处理方法,对大家解决问题具有一定的参考价值,需要的朋友们下面随着小编来一起学习吧!

问题描述

考虑下面的C ++程序,它接受一个文件并打印每一行。这是一个较大的程序片段,我后来根据我看到的文件附加。

Consider the following C++ program, which takes a file and prints each line. It's a slice of a larger program where I later append to the file, based on what I see.

#include <fstream>
using std::fstream;
#include <iostream>
#include <string>
using std::string;

int main()
{
 fstream file("file.txt", fstream::in | fstream::out | fstream::app);

 string line;
 while (std::getline(file, line))
  std::cerr << line << std::endl;

 return 0;
}

现在应用此版本的 file.txt (第一行一个单词,后面跟换行):

Now apply this version of file.txt (One word on the first line, followed by a newline):

Rain

在我的机器上(Snow Leopard),这些都不打印出来。仔细检查,第一次调用getline失败。奇怪的是,如果我添加第二行,它也失败:仍然没有打印!

On my machine (Snow Leopard), this prints out nothing. On closer inspection, the first call to getline fails. Strangely, it also fails if I add a second line: still nothing is printed!

任何人都可以解决这个谜吗?

Can anyone solve this mystery?

推荐答案

当你说:

fstream file("file.txt", fstream::in | fstream::out | fstream::app);

以附加模式打开文件 - 即在结尾。只需以读取模式打开它:

you open the file in append mode - i.e. at the end. Just open it in read mode:

fstream file("file.txt", fstream::in );

或使用ifstream:

or use an ifstream:

ifstream file("file.txt" );

当然,如Earwicker所说,你应该总是测试开放成功。

And of course as Earwicker suggests, you should always test that the open succeeded.

如果您确定以追加模式打开,您可以显式移动读取指针:

If you are determined to open in append mode, you can move the read pointer explicitly:

#include <fstream>
#include <iostream>
#include <string>
using namespace std;

int main() {
    fstream file( "afile.txt", ios::in | ios::out | ios::app );
    if ( ! file.is_open()  ) {
        cerr << "open failed" << endl;
        return 1;
    }
    else {
        file.seekg( 0, ios::beg );   // move read pointer
        string line;
        while( getline( file, line ) ) {
            cout << line << endl;
        }
    }
}

/ strong>似乎在文件打开中使用的标志的组合导致实现特定的行为。上面的代码在Windows上使用g ++,但在Linux上不适用于g ++。

It seems that the combination of flags used in the opening of the file leads to implementation specific behaviour. The above code works with g++ on Windows, but not with g++ on Linux.

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08-22 19:07