为什么我不能在Mac OS X上阅读和追加std :: fstream？

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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