在同一程序中混合cout和wcout [英] Mixing cout and wcout in same program

问题描述

我正在阅读C ++ Cookbook，其中包含以下代码片段：

  // cout< s<< std :: endl; //你不应该能够
 wcout<< ws < std :: endl; // run these at the same time 
  

如果你有兴趣看到实际的例子，的页面中的链接谷歌图书。

另外，我发现这个的 SO问题，似乎声明混合wcout和cout是好的。

从C ++标准[27.4.1]：

相应wide-和窄字符混合操作流遵循相同的语义混合这样的操作上的文件，如ISO C标准的修订1指定

从C标准[7.19.2]：

每个流都有一个方向。后在其上执行任何操作之前一个流与外部文件，但
相关联的，该流是无定向。一旦一个宽
字符输入/输出功能已应用到流无定向，流变成一个面向宽流。同样，一旦一个字节的输入/输出功能具有
被施加到流无定向，流变成一个面向字节的流。
只有调用freopen函数或fwide函数才能改变流的
方向。 （成功调用freopen会删除任何方向。）

字节输入/输出函数不应用于宽定向流和宽
字符输入/输出函数不应该应用于面向字节的流。

所以，标准似乎说，你不应该混合它们。但是，我发现这篇引文从这篇文章：

对于Visual C ++ 10.0，fwide函数被记录为未实现。从实际的角度来看，至少在输出整行的水平上，它显然可以很好地混合使用cout和wcout。所以，幸运的是，Visual C ++显然只是忽略了标准的要求，并且不保持一个不切实际的显式C FILE流定向。

关于gcc我发现这个报价从此处：

这是一个新功能，而不是错误，请参阅libstdc ++ / 11705并且一般在C标准（C99,7.19.2）中搜索关于流定向的
。简而言之，你
不能混合面向字节和宽的I / O。现在，由于libstdc ++ / 11705中指出的
，你可以通过调用std :: ios :: sync_with_stdio（false）来获得接近
期望值的东西。

解决方案

当 cout 或 wcout 第一次调用时， stdout 的方向设置。在 cout 的情况下， stdout 成为面向字节的流，而在 wcout ， stdout 成为面向广泛的流。根据C ++标准[27.4.1]和C标准[7.19.2]，一旦流的方向被设置，你不应该调用与该流的方向不兼容的函数。

I was reading the "C++ Cookbook" which had the following snippet:

// cout  << s  << std::endl;  // You shouldn't be able to
wcout << ws << std::endl;     // run these at the same time

If you're interested in seeing the actual example, here is a link to the page on Google books.

Also, I found this SO question which seems to state that mixing wcout and cout is okay. Could someone explain to me what this comment is talking about?

EDIT

From C++ Standard [27.4.1]:

Mixing operations on corresponding wide- and narrow-character streams follows the same semantics as mixing such operations on FILEs, as specified in Amendment 1 of the ISO C standard.

From C Standard [7.19.2]:

Each stream has an orientation. After a stream is associated with an external file, but before any operations are performed on it, the stream is without orientation. Once a wide character input/output function has been applied to a stream without orientation, the stream becomes a wide-oriented stream. Similarly, once a byte input/output function has been applied to a stream without orientation, the stream becomes a byte-oriented stream. Only a call to the freopen function or the fwide function can otherwise alter the orientation of a stream. (A successful call to freopen removes any orientation.)

Byte input/output functions shall not be applied to a wide-oriented stream and wide character input/output functions shall not be applied to a byte-oriented stream.

So, the standard seems to say that you should not mix them. However, I found this quote from this article:

For Visual C++ 10.0 the fwide function is documented as being unimplemented. And from a practical point of view, at least at the level of outputting whole lines it apparently works fine to intermingle use of cout and wcout. So, happily, Visual C++ apparently just disregards the standard’s requirements and does not maintain an impractical explicit C FILE stream orientation.

And also, concerning gcc I found this quote from here:

This is a (new) feature, not a bug, see libstdc++/11705 and in general search about stream orientation in the C standard (C99, 7.19.2). In a nutshell you cannot mix byte oriented and wide oriented I/O. For now, due to the bug pointed out in libstdc++/11705, you can obtain something close to your expectations by calling std::ios::sync_with_stdio(false); at the beginning of your program.

解决方案

When cout or wcout is called for the first time, the orientation for stdout becomes set. In the case of cout, stdout becomes a byte-oriented stream, and in the case of wcout, stdout becomes a wide-oriented stream. As per the C++ standard [27.4.1] and C standard [7.19.2], once the orientation of a stream is set, you should not call a function which is not compatible with the orientation of that stream.

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