C++ 保存文件为UTF8编码格式

前言

本文是我的第一篇译文，可能翻译不好，将中英同时呈上，便于大家阅读。

本文来自从天堂大鸟的编程博客的保存文件为UTF8格式（Writing
UTF-8 files in C++）.发现的。

原英文地址：http://mariusbancila.ro/blog/2008/10/20/writing-utf-8-files-in-c/

因为最近在使用tinyxml保存xml，发现其并不能将文件转换为utf8格式，一遇到中文显示，就出乱子，所以就百度一下，找到相关内容。觉得《Writing UTF-8 files in C++》比较浅显易读，就翻译了一下。

正文

当你要写下面一段XML文件内容

Let’s say you need to write an XML file with this content:

< ?xml version="1.0" encoding="UTF-8"? >
< root description="this is a naïve example" >
< /root >

如何用C++实现？

How do we write that in C++?

咋一看，你可能试图写成这个样子：

At a first glance, you could be tempted to write it like this:

#include< fstream >

int main()
{
std::ofstream testFile;

testFile.open("demo.xml", std::ios::out| std::ios::binary);

std::string text =
"< ?xml version=\"1.0\" encoding=\"UTF-8\"? >\n"
"< root description=\"this is a naïve example\" >\n< /root >";

testFile << text;

testFile.close();

return0;
}

当你用IE打开时，惊讶的发现不正确显示：

When you open the file in IE for instance, surprize! It's not rendered correctly:



因此，你可能会说：“采用wstring 和 wofstream"。

So you could be tempted to say "let's switch to wstring and wofstream".

int main()
{
std::wofstream testFile;

testFile.open("demo.xml", std::ios::out| std::ios::binary);

std::wstring text =
L"< ?xml version=\"1.0\" encoding=\"UTF-8\"? >\n"
L"< root description=\"this is a naïve example\" >\n< /root >";

testFile << text;

testFile.close();

return0;
}

当你运行程序并打开文件时，仍和以前一样。哦，问题出在哪里了？好了，这个问题既不是ofstream也不是用wofstream来写UTF-8编码格式文件。如果你想要实现真正的UFT-8编码格式，你需要在输出字符串中使用UTF-8。我们可以使用WideCharToMultiByte()。这个API函数可以实现将宽字符串转化为新的字符串（不一定是一个多字节字符集）。该函数的第一个参数是字符集编码类型，对于UTF-8格式我们纸需要使用CP_UTF8.

And when you run it and open the file again, no change. So, where is the problem? Well, the problem is that neither ofstream nor wofstream write the text in a UTF-8 format. If you want the file to really be in UTF-8 format,
you have to encode the output buffer in UTF-8. And to do that we can use WideCharToMultiByte(). This Windows API maps a wide character string to a new character string (which is not necessary from a multibyte character set). The first argument indicates the
code page. For UTF-8 we need to specify CP_UTF8.

下面函数能帮助我们从std::wstring 转成 UTF-8编码的 std::string。

The following helper functions encode a std::wstring into a UTF-8 stream, wrapped into a std::string.

#include< windows.h >

std::string to_utf8(constwchar_t* buffer,int len)
{
int nChars =::WideCharToMultiByte(
CP_UTF8,
0,
buffer,
len,
NULL,
0,
NULL,
NULL);
if(nChars ==0)return"";

string newbuffer;
newbuffer.resize(nChars);
::WideCharToMultiByte(
CP_UTF8,
0,
buffer,
len,
const_cast<char*>(newbuffer.c_str()),
nChars,
NULL,
NULL);

return newbuffer;
}

std::string to_utf8(const std::wstring& str)
{
return to_utf8(str.c_str(),(int)str.size());
}

有了这个基础，你只需要做如下改动：

With that in hand, all you have to do is doing the following changes:

int main()
{
std::ofstream testFile;

testFile.open("demo.xml", std::ios::out| std::ios::binary);

std::wstring text =
L"< ?xml version=\"1.0\" encoding=\"UTF-8\"? >\n"
L"< root description=\"this is a naïve example\" >\n< /root >";

std::string outtext = to_utf8(text);

testFile << outtext;

testFile.close();

return0;
}

现在你再打开文件，你得到你想要的结果：

And now when you open the file, you get what you wanted in the first place.



大功告成！

And that is all!