保存文件为UTF8格式XML file（Writing UTF-8 files in C++）

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 >

How do we write that in C++?

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;
}

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

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;
}

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.

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());
}

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;
}

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

And that is all!