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

爱被打了一巴掌 2022-08-19 01:41 174阅读 0赞

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:

![ieutf8_1.png][]

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.

![ieutf8_2.png][]

And that is all!

[ieutf8_1.png]: /images/20220731/e7277d78ad414008899cd0e0276e6865.png
[ieutf8_2.png]: /images/20220731/5036e205c8e94f91a32a18ee2fe5975a.png