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();
return 0;
}
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();
return 0;
}
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(const wchar_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();
return 0;
}
And now when you open the file, you get what you wanted in the first place.
And that is all!
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