如何在C++中将文件编码格式设置为UTF8

Question

A requirement for my software is that the encoding of a file which contains exported data shall be UTF8. But when I write the data to the file the encoding is always ANSI. (I use Notepad++ to check this.)

What I'm currently doing is trying to convert the file manually by reading it, converting it to UTF8 and writing the text to a new file.

line is a std::string
inputFile is an std::ifstream
pOutputFile is a FILE*

// ...

if( inputFile.is_open() )
{
    while( inputFile.good() )
    {
        getline(inputFile,line);

        //1
        DWORD dwCount = MultiByteToWideChar( CP_ACP, 0, line.c_str(), -1, NULL, 0 );
        wchar_t *pwcharText;
        pwcharText = new wchar_t[ dwCount];

        //2
        MultiByteToWideChar( CP_ACP, 0, line.c_str(), -1, pwcharText, dwCount );

        //3
        dwCount = WideCharToMultiByte( CP_UTF8, 0, pwcharText, -1, NULL, 0, NULL, NULL );
        char *pText;
        pText = new char[ dwCount ];

        //4
        WideCharToMultiByte( CP_UTF8, 0, pwcharText, -1, pText, dwCount, NULL, NULL );

        fprintf(pOutputFile,pText);
        fprintf(pOutputFile,"\n");

        delete[] pwcharText;
        delete[] pText;
    }
}

// ...

Unfortunately the encoding is still ANSI. I searched a while for a solution but I always encounter the solution via MultiByteToWideChar and WideCharToMultiByte. However, this doesn't seem to work. What am I missing here?

I also looked here on SO for a solution but most UTF8 questions deal with C# and php stuff.

Answer 1

On Windows in VC++2010 it is possible (not yet implemented in GCC, as far as i know) using localization facet std::codecvt_utf8_utf16 (ie in C++11). The sample code from cppreference.com has all basic information you would need to read/write UTF-8 file.

std::wstring wFromFile = _T("𤭢teststring");
std::wofstream fileOut("textOut.txt");
fileOut.imbue(std::locale(fileOut.getloc(), new std::codecvt_utf8_utf16<wchar_t>));
fileOut<<wFromFile;

It sets the ANSI encoded file to UTF-8 (checked in Notepad). Hope this is what you need.

Answer 2

On Windows, files don't have encodings. Each application will assume an encoding based on its own rules. The best you can do is put a byte-order mark at the front of the file and hope it's recognized.

Answer 3

AFAIK, fprintf() does character conversions, so there is no guarantee that passing UTF-8 encoded data to it will actually write the UTF-8 to the file. Since you already converted the data yourself, use fwrite() instead so you are writing the UTF-8 data as-is, eg:

DWORD dwCount = MultiByteToWideChar( CP_ACP, 0, line.c_str(), line.length(), NULL, 0 );  
if (dwCount == 0) continue;

std::vector<WCHAR> utf16Text(dwCount);  
MultiByteToWideChar( CP_ACP, 0, line.c_str(), line.length(), &utf16Text[0], dwCount );  

dwCount = WideCharToMultiByte( CP_UTF8, 0, &utf16Text[0], utf16Text.size(), NULL, 0, NULL, NULL );  
if (dwCount == 0) continue;

std::vector<CHAR> utf8Text(dwCount);  
WideCharToMultiByte( CP_UTF8, 0, &utf16Text[0], utf16Text.size(), &utf8Text[0], dwCount, NULL, NULL );  

fwrite(&utf8Text[0], sizeof(CHAR), dwCount, pOutputFile);  
fprintf(pOutputFile, "\n");  

Answer 4

The type char has no clue of any encoding, all it can do is store 8 bits. Therefore any text file is just a sequence of bytes and the user must guess the underlying encoding. A file starting with a BOM indicates UTF 8, but using a BOM is not recommended any more. The type wchar_t in contrast is in Windows always interpreted as UTF 16.

So let's say you have a file encoded in UTF 8 with just one line: "Confucius says: Smile. 孔子说：微笑！." The following code snippet appends this text once more, then reads the first line and displays it in a MessageBoxW and MessageBoxA . Note that MessageBoxW shows the correct text while MessageBoxA shows some junk because it assumes my local codepage 1252 for the char* string.

Note that I have used the handy CA2W class instead of MultiByteToWideChar . Be careful, the CP_Whatever argument is optional and if omitted the local codepage is used.

#include <iostream>
#include <fstream>
#include <filesystem>
#include <atlbase.h>

int main(int argc, char** argv)
{
  std::fstream  afile;
  std::string line1A = u8"Confucius says: Smile. 孔子说：微笑！ 😊";
  std::wstring line1W;

  afile.open("Test.txt", std::ios::out | std::ios::app);
  if (!afile.is_open())
        return 0;

  afile << "\n" << line1A;
  afile.close();

  afile.open("Test.txt", std::ios::in);
  std::getline(afile, line1A);
  line1W = CA2W(line1A.c_str(), CP_UTF8);
  MessageBoxW(nullptr, line1W.c_str(), L"Smile", 0);
  MessageBoxA(nullptr, line1A.c_str(), "Smile", 0);
  afile.close();

  return 0;
}