What is std::wifstream::getline doing to my wchar_t array? It's treated like a byte array after getline returns

Question

I want to read lines of Unicode text (UTF-16 LE, line feed delimited) from a file. I'm using Visual Studio 2012 and targeting a 32-bit console application.

I was not able to find a ReadLine function within WinAPI so I turned to Google. It is clear I am not the first to seek such a function. The most commonly recommended solution involves using std::wifstream.

I wrote code similar to the following:

wchar_t buffer[1024];
std::wifstream input(L"input.txt");

while (input.good())
{
    input::getline(buffer, 1024);
    // ... do stuff...
}

input.close();

For the sake of explanation, assume that input.txt contains two UTF-16 LE lines which are less than 200 wchar_t chars in length.

Prior to calling getline the first time, Visual Studio correctly identifies that buffer is an array of wchar_t. You can mouse over the variable in the debugger and see that the array is comprised of 16-bit values. However, after the call to getline returns, the debugger now displays buffer as if is a byte array.

After the first call to getline, the contents of buffer are correct (aside from buffer being treated like a byte array). If the first line of input.txt contains the UTF-16 string L"123", this is correctly stored in buffer as (hex) "31 00 32 00 33 00"

My first thought was to reinterpret_cast<wchar_t *>(buffer) which does produce the desired result (buffer is now treated like a wchar_t array) and it contains the values I expect.

However, after the second call to getline, (the second line of input.txt contains the string L"456") buffer contains (hex) "00 34 00 35 00 36 00". Note that this is incorrect (it should be [hex] 34 00 35 00 36 00)

The fact that the byte ordering gets messed up prevents me from using reinterpret_cast as a solution to work around this. More importantly, why is std::wifstream::getline even converting my wchar_t buffer into a char buffer anyways?? I was under the impression that if one wanted to use chars they would use ifstream and if they want to use wchar_t they use wifstream...

I am terrible at making sense of the stl headers, but it almost looks as if wifstream is intentionally converting my wchar_t to a char... why??

I would appreciate any insights and explanations for understanding these problems.

Answer 1

wifstream reads bytes from the file, and converts them to wide chars using codecvt facet installed into the stream's locale. The default facet assumes system-default code page and calls mbstowcs on those bytes.

To treat your file as UTF-16, you need to use codecvt_utf16. Like this:

std::wifstream fin("text.txt", std::ios::binary);
// apply facet
fin.imbue(std::locale(fin.getloc(),
          new std::codecvt_utf16<wchar_t, 0x10ffff, std::little_endian>));