Visual Studio C++ 2008 Manipulating Bytes?

Question

I'm trying to write strictly binary data to files (no encoding). The problem is, when I hex dump the files, I'm noticing rather weird behavior. Using either one of the below methods to construct a file results in the same behavior. I even used the System::Text::Encoding::Default to test as well for the streams.

StreamWriter^ binWriter = gcnew StreamWriter(gcnew FileStream("test.bin",FileMode::Create));

(Also used this method)
FileStream^ tempBin = gcnew FileStream("test.bin",FileMode::Create);
BinaryWriter^ binWriter = gcnew BinaryWriter(tempBin);


binWriter->Write(0x80);
binWriter->Write(0x81);
.
.
binWriter->Write(0x8F);
binWriter->Write(0x90);
binWriter->Write(0x91);
.
.
binWriter->Write(0x9F);

Writing that sequence of bytes, I noticed the only bytes that weren't converted to 0x3F in the hex dump were 0x81,0x8D,0x90,0x9D, ... and I have no idea why.

I also tried making character arrays, and a similar situation happens. i.e.,

array<wchar_t,1>^ OT_Random_Delta_Limits = {0x00,0x00,0x03,0x79,0x00,0x00,0x04,0x88};
binWriter->Write(OT_Random_Delta_Limits);

0x88 would be written as 0x3F.

Answer 1

A BinaryWriter() class initialized with a stream will use a default encoding of UTF8 for any chars or strings that are written. I'm guessing that the

binWriter->Write(0x80);
binWriter->Write(0x81);
.
.
binWriter->Write(0x8F);
binWriter->Write(0x90);
binWriter->Write(0x91);

calls are binding to the Write( char) overload so they're going through the character encoder. I'm not very familiar with C++/CLI, but it seems to me that these calls should be binding to Write(Int32), which shouldn't have this problem (maybe your code is really calling Write() with a char variable that's set to the values in your example. That would account for this behavior).

Answer 2

0x3F is commonly known as the ASCII character '?'; the characters that are mapping to it are control characters with no printable representation. As Jon points out, use a binary stream rather than a text-oriented output mechanism for raw binary data.

EDIT -- actually your results look like the inverse of what I would expect. In the default code page 1252, the non-printable characters (i.e. ones likely to map to '?') in that range are 0x81, 0x8D, 0x8F, 0x90 and 0x9D