std::strstream not implementing eof and peek properly?

Question

I have a byte data source defined like this:

char * data;
unsigned int dataSize;

dataSize is non zero and often quite large (megabytes)

The following code works:

std::string str(data, dataSize);
std::istringstream stream(str);
char firstByte = stream.peek();

stream.eof() is false, and firstByte is 1, which is correct

The following code does not work:

std::strstream stream(data, dataSize);
char firstByte = stream.peek();

stream.eof() is true, and firstByte is -1, which is incorrect

I know that strstream is deprecated, but in this case it avoids allocating and copying twice the incoming data, which is nice. But why are peek and eof not working ?

EDIT : If I replace std::strstream by std::istrstream, this works fine. And this is OK as I'm in fact only reading from data. But why std::strstream is not working in that case? I'm just curious.