read does not work with basic_ifstream< unsigned char >

[Luciano Montanaro]

Hi all,
while trying to avoid a cast when reading from a file to a buffrer,
I changed the type of the input file from ifstream to basic_ifstream<
unsigned char >, and suddenly read() does not work anymore.

Is this the expected behaviour? and if it is so, why?

This is a small test program that shows the problem:

#include <fstream>
#include <iostream>

using namespace std;

typedef char mytype;
//typedef unsigned char mytype;

int main(int argc, char **argv)
{
basic_ifstream< mytype > in("testfile", ios::binary);

mytype buf[100];
in.read(buf, 100);
cout << in.gcount() << " bytes read." << endl;

in.close();
}

If mytype is char, the program prints "6" (the number of characters in
testfile), if it is defined as unsigned char, it prints out 0.

Thanks for any clarification,
Luciano

[John Love-Jensen]

Hi Luciano,

You haven't plumbed up the facet, the locale, and/or the codecvt facilities
for your new stream's base type.

Some C++ compilers give you those things for char, and plumb char up to
locale facilities provided by the OS (e.g., the LC_ALL environment
variable).

Most C++ compilers give you those things for char and wchar_t, and, again,
plumb char and wchar_t up to locale facilities provided by the OS.

I don't know of any C++ compilers that provide you those things for unsigned
char, nor do I know of any OS that provides those facilities for unsigned
char.

Take a look at "Standard C++ IOStreams and Locales" by Angelika Langer and
Klaus Kreft, for all the gory details.

You'll be whipping up your own custom std::basic_ifstream<float> and
std::basic_ifstream<MyBigStruct> in no time.

HTH,
--Eljay