Difference in using read/write when stream is open

问题:

In my experiments with the following code snippet, I did not find any particular difference whether i created the streams with/without the ios:binary mode:

int main()
{
    ifstream ostr("Main.cpp", ios::in | ios::binary | ios::ate);
    if (ostr.is_open())
    {
        int size = ostr.tellg();
        char * memBlock = new char[size + 1];
        ostr.seekg(0, ios::beg);
        ostr.read(memBlock, size);
        memBlock[size] = '\0';
        ofstream file("trip.cpp", ios::out | ios::binary);
        file.write(memBlock, size);
        ostr.close();
    }
}

Here I am trying to copy the original source file into another file with a different name.

My question is what is the difference between the read/write calls(which are associated with binary file IO) when an fstream object is opened with/without ios::binary mode ? Is there any advantage of using the binary mode ? when to and when not to use it when doing file IO ?

回答1:

The only difference between binary and text mode is how the '\n' character is treated.

In binary mode there is no translation.

In text mode \n is translated on write into a the end of line sequence.
In text mode end of line sequence is translated on read into \n.

The end of line sequence is platform dependant.

Examples:

ASCII based systems:

LF    ('\0x0A'):      Multics, Mac OS X, BeOS, Amiga, RISC OS
CRLF  ('\0x0a\0x0d'): Microsoft Windows, DEC TOPS-10, RT-11
CR:   ('\0x0D'):      TRS-80, Mac OS Pre X
RS:   ('\0x1E'):      QNX pre-POSIX implementation.

回答2:

When you want to write files in binary, with no modifications taking place to your data, specify the ios::binary flag. When you want to write files in text mode, don't specify ios::binary, and you may get things like line ending translation. If you're on a UNIX-like platform, binary and text formats are the same, so you won't see any difference.