Questions about COSC1030 class code sample

[johntdowney]

Okay, so this project is done and over, so I'm not in a particular rush. I'm just trying to go through this code to figure out why some things are done in it. The code analyzes a BMP file and decodes a hidden message within it. I'll post the entire code and ask questions that have me stumped in between.

// BMPDecoder.cpp
// Retrieve coded message from BMP graphics file.  The "hidden" message is contained in the least significant
// bits of the bitmap RGB data.  If a data item is odd, that's a one-bit, if even, that's a zero-bit. Combine
// the bits from 8 contiguous data items together (big-endian order) to form a character.
// End of message is a '$'.

#include <iostream>
using std::cerr;
using std::cin;
using std::cout;
using std::endl;
#include <fstream>
using std::fstream;
#include <ios>
using std::ios;
using std::ios_base;
#include <string>
using std::string;
#include <limits>
using std::numeric_limits;

enum BMPErrorCode { FILE_OPEN_FAILED, FILE_NOT_BMP };

void OpenInputFile( fstream& In )
{
	string FileName;
	cout << "Enter input file name:  ";
	cin >> FileName;
	In.open( FileName.c_str(), ios::binary | ios::in);
	if ( In.fail() )
		throw( FILE_OPEN_FAILED );
}

void ConfirmBMP( fstream& In )
{
	char tmp1, tmp2;
	In.seekg(0);
	tmp1=In.get();
	tmp2=In.get();
	if ( tmp1!='B' || tmp2!='M' || In.fail() )
		throw( FILE_NOT_BMP );
	
	unsigned int startBitmap;
	In.seekg(10);
	In.read(reinterpret_cast<char* >(&startBitmap),4);
	In.seekg(startBitmap);
	if ( In.fail() )
		throw( FILE_NOT_BMP );
}

^-Here I have a question. I do not understand the purpose of the bolded lines. Why start with an unsigned int, only to reinterpret_cast it to a char pointer? Why read 4 bytes? What does seeking to this value accomplish?

char Decode(char* theBytes)
{
	int tmp=0;
	int thebit;
	for(int i=0; i<8; i++) {
		thebit = static_cast<unsigned char>(theBytes[i])%2;
		tmp = (tmp*2)+(thebit);
	}
	return static_cast<char>(tmp);
}

^-Why static_cast theBytes[i] to an unsigned char?

void ReadDecodeDisplay( fstream& In )
{
	char ByteList[8];
	char TheChar;
	bool done=false;
	In.exceptions( ios_base::failbit );
	while(!done) {
		try {
			In.read(ByteList, 8);
			TheChar=Decode( ByteList );
			if( TheChar=='$' )
				done=true;
			else
				cout << TheChar;
		}
		catch(ios_base::failure) {
			if( In.eof() )
				done=true;
		}
	}
}

int main()
{
	fstream InputFile;
	try {
		OpenInputFile( InputFile );
		ConfirmBMP( InputFile );
		ReadDecodeDisplay( InputFile );
		InputFile.close();
	}
	catch(BMPErrorCode ErrorCode) {
		switch(ErrorCode) {
			case FILE_OPEN_FAILED:
				cerr << "Error attempting to open input file" << endl;
				break;
			case FILE_NOT_BMP:
				InputFile.close();
				cerr << "Error in format of input file, not a BMP" << endl;
				break;
			default:
				cerr << "Unknown error" << endl;
		}
	}
	
	return 0;
}

[S_M_A]

1. The value to be read is a 32 bit integer stored at byte offset 10. To read the integer you would like to do as In.read( &startBitmap,4 ); but unfortunately read can only read bytes. The cast is used to overcome this by telling the compiler that you would like to treat the integer as just any consecutive 4 bytes. Fortunately the bytes are stored in the correct order so there's no need for a byte-swap.

2. theBytes is an array of char's. Most compilers default char to be signed char and as far as I know the modulo-operator is not defined for signed integers. A better way (in my opinion) would have been to declare theBytes unsigned char's instead.

Edit: Tried to find what's said about % and signed values in the standard but failed. However see this link http://en.wikipedia.org/wiki/Modulo_operation