Re: [Newbie] Once more on conversions

From:

Michael DOUBEZ <michael.doubez@free.fr>

Newsgroups:

comp.lang.c++

Date:

Tue, 27 Jan 2009 16:56:34 +0100

Message-ID:

<497f2c97$0$21825$426a34cc@news.free.fr>

Barzo wrote:

Hi,

I'm sorry if my questions are banal...
I have to this 'simple' work:

INPUT: std::string buffer = [11][8E][CD][8D] --> <<CONVERSION>> -->
OUTPUT: std::string = "12,345345"

The buffer arrives from a device that use big-endian.

[11][8E][CD][8D] = 294571405

(294571405 * 90) / 0x80000000 = 12,34534496907....

My routine is (after have readed some posts here..) but it doen't
work:

template< typename T >
std::string ToString( const T& val )
{
  try
  {
    return boost::lexical_cast<std::string>(val);
  }
  catch(const boost::bad_lexical_cast& e)
  {
    // std::cout << e.what();
    return 0;

Here you return a string with NULL as parameter.
This is a candidate for segfault.

Perhaps you where thinking of:
return "0";

  }
}

template<class T>
void fromBytes(unsigned char const* bytes, T* t)
{
    *t = 0;
    for(int i = 0; i != sizeof t; ++i)
        *t |= bytes[i] << 8 * i;
}

float decode_lat(const std::string& buffer, char token_pos)
{
  // VAL = ((L * (2^31))/90) => L = ((VAL * 90)/(2^31))
  // (2^31)=0x80000000

  float lat = 0;
  unsigned long val = 0;

  // Extract the 4 latitude octets from the buffer and
  // convert they into an long value
  fromBytes<unsigned long>(reinterpret_cast<const unsigned char*>
(buffer.substr(token_pos, 4).c_str()), &val);

  if (val == 0x7FFFFFFF)
    lat = 90;
  else
    lat = ((unsigned long)(val * 90) / 0x80000000);

perhaps could you consider modifying your expression to:
lat = unsigned long( val * (90.0/0x80000000) ) );

It would save you possible overflow.

  // The MSBit indicates the sign
  return ( ((buffer[token_pos] & 0x80) == 0x80) ? -lat : lat );
}

void CallerFunction()
{...
  std::string s = ToString<float>( decode_lat(buffer, token_pos) );
}

Could someone explain me what is wrong in this code?
Is this code portable? If not, is there a method to accomplish this?

What doesn't work ?

--
Michael