typecasting

hello! how can i convert unsigned char a[4] to unsigned long. Point is that i need to convert pixel, wich is defined as long to four subpixels and then back again.

The IMO simplest way is using a union:

Code:

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union
{
  unsigned char a[4];
  unsigned long l;
} conv;

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Just assign to one, read the other.

The other way is casting pointers:

Code:

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unsigned long l = *reinterpret_cast<unsigned long *>(a);

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The third way uses shifts (this is the one where you can control endianness):

Code:

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unsigned long l = a[0] | a[1] << 8 | a[2] << 16 | a[3] << 24;

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In all cases, remember that on my Athlon64, gcc will have 8-byte longs, so the result might not be what you expect. If your compiler has it, include <stdint.h> and use uint32_t instead. This header is part of the C99 standard. If it doesn't have it, or you just want to be portable, include <boost/cstdint.hpp> from the Boost libraries. It defines the same types, but in the boost namespace.

Code:

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  unsigned long num;

  num = *reinterpret_cast< unsigned long * >(&a);

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