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I have a question about bit operation in c++,

there is a set of code:

#define INDEX(SRC, DEST) ((U16)SRC | (DEST << 8))

what does this (U16)SRC | (DEST << 8) means?

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  • dpx.googlecode.com/svn-history/r4/trunk/libdpx/… Commented Jan 3, 2012 at 14:18
  • SRC is typecast to (i guess) unsigned int (16bits wide), DEST is shifted 8 bits to the left (same as multiplying DEST by 256) and then the result undergoes an OR operation, so it's like ( SRC | ( DEST * 256 ) ) Commented Jan 3, 2012 at 14:23
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    It's not a very solid macro though. Calling this with INDEX(a,b&c) results in very interesting results if you don't anticipate what order the operators get executed in. Commented Jan 3, 2012 at 14:39
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    By the way, never write macros like this. It can give unexpected results if either argument is an expression (e.g INDEX(s1+s2,d1+d2)). In C++, you should nearly always use an inline function for something like this; if for some reason you have to use a macro, then make sure you put parentheses around every appearance of the macro arguments. Commented Jan 3, 2012 at 14:46

4 Answers 4

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  • (U16)SRC casts SRC to be of type U16.
  • (DEST << 8) does a bitwise shift left of 8 bits.
  • The | operator performs bitwise OR.
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I would guess U16 is also a macro somewhere in the code and it probably designates a 16-bit unsigned integer type(which I deduce from the abbreviation). SRC and DST are the two arguments to the macro expansion the code is defining and (U16)SRC | (DEST << 8) would mean that DEST gets bit shifted 8 bits to the left and then logical or-ed to SRC. Probably the code depends that both SRC and DEST are 8-bit values and this code creates a bit mask that is the result of the appending of the 8-bits of DEST to the 8-bits of SRC.

For instance if (in binary) DEST is 10010101 and SRC is 00001111 then the result is 1001010100001111.

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It casts SRC to a U16 type and performs a bitwise OR with DEST after it has been left-shifted by (8) positions.

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With the code you showed it is a lot of guessing: sure, it looks as if U16 is a macro for unsigned short or uint16_t. If you want to find out, what the define expands to include the definition of the the macro and invoke the macro in a simple test program:

#include "whatever-defines-INDEX.h"

INDEX(10, 1)

Then invoke the compiler with the -Eoption (or the /E option if -E isn't available and you are using Windows): this sends the result of running the preprocessor to the standard output.

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