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Asked: 2026-06-14T08:30:08+00:00

Consider the below code. int value1 = BOOST_BINARY( 100 111000 01 1 110 );

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Consider the below code.

int value1 = BOOST_BINARY( 100 111000 01 1 110 );

Can somebody explain how BOOST_BINARY calculates the int value there?

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  1. Editorial Team

    It is very, very complicated. This BOOST library uses macros constructed by others macros.

    I am going to present you some simplified example of preprocessor programming:

    The simplified question:

    How to make macro adding 0 or 1 at the beginning of rest – like in this examples:

    CAT ( 0 1 ) --> 01
CAT ( 1 100 ) --> 1100

    So – somehow , must be put between them both arguments in sequence. Well I did this:

    // start point - this is our desired format
#define CATS(a,b) a##b
// helper macro to invoke preprocessor on result of a
#define CALLP(a) a
// some helper symbols to build CATS() 
#define CATB (
#define CATE )
#define CATSN CATS
//  I use that only 0 or 1 is expected at the beginning:
//  CATS ( 0, 
#define CAT_B0 CATSN CATB 0,
//  CATS ( 1, 
#define CAT_B1 CATSN CATB 1,

// the final macro:
// Example:
//  CAT(0 XXX) --> CALLP( CAT_B ## 0 XXX  ) )  
// --> CALLP(CAT_B0 XXX ) --> CALLP(CATSN ( 0, XXX ) )
// --> CALLP( CATS(0, XXX) ) --> CATS(0,XXX) --> 0##XXX -> 0XXX 
#define CAT(tt)   CALLP (CAT_B ## tt CATE)

    It really works (at least at gcc): http://ideone.com/EKlTGt

    Now imagine how complicated is work done by BOOST folks in their library? Yes, it is very complicated – but use such preprocessor tricks I presented, and maybe many others – I looked at the source of this BOOST library http://www.boost.org/doc/libs/1_46_1/boost/utility/binary.hpp today for the first time (believe or not).

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