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| Bits Needed To Store A Value At Compile-time Submitted by

Whether you're using bit vectors, manipulating flags or doing some assertions, you might need to know how many bits are needed to store an integer value. This tip will do the job for you at compilation time thanks to template metaprogramming as described in Game Programming Gems 1. It uses a recursive call to NbBits<(x)> to reduce VALUE_TO_NB_BITS(x) to a constant. Compile-time and readability penalities are insignificant compared with execution time and extensibility benefits...
Etranges Team

    /// Implementation use by the Macro VALUE_TO_NB_BITS(x)
    template<unsigned N struct NbBits
    {
            enum
            {
                    Val =  1 + NbBits< (N  1) ::Val
            };
    };

    /// End of recursive calls
    template< struct NbBits<0
    {
            enum

            {
                    Val =  0
            };
    };

    /// Macro which compute number of bits to store value 'x'
    #define VALUE_TO_NB_BITS(x)  ( NbBits<(x)::Val )

    /// Sample
    enum
    {
            VALUE0,
            VALUE1,
            VALUE2,
            VALUE3,
            VALUE4,
            VALUE5,
            NB_VALUES
    };

    #include <stdio.h
    #include <stdlib.h

    /// Main
    int main()
    {
        printf("%d-%d bits, %d-%d bits, %d-%d bits %d-%d bits\n",
                  3, VALUE_TO_NB_BITS(3),
                  4, VALUE_TO_NB_BITS(4),
                  50, VALUE_TO_NB_BITS(50),
                  NB_VALUES-1, VALUE_TO_NB_BITS(NB_VALUES-1)
               );

       // Output :
       // 3-2 bits, 4-3 bits, 50-6 bits 5-3 bits

       return 0;
    }

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