Le 06/02/13 00:59, Christian Henning a écrit :
low_bits_mask_t<Bits>::sig_bits is defined as (~( ~(unsigned_int(0u )) << Bits )) )
which is equivalent to 2^Bits-1 without risk of overflow.
Vicente, what's unsigned_int?
It is an unsigned type with at least Bits bits. It depends on what do you want, speed or size. low_bits_mask_t<Bits>::sig_bits uses the smallest unsigned int and low_bits_mask_t<Bits>::sig_bits_fast uses the fastest one. " template <std::size_t Bits> struct low_bits_mask_t { typedef /implementation-defined-type/ least; typedef /implementation-defined-type/ fast; static const least sig_bits = /implementation-defined/; static const fast sig_bits_fast = /implementation-defined/; static const std::size_t bit_count = Bits; }; Group Bit-Mask Class Template The |boost::low_bits_mask_t| class template provides constants for bit masks equivalent to the value (2^Bits - 1), where |Bits| is the template parameter. The parameter |Bits| must be a non-negative integer from zero to /Max/, where Max is the number of bits supported by the largest, unsigned, built-in integral type. The following table describes the members of |low_bits_mask_t|. *Table 3. Members of the [^boost::low_bits_mask_t] Class Template* Member Meaning |least| The smallest, unsigned built-in type that supports the given bit count. |fast| The easiest-to-manipulate analog of |least|. |sig_bits| A |least| constant of the desired bit-masking value. |sig_bits_fast| A |fast| analog of |sig_bits|. |bit_count| The value of the template parameter, in case its needed from a renamed instantiation of the class template. " Vicente