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
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
