2013/2/5 Christian Henning

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Is there a better way in boost?

Try boost::low_bits_mask_t http://tinyurl.com/bcy22xf

Mhmm, I don't see how that works. How do I get the max value from a low_bit_mask_t? It should be 2^n-1 with n being the number the bits.

low_bits_mask_t<n>::sig_bits Not?

2013/2/5 Christian Henning

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Mhmm, I don't see how that works. How do I get the max value from a low_bit_mask_t? It should be 2^n-1 with n being the number the bits.

low_bits_mask_t<n>::sig_bits

Not?

Yes. Thanks! It works but not for n = 64. It gives a wrong value:

typedef boost::low_bits_mask_t<64> mask_t; mask_t::least oo = mask_t::sig_bits;

Does that work for you?

Yes, I got 18446744073709551615 http://liveworkspace.org/code/3xi9dD$0

Le 05/02/13 16:27, Christian Henning a écrit :

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Is there a better way in boost?

Try boost::low_bits_mask_t http://tinyurl.com/bcy22xf

Mhmm, I don't see how that works. How do I get the max value from a low_bit_mask_t? It should be 2^n-1 with n being the number the bits.

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

Le 06/02/13 00:59, Christian Henning a écrit :

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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

boost::int_max_value_t<V>

http://www.boost.org/doc/libs/1_53_0/libs/integer/doc/html/boost_integer/int...

Robert, are you sure I can get the max value of a 15bit value using int_max_value_t? Christian

On Tue, Feb 5, 2013 at 4:56 PM, Robert Ramey wrote:

Christian Henning wrote:

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boost::int_max_value_t<V>

http://www.boost.org/doc/libs/1_53_0/libs/integer/doc/html/boost_integer/int...

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Robert, are you sure I can get the max value of a 15bit value using int_max_value_t?

whoops - I totally misread your question, I was thinking you wanted the TYPE which would hold a max value corresponding to 15 bits.

It turns out that I've been working in this area recently. I would suggest skipping the mpl layer and using BOOST_STATIC_CONSTANT directly. The reason is that mpl includes some operations like next which might not work for 2^64 etc.

template <int K> struct unsigned_integral_max_value struct { BOOST_STATIC_CONSTANT( boost::uintmax_t, value = ((boost::uintmax_t)1 << K)-1); };

Isn't this pretty much exactly what Christian was using before? If so, I'd guess it still has the same compile-time-overflow issues. But maybe I'm misremembering the original problem :/ - Jeff