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800. Issues in 26.4.7.1 [rand.util.seedseq](6)

Section: 28.5.8.1 [rand.util.seedseq] Status: Resolved Submitter: Stephan Tolksdorf Opened: 2008-02-18 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The for-loop in the algorithm specification has n iterations, where n is defined to be end - begin, i.e. the number of supplied w-bit quantities. Previous versions of this algorithm and the general logic behind it suggest that this is an oversight and that in the context of the for-loop n should be the number of full 32-bit quantities in b (rounded upwards). If w is 64, the current algorithm throws away half of all bits in b. If w is 16, the current algorithm sets half of all elements in v to 0.

There are two more minor issues:

• Strictly speaking end - begin is not defined since InputIterator is not required to be a random access iterator.
• Currently all integral types are allowed as input to the seed_seq constructor, including bool. IMHO allowing bools unnecessarily complicates the implementation without any real benefit to the user. I'd suggest to exclude bools as input.

[ Bellevue: ]

Move to Open: Bill will try to propose a resolution by the next meeting.

[ post Bellevue: Bill provided wording. ]

This issue is made moot if 803 is accepted.

Proposed resolution:

Replace 28.5.8.1 [rand.util.seedseq] paragraph 6 with:

Effects: Constructs a seed_seq object by effectively concatenating the low-order u bits of each of the elements of the supplied sequence [begin, end) in ascending order of significance to make a (possibly very large) unsigned binary number b having a total of n bits, and then carrying out the following algorithm:

for( v.clear(); n > 0; n -= 32 ) v.push_back(b mod 2³²), b /= 2³²;

Rationale:

Addressed by N2836 "Wording Tweaks for Concept-enabled Random Number Generation in C++0X".