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4289. Seed sequence is overspecified

Section: 29.5.3.2 [rand.req.seedseq] Status: New Submitter: Magnus Fromreide Opened: 2025-06-22 Last modified: 2025-07-06

Priority: Not Prioritized

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Discussion:

The requirements on a seed sequence are so strict that it is forbidden to initialize a random number generator directly from a hardware random number generator without lots of boiler plate and intermediary objects.

The main problem is that the seed sequence requirements state that a seed sequence must be "kind of copyable" and that calls to the copy must generate the same output as calls to the original.

If one checks the uses of seed sequences then nothing makes use of this capability so I propose to just drop it.

There has been a previous attempt to handle this issue using P0205 which preserved the old seed sequence and added a new concept that it used to seed generators. That effort stalled with the comment that it should be solved without the new std::seed_adapter. This DR sidesteps that whole issue by simply removing the unused requirements from the seed sequence concept.

I will admit that I am unsure about the deletion of Note 1 but since it only seems to attempt to motivate the copyability I opted to let it go along with the requirements.

Proposed resolution:

This wording is relative to N5008.

1. Modify 29.5.3.2 [rand.req.seedseq] as indicated:

   -1- A seed sequence is an object that consumes a sequence of integer-valued data and produces a requested number of unsigned integer values i, 0 = i < 232, based on the consumed data.

   [Note 1: Such an object provides a mechanism to avoid replication of streams of random variates. This can be useful, for example, in applications requiring large numbers of random number engines. — end note]

   -2- A class S meets the requirements of a seed sequence if the expressions shown in Table 124 [tab:rand.req.seedseq] are valid and have the indicated semantics, and if S also meets all other requirements of 29.5.3.2 [rand.req.seedseq]. In Table 124 [tab:rand.req.seedseq] and throughout this subclause:

   1. (2.1) — T is the type named by S' s associated result_type;

   2. (2.2) — q is a value of type S and r is a value of type S or const S; and

   3. (2.3) — ib and ie are input iterators with an unsigned integer value_type of at least 32 bits;

   4. (2.4) — rb and re are mutable random access iterators with an unsigned integer value_type of at least 32 bits;.

   5. (2.5) — ob is an output iterator; and

   6. (2.6) — il is a value of type initializer_list<T>.

   Table 124 — Seed sequence requirements [tab:rand.req.seedseq]

   Expression	Return type	Pre/post-condition	Complexity
   S::result_type	T	T is an unsigned integer type (6.9.2 [basic.fundamental]) of at least 32 bits.
   S()		Creates a seed sequence with the same initial state as all other default-constructed seed sequences of type S.	constant
   S(ib,ie)		Creates a seed sequence having internal state that depends on some or all of the bits of the supplied sequence [ib, ie).	𝒪(ie - ib)
   S(il)		Same as S(il.begin(), il.end()).	same as S(il.begin(), il.end())
   q.generate(rb,re)	void	Does nothing if rb == re. Otherwise, fills the supplied sequence [rb, re) with 32-bit quantities that depend on the sequence supplied to the constructor and possibly also depend on the history of generate's previous invocations.	𝒪(re - rb)
   r.size()	size_t	The number of 32-bit units that would be copied by a call to r.param.	constant
   r.param(ob)	void	Copies to the given destination a sequence of 32-bit units that can be provided to the constructor of a second object of type S, and that would reproduce in that second object a state indistinguishable from the state of the first object.	𝒪(r.size())