Section: 188.8.131.52 [rand.util.seedseq] Status: C++14 Submitter: Daniel Krügler Opened: 2012-08-18 Last modified: 2016-02-10
Priority: Not Prioritized
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184.108.40.206 [rand.util.seedseq] p1 says upfront:
No function described in this section 220.127.116.11 [rand.util.seedseq] throws an exception.
This constraint seems non-implementable to me when looking especially at the members
template<class T> seed_seq(initializer_list<T> il); template<class InputIterator> seed_seq(InputIterator begin, InputIterator end);
which have the effect of invoking v.push_back() for the exposition-only member of type std::vector (or its equivalent) over all elements of the provided range, so out-of-memory exceptions are always possible and the seed_seq object doesn't seem to be constructible this way.In addition to the potential lack-of-resources problem, the operations of InputIterator might also throw exceptions. Aside to that it should me mentioned, that a default constructor of vector<uint_least32_t> in theory can also throw exceptions, even though this seems less of a problem to me in this context, because such an implementation could easily use a different internal container in seed_seq that can hold this no-throw exception guarantee. Secondly, a slightly different problem category related to exceptions occurs for the member templates
template<class RandomAccessIterator> void generate(RandomAccessIterator begin, RandomAccessIterator end); template<class OutputIterator> void param(OutputIterator dest) const;
where the actual operations performed by the implementation would never need to throw, but since they invoke operations of a user-provided customization point, the overall operation, like for example
copy(v.begin(), v.end(), dest);
could also throw exceptions. In this particular example we can just think of a std::back_insert_iterator applied to a container that needs to allocate its elements used as the type for OutputIterator.Even though Clause 29 [numerics] has mostly stronger exception constraints than other parts of the library the here discussed are overrestrictive, especially since no operation of std::seed_seq except the template generate is actually needed within the library implementation, as mentioned in the discussion of LWG 2124. I suggest to remove the general no-exception constraints for operations of std::seed_seq except for member size() and the default constructor and to provide specific wording for generate() and param() to ensure that the algorithm itself is a nothrow operation, which is especially for generate() important, because the templates specified in 29.6.3 [rand.eng] and 29.6.4 [rand.adapt] also depend on this property indirectly, which is further discussed in LWG 2181. Howard: I suggest to use a different form for the exception specification, something similar to 18.104.22.168 [func.bind.bind] p4:
Throws: Nothing unless an operation on RandomAccessIterator throws an exception.
Daniel:The currently suggested "what and when" form seems a bit more specific and harmonizes with the form used for function template generate_canonical from 22.214.171.124 [rand.util.canonical].
Open an editorial issue on the exception wording ("Throws: What and when").Solution: move to tentatively ready.
Apply to Working Paper
This wording is relative to N3376.
Edit 126.96.36.199 [rand.util.seedseq] p1 as indicated:
-1- No function described in this section 188.8.131.52 [rand.util.seedseq] throws an exception.
Edit 184.108.40.206 [rand.util.seedseq] around p2 as indicated:
-2- Effects: Constructs a seed_seq object as if by default-constructing its member v.
Edit 220.127.116.11 [rand.util.seedseq] around p7 as indicated:
template<class RandomAccessIterator> void generate(RandomAccessIterator begin, RandomAccessIterator end);
-7- Requires: RandomAccessIterator shall meet the requirements of a mutable random access iterator (Table 111) type. Moreover, iterator_traits<class RandomAccessIterator>::value_type shall denote an unsigned integer type capable of accommodating 32-bit quantities.-8- Effects: Does nothing if begin == end. Otherwise, with s = v.size() and n = end - begin, fills the supplied range [begin, end) according to the following algorithm […]
Edit 18.104.22.168 [rand.util.seedseq] around p9 as indicated:
size_t size() const;
-9- Returns: The number of 32-bit units that would be returned by a call to param().-10- Complexity: constant time.
Edit 22.214.171.124 [rand.util.seedseq] around p11 as indicated:
template<class OutputIterator> void param(OutputIterator dest) const;
-11- Requires: OutputIterator shall satisfy the requirements of an output iterator (Table 108) type. Moreover, the expression *dest = rt shall be valid for a value rt of type result_type.-12- Effects: Copies the sequence of prepared 32-bit units to the given destination, as if by executing the following statement:copy(v.begin(), v.end(), dest);