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**Section:** 23 [algorithms] **Status:** C++11
**Submitter:** Alisdair Meredith **Opened:** 2009-09-13 **Last modified:** 2016-02-10

**Priority: **Not Prioritized

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**Discussion:**

There are a number of algorithms whose result might depend on the handling of an empty range. In some cases the result is not clear, while in others it would help readers to clearly mention the result rather than require some subtle intuition of the supplied wording.

23.5.1 [alg.all_of]

Returns:trueifpred(*i)istruefor every iteratoriin the range[first,last), ...

What does this mean if the range is empty?

I believe that we intend this to be `true` and suggest a
non-normative note to clarify:

Add to p1 23.5.1 [alg.all_of]:

[

Note:Returnstrueif[first,last)is empty. —end note]

23.5.3 [alg.none_of]

Returns:trueifpred(*i)isfalsefor every iteratoriin the range[first,last), ...

What does this mean if the range empty?

I believe that we intend this to be `true` and suggest a
non-normative note to clarify:

Add to p1 23.5.3 [alg.none_of]:

[

Note:Returnstrueif[first,last)is empty. —end note]

23.5.2 [alg.any_of]

The specification for an empty range is actually fairly clear in this
case, but a note wouldn't hurt and would be consistent with proposals
for `all_of`/`none_of` algorithms.

Add to p1 23.5.2 [alg.any_of]:

[

Note:Returnsfalseif[first,last)is empty. —end note]

23.5.6 [alg.find.end]

what does this mean if `[first2,last2)` is empty?

I believe the wording suggests the algorithm should return
`last1` in this case, but am not 100% sure. Is this in fact the
correct result anyway? Surely an empty range should always match and the
naive expected result would be `first1`?

My proposed wording is a note to clarify the current semantic:

Add to p2 23.5.6 [alg.find.end]:

[

Note:Returnslast1if[first2,last2)is empty. —end note]

I would prefer a normative wording treating empty ranges specially, but do not believe we can change semantics at this point in the process, unless existing implementations actually yield this result:

Alternative wording: (NOT a note)

Add to p2 23.5.6 [alg.find.end]:

Returns

first1if[first2,last2)is empty.

23.5.7 [alg.find.first.of]

The phrasing seems precise when `[first2, last2)` is empty, but a small
note to confirm the reader's understanding might still help.

Add to p2 23.5.7 [alg.find.first.of]

[

Note:Returnslast1if[first2,last2)is empty. —end note]

23.5.13 [alg.search]

What is the expected result if `[first2, last2)` is empty?

I believe the wording suggests the algorithm should return `last1` in this
case, but am not 100% sure. Is this in fact the correct result anyway?
Surely an empty range should always match and the naive expected result
would be `first1`?

My proposed wording is a note to clarify the current semantic:

Add to p2 23.5.13 [alg.search]:

[

Note:Returnslast1if[first2,last2)is empty. —end note]

Again, I would prefer a normative wording treating empty ranges specially, but do not believe we can change semantics at this point in the process, unless existing implementations actually yield this result:

Alternative wording: (NOT a note)

Add to p2 23.5.13 [alg.search]:

Returns

first1if[first2,last2)is empty.

23.7.4 [alg.partitions]

Is an empty range partitioned or not?

Proposed wording:

Add to p1 23.7.4 [alg.partitions]:

[

Note:Returnstrueif[first,last)is empty. —end note]

23.7.6.1 [includes]

Returns:trueif every element in the range[first2,last2)is contained in the range[first1,last1). ...

I really don't know what this means if `[first2,last2)` is empty.
I could loosely guess that this implies empty ranges always match, and
my proposed wording is to clarify exactly that:

Add to p1 23.7.6.1 [includes]:

[

Note:Returnstrueif[first2,last2)is empty. —end note]

23.7.7.2 [pop.heap]

The effects clause is invalid if the range `[first,last)` is empty, unlike
all the other heap alogorithms. The should be called out in the
requirements.

Proposed wording:

Revise p2 23.7.7.2 [pop.heap]

Requires:The range[first,last)shall be a valid non-empty heap.

[Editorial] Reverse order of 23.7.7.2 [pop.heap] p1 and p2.

23.7.8 [alg.min.max]

`minmax_element` does not clearly specify behaviour for an empty
range in the same way that `min_element` and `max_element` do.

Add to p31 23.7.8 [alg.min.max]:

Returns

make_pair(first, first)iffirst == last.

23.7.10 [alg.lex.comparison]

The wording here seems quite clear, especially with the sample algorithm implementation. A note is recommended purely for consistency with the rest of these issue resolutions:

Add to p1 23.7.10 [alg.lex.comparison]:

[

Note:An empty sequence is lexicographically less than any other non-empty sequence, but not to another empty sequence. —end note]

*[
2009-11-11 Howard changes Notes to Remarks and changed search to
return first1 instead of last1.
]*

*[
2009-11-11 Moved to Tentatively Ready after 5 positive votes on c++std-lib.
]*

**Proposed resolution:**

Add to 23.5.1 [alg.all_of]:

Remarks:Returnstrueif[first,last)is empty.

Add to 23.5.2 [alg.any_of]:

Remarks:Returnsfalseif[first,last)is empty.

Add to 23.5.3 [alg.none_of]:

Remarks:Returnstrueif[first,last)is empty.

Add to 23.5.6 [alg.find.end]:

Remarks:Returnslast1if[first2,last2)is empty.

Add to 23.5.7 [alg.find.first.of]

Remarks:Returnslast1if[first2,last2)is empty.

Add to 23.5.13 [alg.search]:

Remarks:Returnsfirst1if[first2,last2)is empty.

Add to 23.7.4 [alg.partitions]:

Remarks:Returnstrueif[first,last)is empty.

Add to 23.7.6.1 [includes]:

Remarks:Returnstrueif[first2,last2)is empty.

Revise p2 23.7.7.2 [pop.heap]

Requires:The range[first,last)shall be a valid non-empty heap.

[Editorial]

Reverse order of 23.7.7.2 [pop.heap] p1 and p2.

Add to p35 23.7.8 [alg.min.max]:

template<class ForwardIterator, class Compare> pair<ForwardIterator, ForwardIterator> minmax_element(ForwardIterator first, ForwardIterator last, Compare comp);

Returns:make_pair(m, M), wheremis the first iterator in[first,last)such that no iterator in the range refers to a smaller element, and whereMis the last iterator in[first,last)such that no iterator in the range refers to a larger element. Returnsmake_pair(first, first)iffirst == last.

Add to 23.7.10 [alg.lex.comparison]:

Remarks:An empty sequence is lexicographically less than any other non-empty sequence, but not less than another empty sequence.