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4262. copy_if, remove_copy, remove_copy_if, unique_copy have too strong preconditions

Section: 26.7.1 [alg.copy], 26.7.8 [alg.remove], 26.7.9 [alg.unique] Status: New Submitter: Alex Guteniev Opened: 2025-05-12 Last modified: 2025-05-18

Priority: Not Prioritized

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Discussion:

26.7.1 [alg.copy]/16 , 26.7.8 [alg.remove]/11, 26.7.9 [alg.unique]/8.1 all say:

Preconditions: The ranges [first, last) and [result, result + (last - first)) do not overlap.

These algorithms may produce fewer elements than (last - first). If this is known in advance, a smaller output range may be used, so that the precondition will not be satisfied.

Example from LLVM test suite ( /libcxx/test/std/algorithms/alg.modifying.operations/alg.copy/ranges.copy_if.pass.cpp):

std::array<S, 4> in = {{{4, 2}, {1, 3}, {3, 4}, {3, 5}}};
std::array<S, 2> out;
auto ret = std::ranges::copy_if(in.begin(), in.end(), out.begin(), [](int i) { return i == 3; }, &S::val);

I think there should be a weaker precondition, like 26.7.1 [alg.copy]/2:

Preconditions: result is not in the range [first, last).

Proposed resolution:

This wording is relative to N5008.

1. Modify 26.7.1 [alg.copy] as indicated:

   template<class InputIterator, class OutputIterator, class Predicate>
     constexpr OutputIterator copy_if(InputIterator first, InputIterator last,
                                      OutputIterator result, Predicate pred);
   […]
   template<input_range R, weakly_incrementable O, class Proj = identity,
            indirect_unary_predicate<projected<iterator_t<R>, Proj>> Pred>
     requires indirectly_copyable<iterator_t<R>, O>
     constexpr ranges::copy_if_result<borrowed_iterator_t<R>, O>
       ranges::copy_if(R&& r, O result, Pred pred, Proj proj = {});
   

   -15- Let E be: […]

   -16 Preconditions: result is not in the range [first, last)The ranges [first, last) and [result, result + (last - first)) do not overlap.

2. Modify 26.7.8 [alg.remove] as indicated:

   template<class InputIterator, class OutputIterator,
            class T = iterator_traits<InputIterator>::value_type>
     constexpr OutputIterator
       remove_copy(InputIterator first, InputIterator last,
                   OutputIterator result, const T& value);
   […]
   template<input_range R, weakly_incrementable O, class Proj = identity,
            indirect_unary_predicate<projected<iterator_t<R>, Proj>> Pred>
     requires indirectly_copyable<iterator_t<R>, O>
     constexpr ranges::remove_copy_if_result<borrowed_iterator_t<R>, O>
       ranges::remove_copy_if(R&& r, O result, Pred pred, Proj proj = {});
   

   -8- Let E be […]

   […]

   -11 Preconditions: result is not in the range [first, last)The ranges [first, last) and [result, result + (last - first)) do not overlap.

3. Modify 26.7.9 [alg.unique] as indicated:

   template<class InputIterator, class OutputIterator>
     constexpr OutputIterator
       unique_copy(InputIterator first, InputIterator last,
                   OutputIterator result);
   […]
   template<input_range R, weakly_incrementable O, class Proj = identity,
            indirect_equivalence_relation<projected<iterator_t<R>, Proj>> C = ranges::equal_to>
     requires indirectly_copyable<iterator_t<R>, O> &&
             (forward_iterator<iterator_t<R>> ||
             (input_iterator<O> && same_as<range_value_t<R>, iter_value_t<O>>) ||
             indirectly_copyable_storable<iterator_t<R>, O>)
     constexpr ranges::unique_copy_result<borrowed_iterator_t<R>, O>
       ranges::unique_copy(R&& r, O result, C comp = {}, Proj proj = {});
   

   -6- Let pred be […]

   -7- Mandates: […]

   -8 Preconditions:

   1. (8.1) — result is not in the range [first, last)The ranges [first, last) and [result, result + (last - first)) do not overlap.

   2. (8.2) — […]