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<base href="https://bugs.llvm.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Constructors like std::vector::vector may be too liberal (or strict?) with conversions for non-input iterators"
href="https://bugs.llvm.org/show_bug.cgi?id=34898">34898</a>
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<th>Summary</th>
<td>Constructors like std::vector::vector may be too liberal (or strict?) with conversions for non-input iterators
</td>
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<th>Product</th>
<td>libc++
</td>
</tr>
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<th>Version</th>
<td>5.0
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>URL</th>
<td>https://godbolt.org/g/Vge69c
</td>
</tr>
<tr>
<th>OS</th>
<td>Linux
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>All Bugs
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>dlj@google.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org, mclow.lists@gmail.com
</td>
</tr></table>
<p>
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<pre>Example:
=====
template <typename CategoryT>
class Iterator : public boost::iterator_facade<Iterator<CategoryT>, Base,
CategoryT> {
friend class boost::iterator_core_access;
void increment();
bool equal(Iterator const& other) const;
typename Iterator::reference dereference() const;
};
void foo() {
using ForwardIt = Iterator<std::forward_iterator_tag>;
std::vector<Derived> x1(ForwardIt{}, ForwardIt{});
using InputIt = Iterator<std::input_iterator_tag>;
std::vector<Derived> x2(InputIt{}, InputIt{}); // ERROR
}
=====
<a href="https://godbolt.org/g/Vge69c">https://godbolt.org/g/Vge69c</a>
The example is a bit complex, but the upshot is that
`std::vector::vector(Iterator, Iterator)` uses push_back() only for input
iterators. This is in contrast with forward iterators or greater, which use
emplacement, and thus consider the explicit constructors.
The standard would indicate that libc++'s InputIterator behaviour is correct.
Specifically, it gives this example:
X(i, j)
Requires: T shall be EmplaceConstructible into X from *i. [...]
<a href="https://timsong-cpp.github.io/cppwp/n4659/container.requirements#tab:containers.sequence.requirements">https://timsong-cpp.github.io/cppwp/n4659/container.requirements#tab:containers.sequence.requirements</a>
with the constraint:
"... i and j denote iterators satisfying input iterator requirements and refer
to elements implicitly convertible to value_type ..."
<a href="https://timsong-cpp.github.io/cppwp/n4659/container.requirements#sequence.reqmts-3">https://timsong-cpp.github.io/cppwp/n4659/container.requirements#sequence.reqmts-3</a>
This means that constructing a vector from iterator inputs, and using explicit
constructors, is undefined behaviour.</pre>
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