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Always use operator_brackets_proxy in iterator_facade.

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This avoids returning a copy of the value from operator[], which can
be unexpected by users wanting to obtain an address of the value
referenced by the iterator.

We still want to return a proxy instead of the iterator's reference
since the iterator may be holding the value internally, and the iterator
gets destroyed after returning from operator[], so the returned
reference would be dangling.

Closes https://github.com/boostorg/iterator/issues/61.
This commit is contained in:
Andrey Semashev
2025-06-09 03:22:08 +03:00
parent 256004a446
commit 7c77e52509
1 changed files with 2 additions and 40 deletions
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42
include/boost/iterator/iterator_facade.hpp
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@ -368,40 +368,6 @@ private:
Iterator m_iter; Iterator m_iter;
}; };
// A metafunction that determines whether operator[] must return a
// proxy, or whether it can simply return a copy of the value_type.
template< typename ValueType, typename Reference >
struct use_operator_brackets_proxy :
public detail::negation<
detail::conjunction<
std::is_trivially_copyable< ValueType >,
iterator_writability_disabled< ValueType, Reference >
>
>
{};
template< typename Iterator, typename Value, typename Reference >
struct operator_brackets_result
{
using type = typename std::conditional<
use_operator_brackets_proxy< Value, Reference >::value,
operator_brackets_proxy< Iterator >,
Value
>::type;
};
template< typename Iterator >
inline operator_brackets_proxy< Iterator > make_operator_brackets_result(Iterator const& iter, std::true_type)
{
return operator_brackets_proxy< Iterator >(iter);
}
template< typename Iterator >
inline typename Iterator::value_type make_operator_brackets_result(Iterator const& iter, std::false_type)
{
return *iter;
}
// A binary metafunction class that always returns bool. // A binary metafunction class that always returns bool.
template< typename Iterator1, typename Iterator2 > template< typename Iterator1, typename Iterator2 >
using always_bool_t = bool; using always_bool_t = bool;
@ -678,13 +644,9 @@ public:
using difference_type = typename base_type::difference_type; using difference_type = typename base_type::difference_type;
public: public:
typename boost::iterators::detail::operator_brackets_result< Derived, Value, reference >::type operator_brackets_proxy< Derived > operator[](difference_type n) const
operator[](difference_type n) const
{ {
return boost::iterators::detail::make_operator_brackets_result< Derived >( return operator_brackets_proxy< Derived >(this->derived() + n);
this->derived() + n,
std::integral_constant< bool, boost::iterators::detail::use_operator_brackets_proxy< Value, Reference >::value >{}
);
} }
Derived& operator+=(difference_type n) Derived& operator+=(difference_type n)