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Do not require the container iterators to be nothrow move constructible

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iter_impl declared its defaulted move operations noexcept. The exception
specification a defaulted function gets implicitly follows from its members,
here internal_iterator, which holds the object and array iterators. libstdc++
gives std::deque's iterator a user-provided copy constructor without noexcept
before version 11, so the implicit specification is noexcept(false) and does
not match the declared one. That deletes the function -- and with g++ 4.8,
which predates CWG 1778, it is an error outright:

    error: function 'iter_impl<basic_json<std::map, std::deque> >::iter_impl(
    iter_impl&&)' defaulted on its first declaration with an
    exception-specification that differs from the implicit declaration

So std::deque, which this branch documents as a usable array type, could not
be used with an older standard library. Leaving the specification to be
computed cannot mismatch; iteration_proxy_value already spells out the same
condition next door.

The default configuration is unaffected: json::iterator, json::const_iterator
and ordered_json::iterator stay nothrow move constructible and move
assignable, which the test now checks so it cannot regress unnoticed.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann
2026-08-28 19:09:51 +00:00
parent 8ce64b9c16
commit 5a2b8a274d
3 changed files with 28 additions and 4 deletions
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include/nlohmann/detail/iterators/iter_impl.hpp
+7 -2
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@@ -88,8 +88,13 @@ class iter_impl // NOLINT(cppcoreguidelines-special-member-functions,hicpp-speci
iter_impl() = default;
~iter_impl() = default;
iter_impl(iter_impl&&) noexcept = default;
iter_impl& operator=(iter_impl&&) noexcept = default;
// the exception specification is left to be computed rather than declared:
// an array or object type whose iterator is not nothrow move constructible
// (std::deque's is not before libstdc++ 11) would make a declared noexcept
// differ from the implicit one, which deletes the function -- and is an
// error outright with older compilers
iter_impl(iter_impl&&) = default; // NOLINT(hicpp-noexcept-move,performance-noexcept-move-constructor,cppcoreguidelines-noexcept-move-operations)
iter_impl& operator=(iter_impl&&) = default; // NOLINT(hicpp-noexcept-move,performance-noexcept-move-constructor,cppcoreguidelines-noexcept-move-operations)
/*!
@brief constructor for a given JSON instance