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1.. title:: clang-tidy - performance-no-automatic-move2 3performance-no-automatic-move4=============================5 6Finds local variables that cannot be automatically moved due to constness.7 8Under9`certain conditions <https://en.cppreference.com/w/cpp/language/return#automatic_move_from_local_variables_and_parameters>`_,10local values are automatically moved out when returning from a function. A11common mistake is to declare local ``lvalue`` variables ``const``, which12prevents the move.13 14Example `[1] <https://godbolt.org/z/x7SYYA>`_:15 16.. code-block:: c++17 18 StatusOr<std::vector<int>> Cool() {19 std::vector<int> obj = ...;20 return obj; // calls StatusOr::StatusOr(std::vector<int>&&)21 }22 23 StatusOr<std::vector<int>> NotCool() {24 const std::vector<int> obj = ...;25 return obj; // calls `StatusOr::StatusOr(const std::vector<int>&)`26 }27 28The former version (``Cool``) should be preferred over the latter (``NotCool``)29as it will avoid allocations and potentially large memory copies.30 31Semantics32---------33 34In the example above, ``StatusOr::StatusOr(T&&)`` have the same semantics as35long as the copy and move constructors for ``T`` have the same semantics. Note36that there is no guarantee that ``S::S(T&&)`` and ``S::S(const T&)`` have the37same semantics for any single ``S``, so we're not providing automated fixes for38this check, and judgement should be exerted when making the suggested changes.39 40-Wreturn-std-move41-----------------42 43Another case where the move cannot happen is the following:44 45.. code-block:: c++46 47 StatusOr<std::vector<int>> Uncool() {48 std::vector<int>&& obj = ...;49 return obj; // calls `StatusOr::StatusOr(const std::vector<int>&)`50 }51 52In that case the fix is more consensual: just `return std::move(obj)`.53This is handled by the `-Wreturn-std-move` warning.54