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    std::atomic_flag::operator=

    From cppreference.net
     
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    std::atomic_flag
     
    Defined in header <atomic> 
    atomic_flag& operator=( const atomic_flag& ) = delete;
    		(1) (since C++11) 
    atomic_flag& operator=( const atomic_flag& ) volatile = delete;
    		(2) (since C++11)

    std::atomic_flag is not assignable, its assignment operators are deleted.