std::_Placeholder< _Num > - The type of placeholder objects defined by libstdc++.

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                                                    libstdc++                    std::_Placeholder<_Num>(3cxx)

template<int_Num>
       struct std::_Placeholder< _Num >"The type of placeholder objects defined by libstdc++.

       Since
           C++11

       std::_Placeholder< _Num > - The type of placeholder objects defined by libstdc++.

       #include <functional>