`std::iterator` has been [deprecated](http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0174r0.html#2.1) by C++17. It should be removed in preparation for the future, and replaced with explicit `using` declarations.
std::iteratorhas been deprecated by C++17. It should be removed in preparation for the future, and replaced with explicitusingdeclarations.