Function operator()

cpp/src/arrow/compute/expression.cc:1074–1076

    std::unordered_set<Expression, Expression::Hash> set_;

    bool operator()(const Expression& expr) const {
      return set_.find(expr) != set_.end();
    }

    void Add(std::vector<Expression> exprs) {
      std::move(exprs.begin(), exprs.end(), std::inserter(set_, set_.end()));