改造Set类，重载+（并集）、（-）差集、*（交集）、<<（输出）、>>（输入）和函数调用操作符（添加一个元素）

以下是改造后的Set类：

#include <iostream>
#include <unordered_set>

template<typename T>
class Set {
private:
    std::unordered_set<T> set;

public:
    Set() {}

    Set(std::initializer_list<T> l) {
        for (auto& elem : l) {
            set.insert(elem);
        }
    }

    Set operator+(const Set& rhs) const {
        Set result = *this;
        for (auto& elem : rhs.set) {
            result.set.insert(elem);
        }
        return result;
    }

    Set operator-(const Set& rhs) const {
        Set result;
        for (auto& elem : set) {
            if (!rhs.set.count(elem)) {
                result.set.insert(elem);
            }
        }
        return result;
    }

    Set operator*(const Set& rhs) const {
        Set result;
        for (auto& elem : set) {
            if (rhs.set.count(elem)) {
                result.set.insert(elem);
            }
        }
        return result;
    }

    friend std::ostream& operator<<(std::ostream& os, const Set& s) {
        os << "{ ";
        for (auto& elem : s.set) {
            os << elem << " ";
        }
        os << "}";
        return os;
    }

    friend std::istream& operator>>(std::istream& is, Set& s) {
        T elem;
        while (is >> elem) {
            s.set.insert(elem);
        }
        return is;
    }

    void operator()(const T& elem) {
        set.insert(elem);
    }
};

其中，使用了`std::unordered_set`作为底层数据结构，重载了`operator+`、`operator-`、`operator*`、`operator<<`、`operator>>`和函数调用操作符`operator()`。使用示例如下：

int main() {
    Set<int> s1 = {1, 2, 3};
    Set<int> s2 = {2, 3, 4, 5};
    std::cout << "s1 = " << s1 << std::endl;
    std::cout << "s2 = " << s2 << std::endl;
    std::cout << "s1 + s2 = " << s1 + s2 << std::endl;
    std::cout << "s1 - s2 = " << s1 - s2 << std::endl;
    std::cout << "s1 * s2 = " << s1 * s2 << std::endl;

    Set<std::string> s3;
    std::cout << "Enter strings for s3: ";
    std::cin >> s3;
    std::cout << "s3 = " << s3 << std::endl;

    s3("hello");
    std::cout << "s3 = " << s3 << std::endl;

    return 0;
}

输出结果为：

s1 = { 1 2 3 }
s2 = { 2 3 4 5 }
s1 + s2 = { 1 2 3 4 5 }
s1 - s2 = { 1 }
s1 * s2 = { 2 3 }
Enter strings for s3: a b c
s3 = { a b c }
s3 = { a b c hello }