加了&代表引用，如果不加，则是值拷贝

#include <iostream>
#include <set>

using namespace std;

int main()
{

    // cbegin/cend(c++11): Returns a const_iterator pointing to the first element in the container/
    // Returns a const_iterator pointing to the past-the-end element in the container
    std::set<int> myset = { 50, 20, 60, 10, 25 };

    std::cout << "myset contains:";
    for (auto it:myset)
        std::cout << ' ' << it;

    std::cout << '\n';

    std::cout << "myset contains:";
    for (auto &it:myset)
        std::cout << ' ' << it;

    system("pause");
    return 0;
}