为什么 set.begin() 总是返回 const 迭代器而不是标准迭代器?
35 int test(){
36 std::set<int> myset;
37 myset.insert(2);
38 myset.insert(3);
39 int &res = *myset.begin();
40 return res;
41 }
test.cpp:39: error: invalid initialization of reference of type ‘int&’ from expression of type ‘const int’