为什么set.begin()总是返回一个const迭代器而不是一个标准迭代器呢?
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’
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