尝试在多集中插入元素时 C++11 编译错误

Question

代码如下

#include <set>
#include <iostream>
#include <string>
#include <memory>

using namespace std;
class Quote {
    public:
        int getnum() {
            return num;
        }
    private:
        int num;
};

class basket {
    public:
        void add_item(const shared_ptr<Quote> &b) {
            setQuo.insert(b);
        }
    private:
        static bool compare(shared_ptr<Quote> &l, shared_ptr<Quote> &r) {
            return l->getnum() < r->getnum();
        }
        multiset<shared_ptr<Quote>, decltype(compare)*> setQuo{compare};
};


int main()
{
    cout << "start" << endl;
}

我发现setQuo.insert(b);会导致编译错误。编译错误如下

*/usr/include/c++/7/bits/stl_tree.h:2069:51: error: binding reference of type ‘std::shared_ptr<Quote>&’ to ‘const key_type {aka const std::shared_ptr<Quote>}’ discards qualifiers
__x = _M_impl._M_key_compare(__k, _S_key(__x)) ?*

*/usr/include/c++/7/bits/stl_tree.h:1750:10: error: binding reference of type ‘std::shared_ptr<Quote>&’ to ‘const std::shared_ptr<Quote>’ discards qualifiers*

代码看起来很适合我，这个问题让我很困惑。

Answer 1

您的 compare 函数不应更改它正在比较的元素，因此请使用 const& 的参数：

        static bool compare(const shared_ptr<Quote> &l, const shared_ptr<Quote> &r) {
            return l->getnum() < r->getnum();
        }

Demo