一个引用没效果错误示范

#include <iostream>
#include <unordered_map>
using namespace std;
class TestClass
{
public:
    std::unordered_map<int, int>& GetData() { return m_data; }
private:
    std::unordered_map<int, int> m_data;
};

int main()
{
    TestClass instance;
    {
        auto map = instance.GetData();  //引用错误示范, 这边的等号左边map是对右边的复制, 后面添加数据在临时对象上面操作的
        //auto& map = instance.GetData(); //正确用法
        map.insert(std::make_pair(1, 1));
    }
    {
        auto map = instance.GetData();
        int size = map.size();
        cout << "map's size is :" << size << endl; //输出:map's size is :0
    }
    system("pause");
    return 0;
}