我相信这是因为 [] 在map中不是 const ，因为它创建具有默认值的新对，如果您寻址到不存在的对。尝试

I believe that it is because [] in map isn't const, as it creates new pair with default value, if you address to nonexisting one. Try

void pas(const map <string, float> &mm)
{
    cout<<mm.find("a")->second;
    cout<<mm.find("b")->second;
}