你的类，作为一个工具函数，不是一个成员。

You have to define operator== with two Objects outside your class, as a tool function, not a member.

然后让它的朋友只是把函数的声明放在类中。

Then to make it friend just put the declaration of the function inside the class.

：

class Nick {

public:
    friend bool operator== ( const Nick &n1, const Nick &n2);
};


bool operator== ( const Nick &n1, const Nick &n2) 
{
        return n1.username == n2.username;
}

此外，您的查找应如下所示：

Also your find should look like this:

std::find(userlist.begin(), userlist.end(), Nick(username, false) );

不需要新。