返回值可接受为空，用以表示状态失败

举个栗子，

#include <iostream>
#include <optional>using namespace std;class User{string name;optional<string> nickName;optional<int> age;public:User(const string& name,optional<string> nickName,optional<int> age):name(name),nickName(nickName),age(age){}friend ostream& operator << (ostream& stream,const User& user);
};ostream& operator << (ostream& os,const User& user) {os<< user.name << ' ';if(user.nickName) {os<< *user.nickName << ' ';}if(user.age) {os <<"age of" << *user.age;}return os;
}int main() {User peter("Peter", "Spiderman", 20);User chris("Chris", nullopt, nullopt);cout << peter << endl;cout << chris << endl;return 0;
} 

Peter Spiderman age of20
Chris