invalid operands to binary expression ('std::ostream' (aka 'basic_ostream<char>') and 'void')报错

class birthday
{
private:
    DateV2 onedate;

public:
    birthday();
    birthday(int by, int bm, int bd);
    void input();
    void output();
    void printdate();
};
birthday::birthday(int by, int bm, int bd) : onedate(by, bm, bd) {}
void DateV2::output(){}
birthday::birthday() : onedate(2004, 7, 10){};
void birthday::printdate()
{
    cout << "My boirthday is:" << endl;
    cout << onedate.output()<<endl;
}

关于这一行代码
cout << onedate.output()<<endl;
他一直报错，显示
invalid operands to binary expression ('std::ostream' (aka 'basic_ostream') and 'void')
不知道怎么改了，能帮忙解答一下吗

你这个接口没有返回值