c++ - Was not declared in the scope 错误

Question

我有一个保存学生数据的类(class)。在存储之前，将首先检查 ID 是否有效。在重载运算符 >> 时，我调用了 validate_id 函数。我已经将它声明为 friend ，但是在编译时，它说 'validate_id was not declared in scope 。是因为它是静态函数吗？

#include <iostream>
#include <string>
#include <locale>  
#include <utility>
#include <algorithm>
#include <stdexcept>


typedef std::pair<std::string, std::string> Name;

class Student {

   public:
       Student(){};

       bool operator <( const Student& rhs ) const {
          return ( id_ < rhs.id_ );
        }

   friend std::ostream& operator <<(std::ostream& os, const Student& s);
   friend std::istream& operator >>(std::istream& is, Student& s);

   private:
    std::string     id_;
    Name        name_;

    static bool validate_id(const std::string& id){
        if(id.length() != 9 && id[0] != 'a')
            return false;

        for(size_t i = 1, sz = id.length(); i < sz; i++){
            if(!isdigit(id[i]))
              return false;
        }

            return true;
    }

};


std::ostream& operator <<(std::ostream& os, const Student& s){
    return os << s.id_ << " " << s.name_.first << " " << s.name_.second;
}

std::istream& operator >>(std::istream& is, Student& s){
     is >> s.id_ >> s.name_.first >> s.name_.second;

    if(!validate_id(s.id_))  // error here, says validate_id is not in scope
        throw std::invalid_argument( "invalid ID" );


    return is;
}

Answer 1

validate_id 是Student 的静态成员，所以需要使用类的作用域来命名:

if(!Student::validate_id(s.id_))
    ^^^^^^^^^