Почему высвечивается ошибка error: request for member ‘push_salary’ in ‘salary’, which is of non-class type ‘int()’

#include<vector>
#include<iostream>
#include<string>
#include <ctime>
using namespace std;
 
struct date
{
  int day;
  int month;
  int year;
 
  date()
  {
    day = 0;
    month = 0;
    year = 0;
  }
  date(const date& other)
  {
    day = other.day;
    month = other.month;
    year = other.year;
  }
  date(int day, int month, int year)
  {
    this->day = day;
    this->month = month;
    this->year = year;
  }
  void setDay(int day) {
    this->day = day;
  }
  int getDay() {
    return  day;
  }
 
  void setMonth(int month) {
    this->month = month;
  }
  int getMonth() {
    return  month;
  }
 
 
  void setYear(int year) {
    this->year = year;
  }
  int getYear() {
    return  year;
  }
};
 
 
class PERSON
{
 
protected:
  string name;
  string surname;
  string middle_n;
  string gender;
  date birthday;
public:
  friend istream& operator>>(istream&, PERSON&);
  friend ostream& operator<<(ostream&, PERSON&);
  PERSON()
  {
    name = "NULL";
    surname = "NULL";
    middle_n = "NULL";
    gender = "NULL";
    birthday.day = 0;
    birthday.month = 0;
    birthday.year = 0;
  }
 
  //операция разрешения области действия ::
 
  PERSON(string name, string surname, string middle_n, string gender, date birthday)
  {
    this->name = name;
    this->surname = surname;
    this->middle_n = middle_n;
    this->gender = gender;
    this->birthday = birthday;
  }
 
  void setName(string name) {
    this->name = name;
  }
  string getName()
  {
    return name;
  }
  void setSurname(string surname) {
    this->surname = surname;
  }
  string getSurname()
  {
    return surname;
  }
  void setMiddle_n(string middle_n) {
    this->middle_n = middle_n;
  }
  string getMiddle_n()
  {
    return middle_n;
  }
 
  void setGender(string gender) {
    this->gender = gender;
  }
  string getGender()
  {
    return gender;
  }
  void setDay(int day) {
    this->birthday.day = day;
  }
  int getDay() {
    return  birthday.day;
  }
 
  void setMonth(int month) {
    this->birthday.month = month;
  }
  int getMonth() {
    return  birthday.month;
  }
 
 
  void setYear(int year) {
    this->birthday.year = year;
  }
  int getYear() {
    return  birthday.year;
  }
 
 
  // Getter
  void printperson()
  {
    cout << name << surname << middle_n << gender << birthday.day << birthday.month << birthday.year;
  }
};
 
istream& operator >> (istream& is, PERSON& first)
{
  is >> first.name >> first.surname >> first.middle_n >> first.gender >> first.birthday.day;
  return is;
}
ostream& operator<<(ostream& os, PERSON& first)
{
  os << first.name << first.surname << first.middle_n << first.gender << first.birthday.day;
  return os;
}
 
 
 
class STUDENT : public PERSON
{
protected:
 
  PERSON person1;
  string institutename;
  string faculty;
  int course;
  int group;
 
 
public:
  int ID;
  friend bool operator>(const STUDENT& first, int value);
  friend istream& operator>>(istream&, STUDENT&);
  friend ostream& operator<<(ostream&, STUDENT&);
 
  STUDENT() : PERSON() {
 
    institutename = "null";
    faculty = "null";
    course = 0;
    group = 0;
    ID = 0;
 
  }
  STUDENT(PERSON person1, string institutename, string faculty, int course, int group, int ID) :PERSON(person1)
  {
 
    this->institutename = institutename;
    this->faculty = faculty;
    this->course = course;
    this->group = group;
    this->ID = ID;
  }
  STUDENT(string name, string surname, string middle_n, string gender, date birthday, string institutename, string faculty, int course, int group, int ID) :PERSON(name, surname, middle_n, gender, birthday)
  {
 
    this->institutename = institutename;
    this->faculty = faculty;
    this->course = course;
    this->group = group;
    this->ID = ID;
 
 
  }
 
  void setInstitutename(string institutename) {
    this->institutename = institutename;
  }
  string getInstitutename()
  {
    return institutename;
  }
  void setFaculty(string faculty) {
    this->faculty = faculty;
  }
  string getFaculty()
  {
    return faculty;
  }
  void setCourse(int course) {
    this->course = course;
  }
  int getCourse()
  {
    return course;
  }
 
 
void setGroup(int group) {
    this->group = group;
  }
  int getGroup()
  {
    return group;
  }
 
  void setID(int ID) {
    this->ID = ID;
  }
  int getID()
  {
    return ID;
  }
  void printf() {
 
    cout << name << surname << middle_n << birthday.day << faculty << course << institutename;
  }
};
 
struct mark_subj 
{
  int mark;
  int id;
  string namesubj;
  mark_subj()
  {
    id = 0;
    mark = 0;
    namesubj = "null";
  }
  mark_subj(const mark_subj& other)
  {
    mark = other.mark;
    id = other.id;
    namesubj = other.namesubj;
  }
  mark_subj(int mark, int id, string namesubj)
  {
    this->id = id;
    this->mark = mark;
    this->namesubj = namesubj;
  }
 
 
};
class GROUP : public STUDENT
{
  mark_subj pt_mark;
  int a;
public:
  GROUP()
  {
 
  }
  GROUP(string name, string surname, string middle_n, string gender, date birthday, string institutename, string faculty, int course, int group, int ID) :STUDENT(name, surname, middle_n, gender, birthday,
    institutename, faculty, course, group, ID)
  {
 
  }
  vector <mark_subj> mark;
  vector <STUDENT> vec;
  
  void enterdata()
  {
    string name1; string surname1; string middle_n1; string gender1; int day1 = 0; int month1 = 0; int year1 = 0;
    string institutename; string faculty; int course = 0; int group = 0; int ID = 0;
 
    cout << "enter name: ";             cin >> name1;
 
    cout << "enter surname: ";          cin >> surname1;
    cout << "enter middle name: ";      cin >> middle_n1;
    cout << "enter gender: ";           cin >> gender1;
    cout << "enter day of birth: ";             cin >> day1;
    cout << "enter month of birth : ";            cin >> month1;
    cout << "enter year of birth : ";             cin >> year1;
    cout << "enter institute name: ";   cin >> institutename;
    cout << "enter faculty: ";          cin >> faculty;
    cout << "enter course: ";           cin >> course;
    cout << "enter group: ";            cin >> group;
    cout << "enter ID: ";               cin >> ID;
 
 
    date date1(day1, month1, year1);
    PERSON person(name1, surname1, middle_n1, gender1, date1);
    STUDENT student(person, institutename, faculty, course, group, ID);
 
    vec.push_back(student);
    cout << endl << "data added" << endl;
  }
  void printdata()
  {
    for (int i = 0; i < vec.size(); i++)
    {
      cout << vec[i] << endl;
    }
  }
  void checkindex()
  {
    int index = 0;
    cout << "Vvedite index:"; cin >> index;
    cout << vec.at(index) << endl;
  }
 
  void dellastdata()
  {
    vec.pop_back();
    cout << "last element removed" << endl;
  }
  void changeindexelement()
  {
    vector<STUDENT>::iterator it = vec.begin();
    int index = 0;
    cout << "Vvedite index:"; cin >> index;
    advance(it, index);
    string name1; string surname1; string middle_n1; string gender1; int day1 = 0; int month1 = 0; int year1 = 0;
    string institutename; string faculty; int course = 0; int group = 0; int ID = 0;
 
    cout << "enter name: ";             cin >> name1;
 
    cout << "enter surname: ";          cin >> surname1;
    cout << "enter middle name: ";      cin >> middle_n1;
    cout << "enter gender: ";           cin >> gender1;
    cout << "enter day of birth: ";             cin >> day1;
    cout << "enter month of birth : ";            cin >> month1;
    cout << "enter year of birth : ";             cin >> year1;
    cout << "enter institute name: ";   cin >> institutename;
    cout << "enter faculty: ";          cin >> faculty;
    cout << "enter course: ";           cin >> course;
    cout << "enter group: ";            cin >> group;
    cout << "enter ID: ";               cin >> ID;
 
 
    date date1(day1, month1, year1);
    PERSON person(name1, surname1, middle_n1, gender1, date1);
 
    STUDENT student(person, institutename, faculty, course, group, ID);
 
    vec.push_back(student);
    cout << endl << "data added" << endl;
    vec.insert(it, student);
    cout << "data added" << endl;
 
 
}
  void deleteindexelement()
  {
    vector<STUDENT>::iterator it = vec.begin();
    int index = 0;
    cout << "Vvedite index:"; cin >> index;
    advance(it, index);
    vec.erase(it);
    cout << " element with index" << index << " removed" << endl;
  }
 
  void cleardata()
  {
    vec.clear();
    cout << "vector size now:" << vec.size();
  }
  void mark1() 
  {
    int size = 0;
    int id = 0;
    cout << "id";
    cin >> id;
    for (int i = 0; i < vec.size(); i++) 
    {
      if (vec[i].ID == id) 
      {
        cout << "size: ";
        cin >> size;
        for (int i = 0; i < size; i++) 
        {
          int mark1 = 0; string namesubj1;
          cout << "mark: "; cin >> mark1;
          cout << "name_subj: "; cin >> namesubj1;
 
          mark_subj a(mark1, id, namesubj1);
          mark.push_back(a);
        }
      }
    }
    
  }
  void checkmark() 
  {
    int id = 0;
    cout << "id";
    cin >> id;
    cout << "mark student with id" << id << endl;
    cout << "mark:\t" << "name subj:\t" << endl;
    for (int i = 0; i < mark.size(); i++)
    {
      if (mark[i].id == id)
      {
        cout << mark[i].id << "\t" << mark[i].namesubj << "\t" << endl;
      }
        
    }
      
  
  }
  void poorgrades() 
  {
    int id = 0;
    cout << "id";
    cin >> id;
    double sumgrades = 0;
    int k = 0;
    int average = 0;
    for (int i = 0; i < mark.size(); i++)
    {
      if (mark[i].id == id)
      {
        k += 1;
        sumgrades += mark[i].mark;
      }
 
    }
 
    average = sumgrades / k;
    cout << "average all grades: " << average << endl;
    if (average >= 2.5)
    {
      cout << "normal grades" << endl;
    }
    else cout << "poor grades" << endl;
  }
 
};
 
 
istream& operator>>(istream& is, STUDENT& first)
{
  is >> (PERSON&)first;
  is >> first.institutename >> first.faculty >> first.course >> first.group >> first.ID;
  return is;
}
 
ostream& operator<<(ostream& out, STUDENT& first)
{
  out << (PERSON&)first;
  out << first.institutename << first.faculty << first.course << first.group << first.ID;
  return out;
}
 
 
int main()
{
  GROUP a;
  /*mark_subj a2;*/
  int key = 0;
  do {
    cout << "menu:" << endl;
    cout << "to enter data enter 1" << endl;
    cout << "to look all data enter 2" << endl;
    cout << "to look data by index enter 3" << endl;
    cout << "to enter mark enter 4" << endl;
    cout << "to delete last data enter 5" << endl;
    cout << "to change data with index enter 6" << endl;
    cout << "to delete data with index enter 7" << endl;
    cout << "to clear data enter 8" << endl;
    cout << "to complete work enter 0" << endl;
    cin >> key;
    switch (key)
    {
    case 1: a.enterdata(); break;
    case 2: a.printdata(); break;
    case 3: a.checkindex(); break;
    case 4: a.mark1(); break;
    case 5: a.dellastdata(); break;
    case 6: a.changeindexelement(); break;
    case 7: a.deleteindexelement(); break;
    case 8: a.cleardata(); break;
    case 9: a.checkmark(); break;
    case 10:a.poorgrades(); break;
    }
  } while (key);
 
}