help me with functors

[jinanzhaorenbo]

Hi, the following is my code, i have two questions:

first, in line 7, why can't i make ba1 and ba2 to be reference?
second, there are some standard comparison functors for sort algorithm, what is the difference between greater<T>() and greater_equal<T>(), and less<T>() and less_equal<T>()?

  1 #include <iostream>
  2 #include "bank_acc.cpp"
  3 #include <vector>
  4 #include <algorithm>
  5 using namespace std;
  6
  7 bool richer (bankAcct ba1, bankAcct ba2){
  8     return ba1.getBalance() > ba2.getBalance();
  9 }
 10
 11 int main(){
 12     vector<bankAcct> baV;
 13
 14     double balance;
 15     for (int i = 1; i <= 5; i++){
 16         cout << "Bank acct No." << i << endl;
 17         cout << "Balance: ";
 18         cin >> balance;
 19         baV.push_back(bankAcct (i, balance));
 20     }
 21
 22     cout << endl << "Sort bank accts in terms of balance in descending order.\n";
 23     sort(baV.begin(), baV.end(), richer);
 24     for (int i = 0; i < baV.size(); i++)
 25         cout << baV[i].toString() << endl;
 26
 27     return 0;
 28 }

[Cubbi]

1. of course you can use references:

bool richer (const bankAcct& ba1, const bankAcct& ba2){
    return ba1.getBalance() > ba2.getBalance();
}

2. greater<T>() sorts in descending order, less<T>() sorts in ascending order and is used by default. less_equal<T>() and greater_equal<T>() are not for use with sort()

Incidentally, #including a cpp file is very strange

[jinanzhaorenbo]

I have tried your code, but i get the following compilation error:
sort_ba.cpp:8:27: error: passing ‘const bankAcct’ as ‘this’ argument of ‘double bankAcct::getBalance()’ discards qualifiers
Why?

[Cubbi]

Why?

Because your forgot to make getBalance() a const member function, as in
double getBalance() const { return whatever; }

[jinanzhaorenbo]

why must i put 'const' in this case?