Question

I can get a method of a class in a set iterator ?

#include <iostream>
#include <string>
#include <set>
class student{
  public:
     student(std::string n){
        name=n;
     }
     void print(){
        std::cout << name << std::endl;
     }
     bool operator < (const student & s1){ return true;}
     bool operator = (const student & s1){ return true;}
  private:
     std::string name;
};
int main(){
  std::set<student> studs;
  studs.insert(student("name01"));
  studs.insert(student("name02"));
  std::set<student>::iterator it;
  for(it = studs.begin(); it != studs.end(); it++)
      (*it).print() ;
}

I get this error

students.cpp: In function ‘int main()’:  
students.cpp:22: error: passing ‘const student’ as ‘this’ argument of ‘void student::print()’ discards qualifiers
/usr/include/c++/4.2.1/bits/stl_function.h: In member function ‘bool std::less<_Tp>::operator()(const _Tp&, const _Tp&) const [with _Tp = student]’:
/usr/include/c++/4.2.1/bits/stl_tree.h:982:   instantiated from ‘std::pair<typename std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::iterator, bool> std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_M_insert_unique(const _Val&) [with _Key = student, _Val = student, _KeyOfValue = std::_Identity<student>, _Compare = std::less<student>, _Alloc = std::allocator<student>]’
/usr/include/c++/4.2.1/bits/stl_set.h:307:   instantiated from ‘std::pair<typename std::_Rb_tree<_Key, _Key, std::_Identity<_Key>, _Compare, typename _Alloc::rebind<_Key>::other>::const_iterator, bool> std::set<_Key, _Compare, _Alloc>::insert(const _Key&) [with _Key = student, _Compare = std::less<student>, _Alloc = std::allocator<student>]’
students.cpp:18:   instantiated from here
/usr/include/c++/4.2.1/bits/stl_function.h:227: error: passing ‘const student’ as ‘this’ argument of ‘bool student::operator<(const student&)’ discards qualifiers

with

     bool operator<(const student & s1) const { return true;}  
     bool operator==(const student & s1) const { return true;}  

now work!! O_o’,

#include <iostream>
#include <string>
#include <set>
class student{
  public:
     student(std::string n){
        name=n;
     }
     void print() const {
        std::cout << name << std::endl;
     }
     bool operator<(const student & s1) const { return true;}
     bool operator==(const student & s1) const { return true;}
  private:
     std::string name;
};
int main(){
  std::set<student> studs;
  studs.insert(student("name01"));
  studs.insert(student("name02"));
  std::set<student>::iterator it;
  for(it = studs.begin(); it != studs.end(); it++)
      it->print() ;
}

Answer 1

You need to add a const qualifer to your print member function:

void print() const
{
  std::cout << name << std::endl;
}

Objects in an std::set are necessarily const, since they are used as keys. When an object (or reference) is constant, you can only call member functions of that object which are declared with the const qualifier.

You also want const qualifiers on both the == and < operator overload functions. (And don’t forget to change = to == as pointed out in the comments.)