error: passing xxx as 'this' argument of xxx discards qualifiers

Question

#include <iostream> #include <set>  using namespace std;  class StudentT {  public:     int id;     string name; public:     StudentT(int _id, string _name) : id(_id), name(_name) {     }     int getId() {         return id;     }     string getName() {         return name;     } };  inline bool operator< (StudentT s1, StudentT s2) {     return  s1.getId() < s2.getId(); }  int main() {      set<StudentT> st;     StudentT s1(0, "Tom");     StudentT s2(1, "Tim");     st.insert(s1);     st.insert(s2);     set<StudentT> :: iterator itr;     for (itr = st.begin(); itr != st.end(); itr++) {         cout << itr->getId() << " " << itr->getName() << endl;     }     return 0; } 

In line:

cout << itr->getId() << " " << itr->getName() << endl; 

It give an error that:

../main.cpp:35: error: passing 'const StudentT' as 'this' argument of 'int StudentT::getId()' discards qualifiers

../main.cpp:35: error: passing 'const StudentT' as 'this' argument of 'std::string StudentT::getName()' discards qualifiers

What's wrong with this code? Thank you!

Answer 1

The objects in the std::set are stored as const StudentT. So when you try to call getId() with the const object the compiler detects a problem, mainly you're calling a non-const member function on const object which is not allowed because non-const member functions make NO PROMISE not to modify the object; so the compiler is going to make a safe assumption that getId() might attempt to modify the object but at the same time, it also notices that the object is const; so any attempt to modify the const object should be an error. Hence compiler generates an error message.

The solution is simple: make the functions const as:

int getId() const {     return id; } string getName() const {     return name; } 

This is necessary because now you can call getId() and getName() on const objects as:

void f(const StudentT & s) {      cout << s.getId();   //now okay, but error with your versions      cout << s.getName(); //now okay, but error with your versions } 

As a sidenote, you should implement operator< as :

inline bool operator< (const StudentT & s1, const StudentT & s2) {     return  s1.getId() < s2.getId(); } 

Note parameters are now const reference.