error: passing xxx as 'this' argument of xxx discards qualifiers
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.
Member functions that do not modify the class instance should be declared as const
:
int getId() const {
return id;
}
string getName() const {
return name;
}
Anytime you see "discards qualifiers", it's talking about const
or volatile
.