Error: Binding reference of type .. to const

I’m trying to overload operator<<. When trying I got an error saying

Error: Passing const as this argument discards qualifiers

So I added const to my functions but now I’m getting this error:

Binding reference of type .. to const.

Main.cpp

ostream& operator<<(ostream& ostr, const Student& stud){
    float mo = 0;
    int quantity = stud.get_grade().size();\
.
.
.

Get_grade Function

vector<pair<Subject *, float>>& Student::get_grade() const{
    return grade;
}

Error

binding reference of type ‘std::vector<std::pair<Subject*, float> >&’ to ‘const std::vector<std::pair<Subject*, float> >’ discards qualifiers
| return grade;

Grade is a vector

>Solution :

get_grade is a const member function meaning that the type of this pointer inside it is const Student* which in turn means that the data member grade is treated as if it were itself const. But the problem is that the return type of your function is an lvalue reference to non-const std::vector meaning it cannot be bound to a const std::vector.

To solve this just add a low-level const in the return type of the function as shown below:

vvvvv------------------------------------------------------------->low-level const added
const vector<pair<Subject *, float>>& Student::get_grade() const{
    return grade;
}